Jam calculator

Jam sets at the altitude you actually cook at

Give it the fruit, the prepared weight and your altitude. You get the sugar and lemon from the published recipe for that fruit, and the temperature jam sets at where you are — which is not the 105 °C in the French books if you live on a plateau.

Your pan

Prepared fruit (g)

Peeled, pitted and cut already. This is the weight the ratios were written against.

Altitude (m)

Of the place you will be cooking. It moves both the set point and the canning time.

Fruit

Per the NCHFP, this fruit always needs added acid, pectin or both. The lemon in the recipe is not seasoning: it is what makes the jam set. NCHFP, Jellied Product Ingredients — Pectin and Acid Content of Common Fruits

Embrapa table — pectin poor, acidity medium. This fruit appears in both classifications, the American and the Brazilian, and the two agree. Row reproduced from Jackix (1988) by Embrapa. Third-hand citation.

How much sugar

The ratio the Blue Chair Jam Cookbook publishes for this fruit, against the weight of prepared fruit. Every fruit has its own, and that is what an average would hide.

What to weigh

White cane sugar645 g

Strained lemon juice: 97 g
Apple jelly: 200 g
Powdered pectin, if you use it: 3.2 g – 9.7 g
Sugar against fruit: 64.5%
Yields, roughly: 3 jars of 240 ml
Water to boil off: 422 g – 499 g
Target pH: 3.0 – 3.2

Saunders, p. 178 · Embrapa, Documentos 138, § Pectina — quantidade a ser acrescentada, § Formulação — geleia de mirtilo e geleia de amora-preta · Embrapa, Documentos 29, §1 Introdução — "geralmente 1% é suficiente para produzir uma geléia firme", §1 Introdução — janela de pH da geleificação, §2.7 Adição de acidulantes — pH final de 3,0 a 3,2

Sugar against fruit

The source ratio

Cited recipe: 64.5% · Shelf life the book declares: 6 months

When to stop cooking

Setting point: 104.4 °C
Water boils at: 100.0 °C
Boiling-water canner: 5 min

Eight degrees Fahrenheit above boiling. Go past it, Saunders warns, and you get an "irrevocably tough, leathery" preserve — with no way back.

Half-pint or pint jars, jam without added pectin. NCHFP Table 2, which raises the time precisely because water boils cooler up there.

NCHFP, Testing Jelly without Added Pectin, Making Jam without added Pectin — tabela 2 · Saunders, p. 26 · Ferber, cap. "Le sucre et la cuisson"

Ferber, cap. "Le sucre et la cuisson", receita "Griottes"

The set point, at your altitude

Ferber says cook to 105 °C and Saunders says 220 °F. Same point, and both are sea-level numbers. The NCHFP is the only one that writes the rule in a form that survives moving house.

The set point, at your altitude
m	0	305	610	914	1219	1524	1829	2134	2438
°F	220	218	216	214	212	211	209	207	205
°C	104.4	103.3	102.2	101.1	100.0	99.4	98.3	97.2	96.1

Two honest caveats. The NCHFP table is American and the boiling-water canner is its method: Saunders sterilises in the oven and Ferber fills to the brim, caps hot and inverts the jar — all three work, and the NCHFP does not endorse inverting. And reaching the setting point depends on the mixture having enough sugar to get there; with little sugar the thermometer only climbs after the fruit has already overcooked.

What the Brazilian sources add

Two free publications from Embrapa, Brazil’s public agricultural research agency, closed three gaps this research had been carrying in the open: fruit that grows here, the dose for powdered pectin, and the pH window.

The legal ruler

Brazil’s definition of jam does not talk about recipes. It talks about parts of fruit to parts of sugar, and about how much soluble solid the product must hold. It is a ruler for industrial labelling, which is why it asks for more sugar than any home recipe on this shelf.

The legal ruler
Class	Fruit : sugar	Sugar against fruit	Soluble solids, minimum
Extra	50 : 50	1.00	65%
Standard	40 : 60	1.50	62%
Standard quince, orange and apple	35 : 65	1.86	62%

Notice where they meet: the 65% soluble solids of extra jam are exactly the 65% sugar Christine Ferber names as the keeping point, and neither source knows the other. That is the second, independent source the water-to-boil-off estimate never had.

