Free jam jar calculator. Enter fruit in kilograms to find how many 500 gram jars your jam fills and the sugar you need for the batch.
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Enter fruit weight in kilograms. This jam jar calculator plans 2 jars of 500 grams and 750 grams of sugar per kilogram of fruit, so 2 kilograms of fruit plan 4 jars and 1500 grams of sugar. Weigh the fruit after preparation, since stones, cores and hulls change the weight that actually goes into the pot.
The classic jam ratio is close to equal weights of prepared fruit and sugar, often 1 kilogram of fruit to 750 grams to 1000 grams of sugar. This calculator uses 750 grams of sugar per kilogram of fruit, a middle ratio that suits most berries and stone fruit. Very tart fruit, such as blackcurrants, may need the full 1000 grams, while sweet ripe fruit may set well at 700 grams. Sugar is not only for sweetness: it helps the jam set, preserves it, and gives the finished jam its body.
Pectin is the natural gelling agent in fruit, and it decides whether a jam sets without help. High pectin fruit includes plums, grapes, quinces, blackcurrants and crab apples, and usually needs no added pectin. Medium pectin fruit includes raspberries, blackberries and figs, which may need a little help. Low pectin fruit includes strawberries, peaches, cherries and blueberries, which usually need added pectin, often one packet per 4 pounds of fruit in competitor tables. Very low pectin fruit, such as apricots, almost always needs added pectin.
Strawberry jam is the classic low pectin case. Strawberries are low in pectin and low in acid, so a strawberry jam recipe usually adds pectin and lemon juice together. Lemon juice adds acid, which helps pectin form its gel, and it brightens the flavour. As a guide, add the juice of one lemon per kilogram of strawberries, plus the pectin amount your pectin packet recommends for that batch size. Without that help, strawberry jam often stays runny even after a long boil.
Jam sets at its setting point, about 104 to 105 degrees C on a thermometer, or about 8 degrees above the boiling point of water at altitude. If you have no thermometer, use the cold plate test: put a small spoonful on a cold saucer, wait 30 seconds, then push it with a finger. If the surface wrinkles, the jam has reached its setting point. If it stays liquid, boil for 2 more minutes and test again. The spoon sheet test is a third check: jam that sheets off a spoon in a single sheet, rather than dripping in separate drops, is ready.
In this calculator, 1 kilogram of fruit plans 2 jars of 500 grams. Two kilograms plan 4 jars, and 5 kilograms plan 10 jars. Use this yield guide to see how much jam a batch can make before you buy jars. Competitor guides using smaller jars give higher counts: 1 kilogram of fruit with 800 grams of sugar gives roughly 1.5 to 1.8 kilograms of finished jam, which fills 4 to 5 jars of 340 grams, or about 4 jars of 375 grams. The finished jam weight is fruit plus sugar minus evaporation loss, about one fifth for a traditional long boil and about one tenth for a short boil pectin jam. Always sterilise one more jar than you expect to fill.
Sterilise jars before filling, using any of three methods. Oven method: wash jars, place them in a cold oven, heat to 120 degrees C for 15 minutes, and fill them while hot. Boiling method: boil jars in water for 10 minutes and lift them out with tongs. Dishwasher method: run the hottest cycle and use the jars straight from the machine while hot. Sterilise lids as well, fill jars to within about 6 millimetres of the rim, wipe rims clean, and seal at once so the vacuum forms as the jam cools.
A low sugar jam ratio cuts sugar well below the classic ratio, but it changes setting and shelf life, so it needs a tested low sugar pectin and its own recipe. Ordinary pectin will not set a low sugar jam reliably. Follow the pectin packet for the exact fruit, sugar and lemon juice amounts, and store low sugar jam in the fridge rather than the cupboard unless the recipe says it is shelf stable. Never guess a low sugar ratio for a jam you plan to sell or store for a year.
Marmalade follows the same fruit to sugar thinking as jam, with citrus peel supplying plenty of pectin, so it usually sets well at a near equal fruit to sugar ratio. For scaling, jam scales cleanly in small batches: double the fruit, double the sugar, and use a wider pot rather than a deeper one, so evaporation stays fast. Very large batches take longer to reach setting point and can darken, so most guides suggest cooking no more than 2 kilograms of fruit at a time.
Sealed homemade jam kept in a cool dark cupboard usually lasts 12 to 18 months. Once opened, keep it in the fridge and use it within 4 to 6 weeks. Low sugar jam lasts less long, so check its recipe for storage advice. Discard any jar with mould, a broken seal, fizzing, or an off smell, and never scrape mould off and eat the rest. Label every jar with the fruit and the date you made it.
In this calculator, 1 kg of fruit plans 2 jars of 500 grams. With smaller 340 gram jars, the same batch fills about 4 to 5 jars, since finished jam weighs about 1.5 to 1.8 kg.
About 750 grams in this calculator, within the classic 750 to 1000 gram range. Tart fruit may need up to 1000 grams, while sweet fruit may set at 700 grams.
Close to equal weights of fruit and sugar, often 1 to 0.75 or 1 to 1. This calculator uses 1 kg of fruit to 750 grams of sugar.
Usually yes. Strawberries are low in pectin, so add pectin plus lemon juice: about one lemon per kilogram of strawberries, plus the pectin packet amount for the batch.
About 104 to 105 degrees C. Without a thermometer, use the cold plate test: a spoonful on a cold saucer should wrinkle when pushed after 30 seconds.
Oven method: 120 degrees C for 15 minutes. Boiling method: 10 minutes in boiling water. Dishwasher method: hottest cycle, used while hot. Fill jars while they are hot.
Only with a tested low sugar pectin recipe. Ordinary pectin will not set a low sugar jam reliably, and low sugar jam usually needs fridge storage.
Sealed jars last about 12 to 18 months in a cool dark cupboard. Opened jars last 4 to 6 weeks in the fridge. Discard any jar with mould or a broken seal.