Jam Making Calculator
Pick your fruit and how much you have, and get the sugar, pectin, and lemon juice to add, how many jars it makes, and the temperature jam sets at where you live. Adjusts for your altitude. Nothing uploaded.
Units
Sugar ratio ?
= 1000 g sugar
Advanced options
Set point target ?
Saved recipes
Makes about - jam, around - of this size. About - of water boils off (- finished).
Your jam recipe
- - fruit, prepared
- - sugar
- - powdered pectin ?
- - lemon juice
Cook, stirring, until it reaches
Learn more: why jam sets (or doesn't)
Pectin, sugar and acid work together
The ThermoWorks guide to pectin and jam temperatures says pectin needs sugar, acid and the right temperature to gel. It puts the sugar level at above 60%, so that water binds to the sugar and not to the pectin.
That is why lemon juice shows up even for pectin-rich fruit. The calculator tracks pectin and acid separately. Apple is high in pectin but only medium in acid, so it needs no added pectin but still gets 15 ml of lemon juice per kilogram of fruit. A low-acid fruit such as peach or blueberry gets 30 ml per kilogram.
The pectin amounts are the tool's defaults: 15 g of powdered pectin per kilogram for low-pectin fruit, 8 g for medium and none for high. Brands differ a lot, so follow the packet if it disagrees.
How much water boils off
Take 1000 g of strawberries (the tool assumes 8% natural sugar, or 80 g) and 1000 g of added sugar. That starts at 2000 g with 1080 g of sugar, a 54.0% concentration. Boiling removes water until the sugar share reaches your target, 65% by default.
The finished jam then weighs 1080 / 0.65, about 1662 g, so roughly 338 g of water boils away. The tool marks a batch as shelf-stable in its own terms when the final concentration is at least 60%, in line with the ThermoWorks figure above. It is a set guide, not a canning safety rule, so follow a tested canning method for storage.
Setting temperature and altitude
The tool takes the gel point as 8 F above the local boiling point of water, which is 220 F (104.4 C) at sea level. It lowers the boiling point by 1.8 F per 1000 ft of elevation. At 5000 ft that gives a boiling point of 203 F and a set point of 211 F (99.4 C). ThermoWorks rounds the drop to about 2 F per 1000 ft, so expect a degree of difference from other charts.
FAQ
Why does the recipe still ask for lemon juice on high-pectin fruit?
Pectin and acid are tracked separately. A fruit such as apple can be rich in pectin and only medium in acid, so the tool adds lemon juice for the acid side even when it adds no pectin.
Does altitude change when jam is done?
Yes. Water boils at a lower temperature as you climb, and the tool sets the gel point 8 F above that. At sea level it is 220 F (104.4 C) and it falls about 1.8 F for every 1000 ft. A cold-plate wrinkle test is a useful check if you have no thermometer.
Why is my jam thinner than expected?
Common causes are too little sugar, too little acid or stopping before the set point. If the final sugar concentration is under 60%, ThermoWorks says gelling is unlikely to work, so check the batch figure in the tool, then boil longer or add pectin.