Embrapa, Documentos 29, §1 Introdução — classificação legal de geléia comum e extra

The dose for powdered pectin

From 0.5% to 1.5% against the sugar in the formulation — the base is the sugar, and that is the detail a hurried reading loses. The document checks itself: of the eleven weighed formulations it publishes, ten land inside its own band and the eleventh misses by five hundredths of a point. Where you land inside the band depends on the fruit’s own pectin, and that is what the table below is for.

Embrapa, Documentos 138, § Pectina — quantidade a ser acrescentada · Embrapa, Documentos 29, §1 Introdução — "geralmente 1% é suficiente para produzir uma geléia firme"

The pH window

The gel forms around pH 3, and above pH 3.4 it does not form at all. In the finished jam the target is 3.0 to 3.2. For anyone with strips or a meter there is a bench rule worth more than any dose: measure the pulp before you start, and if it already sits between 3.0 and 3.3, do not acidify. How much acid to add when it sits outside remains unpublished, and so it stays off the screen.

Total acidity in the finished jam should sit between 0.5% and 0.8%. Above 1% you get syneresis — the jam weeping liquid in the jar — the same fault the NCHFP already blamed on excess acid, now with a threshold attached.

Embrapa, Documentos 29, §1 Introdução — janela de pH da geleificação, §2.7 Adição de acidulantes — pH final de 3,0 a 3,2, §1 Introdução — acidez total e sinérese · Embrapa, Documentos 138, § Formulação — geleia de mirtilo e geleia de amora-preta

The fresh jam, and why it is not a preserve

The biodiversity cookbook from Brazil’s environment ministry publishes weighed jams for fruit no preserving book on this shelf covers: umbu, pitanga, cerrado and wild passion fruit, cerrado pear, jaboticaba and cashew apple. Quantities in grams, from a free official publication, and it is what those fruits needed to step off the norm’s generic ruler.

What they are not is preserves. The method stops at 65–70 °C, where pectin dissolves — the setting point is thirty-five degrees above that. No jars, no water bath, no shelf life declared. It is a plated component, and the page says so every time one of those fruits is chosen.

The four pectin doses in the cookbook — 1.33%, 1.33%, 1.00% and 0.50% against the sugar — all land inside the band Embrapa publishes. A third institution, a third kind of publication, four out of four.

MMA, Biodiversidade 52, p. 312, p. 324, p. 760, p. 714, p. 226, p. 228, p. 172

Table 1, in full

Thirty-eight fruits classified on two axes. It is here whole because a trimmed table is a table we edited, and because variety matters: ripe fig and green fig are not the same row, and do not hold the same pectin.

Table 1, in full
Fruit	Pectin	Acidity	Row comes from
Pineapple	poor	high	Jackix (1988)
Acerola	poor	medium	Embrapa
Japanese plum	rich	high	Jackix (1988)
Araçá (purple guava)	rich	high	Embrapa
Banana (d’água or nanica)	medium	low	Embrapa
Cajá-manga (ambarella)	poor	high	Embrapa
Cashew apple	poor	medium	Jackix (1988)
Persimmon	poor	low	Jackix (1988)
Sour starfruit	poor	medium	Jackix (1988)
Sweet starfruit	poor	low	Jackix (1988)
Ripe fig	poor	low	Jackix (1988)
Green and half-ripe fig	rich	low	Jackix (1988)
Sugar apple	poor	medium	Embrapa
Red guava	rich	medium	Jackix (1988)
Currant	rich	high	Jackix (1988)
Jaboticaba, common	poor	medium	Jackix (1988)
Jaboticaba, ponhema	poor	high	Jackix (1988)
Jaboticaba sabará, skin on	medium	high	Jackix (1988)
Jaboticaba sabará, peeled	poor	low	Jackix (1988)
Orange, whole fruit	rich	high	Jackix (1988)
Lemon	rich	high	Jackix (1988)
Tart apple	medium	high	Jackix (1988)
Sweet apple	rich	medium	Jackix (1988)
Papaya	poor	low	Jackix (1988)
Mango, espada	medium	high	Jackix (1988)
Mango, espadão	rich	high	Jackix (1988)
Passion fruit, juice	poor	high	Embrapa
Quince	rich	medium	Jackix (1988)
Strawberry	poor	medium	Jackix (1988)
Loquat	medium	high	Jackix (1988)
Ripe water pear	poor	low	Jackix (1988)
Ripe yellow peach	poor	low	Jackix (1988)
Green peach	rich	high	Embrapa
Pitanga (Suriname cherry)	medium	high	Jackix (1988)
Pomegranate	poor	medium	Embrapa
Grape: ananás, catawba, empire state	poor	high	Jackix (1988)
Grape: isabel and niágara	medium	high	Jackix (1988)
Uvaia	poor	high	Jackix (1988)

