Jam Jar Quantity Calculator

The approximate yield of jam is about 85% of the combined weight of fruit and sugar. The remaining 15% is water that evaporates during the boiling process. So 1kg fruit + 1kg sugar gives roughly 1.7kg (1,700g) of jam.

Leave about 5mm of headspace at the top of each jar. This allows for a proper vacuum seal to form as the jam cools. The lid should create a slight concave shape when sealed properly, indicating a good seal.

The most popular size for homemade jam in the UK is the 1lb jar, which holds about 454g of jam (its water capacity is around 380ml). For gifts, the 8oz / half-pint jar (~227g) is a nice size, and 12oz jars hold about 340g. Small hexagonal jars hold roughly 190g. UK jars are graded by the weight of preserve they hold, not their water volume. All jars should have new, unused lids for a proper seal.

A "1lb jam jar" is named for the weight of jam it holds when filled to the shoulder: about 454g (1lb). Its actual water capacity is only around 380ml, because jam is denser than water (roughly 1.3g/ml). That is why this calculator divides your estimated yield weight by the jar grade in grams — 1700g of jam ÷ 454g per jar ≈ four 1lb jars, matching the well-known rule that 1kg fruit plus 1kg sugar fills about four 1lb jars.

Always prepare 1-2 more jars than you think you will need. Yield can vary depending on how long you boil the jam and the water content of your fruit. Having spare sterilised jars ready avoids scrambling if you have more jam than expected.