Jam Sugar Ratio Calculator

Equal weights, 1kg of sugar to 1kg of prepared fruit, is the standard for high and medium pectin fruit: cooking apples, citrus, gooseberries, blackcurrants, plums, raspberries and apricots. Low-pectin fruit such as strawberries, cherries and rhubarb takes LESS sugar, around 750g per kilo, not more. OAKDEN puts it as "1kg of Fruit to 1kg of Sugar (high pectin) or 1kg Fruit to 750g Sugar (low pectin)". Gooseberry and blackcurrant jam is traditionally taken as high as 1.5kg of sugar per kilo of fruit, which makes a bigger batch out of the same fruit. Allotment Garden reports no problem setting or storing high-pectin jams at equal weights and no improvement to the flavour from the extra sugar, so this page uses equal weights and shows the published range beside the answer.

Pectin is what sets jam, and unless you use jam sugar every bit of it comes from the fruit. The finished jam is boiled to a fixed concentration, around 65% soluble solids, so the sugar you add decides how big the finished batch is: 1kg of sugar makes about 1.5kg of jam whatever fruit it came from. Add more sugar to a fixed quantity of fruit and the same fixed quantity of pectin has to set a larger batch, so it ends up more dilute and the set is looser. That is why the traditional rule runs the way it does. Allotment Garden states it plainly: with a fruit that is low in pectin, "it is advisable to increase the proportion of fruit to sugar to allow more pectin to be extracted".

Pectin is a natural substance found in fruit that causes jam to set. Fruits high in pectin (cooking apples, citrus peel, gooseberries, blackcurrants) set easily. Low-pectin fruits (strawberries, cherries, rhubarb) need added pectin, jam sugar or a smaller sugar charge to achieve a good set. Where the fruit supplies all the pectin, which is the case with granulated and preserving sugar, low-pectin fruit is given less sugar, never more. Jam sugar brings pectin of its own, so choosing it here puts the charge back up to equal weights.

Regular granulated sugar works perfectly for high-pectin fruits. Jam sugar (which has pectin added) is useful for low-pectin fruits as it simplifies the process, and on this page choosing it raises the charge on a low-pectin fruit from 750g back to 1000g per kilo, because the added pectin is what lets the bigger batch set. Preserving sugar has larger crystals that dissolve slowly and produce less scum, but it has no added pectin, so it does not change the quantity.

Lemon juice adds acid, which helps the pectin set, helps draw it out of the fruit, and brightens the flavour. Allotment Garden lists cooking apples, blackcurrants, redcurrants, cranberries, damsons, gooseberries, loganberries, plums and raspberries as carrying enough acid already, and bilberries, late blackberries, blueberries, sweet cherries, strawberries, peaches and pears as generally lacking it, which is largely the same set as the low-pectin fruits. So this calculator adds none for high-pectin fruit, treats it as optional for medium, and recommends it for low. Two tablespoons is about the juice of one lemon, so the default rate of 1 tbsp per 500g works out at roughly one lemon per kilo of fruit.

The finished weight is set by concentration, not by a fixed percentage. Sugar does not evaporate during boiling, only water does, so jam is boiled until the soluble solids reach about 65% (the set point at 104-105 °C). That means roughly the sugar weight divided by 0.65: 1kg of sugar gives about 1.5kg of jam. The Estimated Yield on this page counts the sugar you add and nothing else, so treat it as an estimate rather than a promise. The fruit brings its own sugars, which push the finished weight up, while skimming off the scum, jam left in the pan and boiling past the set point all take weight away. Measured against the published yields on Allotment Garden it lands about 2.5% ABOVE their three equal-weight recipes (1.8kg of sugar, "Makes around 6 lbs (2.7 kg) of jam") and about 23% BELOW their cherry jam (1.125kg of sugar, "Makes around 5 lbs (2.25 kgs)"), so sterilise a spare jar. UK law (the Jam and Similar Products (England) Regulations 2003) sets a 60% soluble-solids minimum, so a 1kg-sugar batch should never end up heavier than about 1.67kg, or it will be under-set and spoil faster.

On this page the sugar charge sets the SIZE of the batch, not how sweet the jam is. The pan is boiled to a fixed concentration, so 750g of sugar per kilo of low-pectin fruit makes a smaller batch at the same sweetness and the same keeping quality, and that 750g is the traditional figure for those fruits rather than a reduction. Choosing jam sugar here puts the charge back UP to 1000g per kilo, because its added pectin can set the bigger batch: it gives you more jam, not less sugar. A genuinely less-sweet jam means a lower finished concentration instead. UK law requires jam to reach 60% soluble solids, while a product labelled "reduced sugar" may be 25% to 50% (Jam and Similar Products (England) Regulations 2003, Schedule 1, note 2(b)). Sugar at those lower concentrations is not preserving the fruit the way it does at 60%, which is what OAKDEN means by "low-sugar recipes will effect the keeping quality of the preserve", so a home-made reduced-sugar jam is a fridge jam. This calculator does not model one: Target Soluble Solids holds at the 60% legal minimum, and the answer now names the figure it used when it has raised yours to it.