Thirty of the thirty-eight rows are Jackix’s (1988), reproduced by Embrapa; the origin column says which. Until that book reaches the shelf, the citation for those rows is third-hand, and the table says so rather than hiding it.

Embrapa, Documentos 29, Tabela 1 — classificação de algumas frutas segundo teores de pectina e acidez

A second table for the setting point, this one in metres

Embrapa converts boiling temperature into °Brix by altitude, in metres and in Celsius. It is not the same quantity as the American table — one says where the gel forms, the other says when the syrup reaches a given concentration — and that is precisely why their landing so close together is worth something.

A second table for the setting point, this one in metres
°Brix	0 m	500 m	1,000 m	1,500 m	2,000 m
50	102.2	100.5	98.8	97.1	95.4
60	103.7	102.2	100.3	98.6	96.9
62	104.1	102.4	100.7	99.0	97.3
64	104.6	102.9	101.2	99.5	97.8
66	105.1	103.4	101.7	100.0	98.3
68	105.7	104.0	102.3	100.6	98.9
70	106.4	104.7	103.0	101.3	99.6

At 65 °Brix, interpolating between the 64 and 66 rows, the gap against what this calculator returns stays under seven tenths of a degree across the whole range. Rewritten as how far the setting point sits above boiling at sea level: NCHFP, 4.44 °C; Embrapa, 4.85 °C; Ferber, 5.00 °C. The Brazilian table stops at 2,000 m, and above that the page shows only the American one — extrapolating someone else’s table is not citing, it is inventing.

Embrapa credits this table onward itself: "taken from a fruit processing course".

Embrapa, Documentos 138, § Determinação da temperatura de ebulição — Tabela 1, conversão em °Brix

How the calculation works

The sugar figure is a rule of three against the weight of prepared fruit, which is the base Saunders insists on. What the calculator brings is not the arithmetic: it is where the ratio comes from. Each fruit carries the recipe published for it, with a page number, and the ratio comes out of that recipe — apricot at 0.42, strawberry at 0.65, raspberry at 1.00. An average of those nine would be nobody’s source.

Saunders writes in pounds and ounces, and none of it needed converting. A ratio between two masses has no unit: 40 ounces of sugar to 62 of fruit is the same 0.6452 in grams, in ounces or in stones. That is why the calculator’s test can reproduce the book’s recipe line by line with no conversion factor anywhere in the path.

The setting point is the part no book hands over ready-made. The NCHFP publishes a table of temperature against altitude, in feet and Fahrenheit, and the calculator interpolates that table for the altitude you enter. A curiosity of the source: the NCHFP text sums it all up as "subtract 2 degrees F" per thousand feet, but the table on the same page drops only one degree between 4,000 and 5,000 feet and stays a degree above the rule from there on. The table is the one that is right — water’s boiling point does not fall in a straight line with altitude. Below 1,219 m the two agree, which covers nearly every large Brazilian city.

Brazilian fruit comes in by another road, and the road is stated on screen. Guava, jaboticaba and passion fruit have no weighed recipe in any book on this shelf — what exists is Table 1 from Embrapa, which classifies 38 fruits by pectin and acidity on two separate axes, and the Brazilian legal definition, which fixes the ratio in parts of fruit to parts of sugar. So the classification comes from one source and the ratio from another, and the page names both. What Embrapa does not publish — yield in jars, shelf life, a lemon dose — simply does not appear for those fruits, rather than being borrowed from a fruit that has nothing to do with them.

