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
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: sugar, pectin and the gel point
How the calculator turns fruit into a recipe
Sugar is set as a share of your fruit weight - 100% means equal weights of sugar and fruit, the traditional 1:1 jam. Yield comes from sugar concentration: jam is "done" when the soluble solids reach your set-point target (65% by default), so the finished weight equals the total sugar divided by that fraction, capped at the starting weight since boiling can only remove water, not add it. For 1 kg of strawberries plus 1 kg of sugar, that works out to roughly 1.66 kg of jam with about 340 g of water boiled off - close to the classic "1 kg fruit plus 1 kg sugar makes about 1.6 kg of jam" rule.
Pectin, acid and the set
Jam sets when pectin, acid and sugar are all in balance. High-pectin fruits - apple, quince, blackcurrant, gooseberry, citrus - gel on their own. Low-pectin fruits such as strawberry, peach and fig need help, so the calculator suggests around 15 g of classic powdered pectin per kg for low-pectin fruit and about 8 g per kg for medium. Acid does the same job for the set and for safety: low-acid fruit gets about 30 ml of lemon juice per kg, medium-acid about 15 ml, and naturally tart fruit none. Powdered-pectin strengths vary a lot between brands, so treat the figure as a starting point and follow the dose on your packet.
The gel point and your altitude
At sea level jam gels at 220 deg F (104.5 deg C), which is 8 deg F above the boiling point of water. Water boils cooler as you climb - roughly 1.8 deg F lower per 1000 feet - so the set point falls with it. At 5000 feet the calculator targets about 211 deg F (99.4 deg C). If you do not have a thermometer, use the cold-plate wrinkle test: chill a saucer, spoon on a little jam, and if it wrinkles when you push it the jam is set.
FAQ
How much sugar do I need for jam?
Traditional jam uses equal weights of sugar and fruit (a 1:1 ratio), which sets firmly and keeps for a year unopened. You can drop to 75% or 50% of the fruit weight for a softer, fresher-tasting jam, but lower sugar means a weaker set and a shorter fridge life unless you use a low-sugar pectin. This calculator shows the sugar weight live as you move the ratio slider.
At what temperature does jam set?
Jam reaches its gel point at 220 degrees F (104.5 degrees C) at sea level, which is 8 degrees F above the boiling point of water. At altitude water boils cooler, so the set point drops about 1.8 degrees F per 1000 feet of elevation. Enter your altitude and the calculator gives the exact target temperature in both Celsius and Fahrenheit.
Which fruits need added pectin for jam?
High-pectin fruits like apple, quince, blackcurrant, gooseberry and citrus set on their own with enough sugar and acid. Low-pectin fruits like strawberry, cherry, peach and fig usually need added powdered pectin or a high-pectin fruit blended in. The calculator labels your fruit's natural pectin level and suggests a powdered-pectin amount when one is needed.