The water to boil off is the one estimate on the page, and it combines two of Ferber’s numbers: the 65% sugar of a keeping preserve and the 10% to 15% the fruit already brings. It comes out as a range because the second number is a range. It serves one purpose, but an important one: showing why low-sugar jam cooks so much longer. With less sugar, more water has to leave to reach the same 65% — and that is where the fruit falls apart. Since September 2026 that 65% is no longer a single-source figure: it is also the minimum soluble solids Brazilian law demands of extra jam, written in 1988 with no knowledge of Ferber.

Common questions

How much sugar for 1 kg of fruit?

It depends on the fruit, and that is the honest answer. In the Blue Chair Jam Cookbook it runs from 417 g per kilo for apricots to a full kilo per kilo for raspberries. Christine Ferber uses 800 g per kilo through nearly her whole book. This calculator gives you the ratio from the cited recipe for the fruit you picked, and shows Ferber alongside it.

What temperature does jam set at?

Eight degrees Fahrenheit above wherever water boils in your kitchen — 104.4 °C at sea level, about 100 °C in Brasília. The 105 °C the French books quote is a sea-level number for Europe. Enter your altitude and the calculator gives you your own.

Can I make jam with less sugar?

You can, but the product changes category. The NCHFP is blunt: sugar is the preservative, and "too little sugar prevents gelling and may allow yeasts and molds to grow". With little sugar you have a refrigerator spread to eat within a month, not a shelf-stable preserve.

Do I need store-bought pectin?

For NCHFP Group I fruits, no: they have pectin and acid to spare. Group III always needs added acid, pectin or both. The calculator offers Ferber’s way out — borrowing pectin from 200 g of apple jelly per kilo of fruit rather than reaching for the powder.

What is the lemon juice for?

Two things at once: "Acid is needed both for gel formation and flavor", says the NCHFP. Without enough acid the pectin never forms a network and the jam will not set. With too much, the network turns unstable and the jam weeps in the jar.

Can I make guava, jaboticaba or passion fruit jam?

You can, and as of September 2026 the calculator handles those fruits. What changes is where the number comes from: no book on this shelf publishes a weighed guava recipe, so the classification comes from Table 1 of Embrapa, Brazil’s public agricultural research agency — red guava is rich in pectin and medium in acidity — and the ratio comes from the Brazilian legal definition of jam. What Embrapa does not publish, such as yield in jars and shelf life, simply does not appear.

How much powdered pectin do I use?

From 0.5% to 1.5% of the sugar, not of the fruit. The base is what people get wrong: in a jam with 1 kg of sugar that is 5 to 15 g of pectin. Embrapa publishes the band and the formulations in the same document obey it. Where you land inside the band depends on the fruit’s own pectin, which is why the calculator shows the classification beside the dose instead of naming one figure.

What pH does jam need to set?

The gel forms around pH 3, and above 3.4 it does not form; in the finished jam the target is 3.0 to 3.2. There is a bench rule worth more than any lemon dose: measure the pulp before you start, and if it already sits between 3.0 and 3.3, do not acidify. How much acid to add when it sits outside is what no source publishes, so the calculator does not say.

Why does the recipe ask for prepared fruit?

Because that is the weight the ratio was written against. Saunders is explicit: "always base the amount of sugar on the total weight of raw prepared fruit being used". Weigh after peeling, pitting and cutting — not the bag from the market.

Where the sources disagree

Three books, two official agencies, and seven disagreements worth knowing about.

Topic	What each source says	What the calculator does
Setting temperature	Ferber: 105 °C. Saunders: 220 °F, which is 104.4 °C. NCHFP: 8 °F above wherever water boils, with a table by altitude.	Works it out from your altitude, always. The first two are the same point measured at sea level; only the third moves cities with you.
How much sugar	Ferber writes "un poids de sucre plus ou moins égal à celui du fruit", yet 93 of her 218 recipes use 800 g per kilo. Saunders ranges from 0.42 to 1.00 depending on the fruit.	Uses the cited recipe for that fruit, and offers Ferber’s measured 0.80 alongside. Her prose and her bench do not agree, and we say so.
Sealing the jar	NCHFP: boiling-water canner, 5 to 15 min by altitude. Saunders: oven at 250 °F. Ferber: fill to the brim, cap hot and invert.	Reports the NCHFP time, the only one with a table by altitude, and says the other two methods belong to the authors cited. The NCHFP does not endorse inverting.
Jam from a plating book and jam from a preserving book	The MMA cookbook publishes jams between 0.40 and 0.77 sugar against fruit. Brazilian law requires at least 1.00 before the product may be called extra jam, and Saunders works between 0.42 and 1.00 in recipes that go into jars.	Shows all three rulers and says they measure different products. An environment ministry cookbook prints as jam what the labelling norm would not allow — and it is right, because its jam is eaten the same day. The calculator gives the ratio from whichever source covers that fruit, with the warning that source justifies.
How much sugar, again: recipe or norm	Saunders ranges from 0.42 to 1.00 sugar against fruit. Ferber uses 0.80. Brazilian law requires at least 1.00 before the product may be called extra jam, and 1.50 for standard jam.	Shows both rulers and says they measure different things. The norm classifies a labelled industrial product; Saunders and Ferber write home recipes. Neither is wrong, and the site does not quietly pick one.
Where the gel forms, in metres and Celsius	NCHFP: 8 °F above boiling, tabled in feet and Fahrenheit. Embrapa: boiling temperature by °Brix, in metres and Celsius, up to 2,000 m. Ferber: 105 °C.	Computes from the NCHFP table, which reaches higher, and publishes the Brazilian one alongside as a check. The two land within seven tenths of a degree of each other across the shared range, with neither knowing the other exists.
Two classifications of fruit	NCHFP: three groups on one axis, folding missing acid together with missing pectin ("acid, pectin or both"). Embrapa: two separate axes, pectin and acidity, three levels each.	Shows whichever one the source for that fruit publishes, and both where both exist. Translating one into the other would invent an equivalence neither claims. On the three fruits where they meet — strawberry, ripe peach, ripe fig — they agree.
Pectin in guava	The NCHFP puts guava in Group III, "always needs added acid, pectin or both". McGee says the Spanish exploited its high pectin content to make a New World quince paste.	Offers guava as of September 2026, and offers it because Embrapa broke the tie: "red guava, ripe and half-ripe — pectin rich, acidity medium". The NCHFP’s group III is "acid, pectin or both", and the reading that acid is what is missing stopped being a guess.

Glossary

Prepared fruit#: The weight that counts: fruit already peeled, pitted and cut, with whatever juice it released. Not the shopping weight, and the gap is wide — Saunders’s peach jam starts at 6.5 pounds at the market to arrive at 5.5 prepared.Saunders, p. 22
Setting point#: The temperature at which the mixture, left to cool undisturbed, forms a jelly. It is not a fixed number: it is about 4.4 °C above wherever water boils in your kitchen, which moves with altitude.Saunders, p. 26 · NCHFP, Testing Jelly without Added Pectin · Ferber, cap. "Le sucre et la cuisson"
Nappé#: The French name for the same point, tested without a thermometer: the syrup coats the back of the spoon in a film that does not run off. "Vérifiez la nappe", Ferber writes at the end of almost every recipe.Ferber, cap. "Le sucre et la cuisson"
Sheet test#: You dip a cool metal spoon and watch how the syrup falls. While it drips in separate drops, it is not ready; when two drops join and run off the edge as one sheet, it has arrived.NCHFP, Testing Jelly without Added Pectin · Saunders, p. 35
Freezer test#: Metal spoons wait in the freezer from before the pan goes on. You put half a spoonful of jam on one, return it for three or four minutes and tilt it: if it does not run, it is done.Saunders, p. 34 · NCHFP, Testing Jelly without Added Pectin
Pectin#: The cell-wall substance in fruit that forms the network of the gel. It needs sugar and acid to work, and it peaks in just-ripe fruit — underripe and overripe will not gel.NCHFP, Jellied Product Ingredients · Saunders, p. 23
Pectin group#: The official grouping of fruit into three classes, by whether they hold their own pectin and acid (I), may need one of the two (II), or always need added acid, pectin or both (III). It is the American ruler, and it has a single axis: group III does not say which one is missing. The Brazilian one separates them.NCHFP, Jellied Product Ingredients — Pectin and Acid Content of Common Fruits
Embrapa Table 1#: The Brazilian classification: 38 fruits on two independent axes, pectin (rich, medium, poor) and acidity (high, medium, low). It is what lets you say of guava that pectin is plentiful and acid is short, something the American group III cannot distinguish. Thirty of the rows are reproduced from Jackix (1988).Embrapa, Documentos 29, Tabela 1 — classificação de algumas frutas segundo teores de pectina e acidez
Extra and standard jam#: The two classes in Brazilian food law, defined by ratio: extra is fifty parts fruit to fifty parts sugar, standard is forty to sixty. Quince, orange and apple may go to thirty-five by sixty-five. It is a ruler for industrial labelling, not for a home recipe — which is why it asks for more sugar than the books do.Embrapa, Documentos 29, §1 Introdução — classificação legal de geléia comum e extra
Soluble solids#: How much of the finished product is dissolved matter, nearly all of it sugar, read in degrees Brix on a refractometer. The Brazilian norm requires at least 62% in standard jam and 65% in extra; Ferber arrives at the same 65% calling it sugar content. It is the number that decides whether the jam keeps outside the fridge.Embrapa, Documentos 29, §1 Introdução — classificação legal de geléia comum e extra · Ferber, cap. "Le sucre et la cuisson"
Apple jelly#: Pectin borrowed from another fruit instead of bought as powder. Ferber uses 200 g per kilo in fruits that will not gel alone — pears, cherries and sour cherries among them — and in red-fruit jams accepts redcurrant jelly in its place.Ferber, cap. "Le sucre et la cuisson", receita "Griottes"
Syneresis#: The jam weeping liquid in the jar after it is made. The causes the NCHFP lists are excess acid, which makes the pectin unstable, and storage that is too warm or fluctuates in temperature. Embrapa supplies the threshold that was missing: above 1% total acidity in the finished jam, syneresis follows.NCHFP, Causes and Possible Solutions for Problems with Jellied Fruit Products · Embrapa, Documentos 29, §1 Introdução — acidez total e sinérese
Marmalade and marmelada#: Two different things with one origin. The word is Portuguese and named a quince paste; in English, marmalade became the translucent jelly with citrus peel suspended in it, after the sour orange displaced the quince in the 18th century.McGee, cap. 7, "A Survey of Common Fruits" — "Citrus Fruits" · NCHFP, Types of Jellied Products

Sources for this calculator

Two preserving books, a McGee, an American agency and two Embrapa publications. The ratios come from Saunders’s weighed recipes and Ferber’s house proportion; the temperature by altitude and the keeping rules from the NCHFP; the Brazilian fruit, the legal ruler, the pectin dose and the pH window from Embrapa.

The Blue Chair Jam Cookbook
Saunders · Andrews McMeel
p. 178 · p. 234 · p. 268 · p. 168 · p. 246 · p. 197 · p. 224 · p. 183 · p. 265 · p. 26
Manual para a produção de geléias de frutas em escala industrial
Embrapa, Documentos 29 · Embrapa Agroindústria de Alimentos
Tabela 1 — classificação de algumas frutas segundo teores de pectina e acidez · §1 Introdução — classificação legal de geléia comum e extra · §1 Introdução — "geralmente 1% é suficiente para produzir uma geléia firme" · §1 Introdução — janela de pH da geleificação · §2.7 Adição de acidulantes — pH final de 3,0 a 3,2 · §1 Introdução — acidez total e sinérese
Biodiversidade brasileira: sabores e aromas
MMA, Biodiversidade 52 · Ministério do Meio Ambiente
p. 172 · p. 312 · p. 324 · p. 226 · p. 714 · p. 228 · p. 760
National Center for Home Food Preservation
NCHFP · USDA
Jellied Product Ingredients — Pectin and Acid Content of Common Fruits · Testing Jelly without Added Pectin · Making Jam without added Pectin — tabela 2
Mes confitures
Ferber · Payot
cap. "Le sucre et la cuisson" · receita "Griottes"
Keys to Good Cooking
McGee · Penguin
p. 104
Preparo artesanal de geleias e geleiadas
Embrapa, Documentos 138 · Embrapa Clima Temperado
§ Pectina — quantidade a ser acrescentada · § Formulação — geleia de mirtilo e geleia de amora-preta · § Determinação da temperatura de ebulição — Tabela 1, conversão em °Brix

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