Jam Sugar Ratio Calculator

How the calculator works

This calculator does not invent a ratio. It holds the three tested recipes that the National Center for Home Food Preservation publishes for jam made without added pectin, and it multiplies them by a whole number of batches.

total = amount for one batch x number of batches

One batch is:

• Berries: 4 cups of crushed fruit, 4 cups of sugar, no lemon juice. Makes 3 to 4 half-pints.
• Apricots: 4 to 4 1/2 cups of crushed fruit, 4 cups of sugar, 2 tablespoons of lemon juice. Makes 5 to 6 half-pints.
• Peaches: 5 1/2 to 6 cups of crushed fruit, 4 to 5 cups of sugar, 2 tablespoons of lemon juice. Makes 6 to 7 half-pints.

“Berries” in the source covers blackberries, boysenberries, dewberries, gooseberries, loganberries, raspberries and strawberries.

If you enter a part batch, such as 1.5, the calculator rounds up to 2. The recipes are tested at these sizes, and the calculator keeps to them.

The second thing it does is look up your elevation in the source’s table, and return the temperature at which the jam gels and the time the jars spend in the canner.

Worked example

You have a flat of strawberries and want about 10 half-pint jars.

1. One batch of berry jam makes 3 to 4 half-pints.
2. Three batches make 9 to 12, which covers 10.
3. Fruit: 3 x 4 = 12 cups of crushed strawberries.
4. Sugar: 3 x 4 = 12 cups.
5. Lemon juice: none.

Twelve cups of sugar is the number to check before you start. If your kitchen is at 3,000 feet, the gel point is 214 °F and the jars are processed for 10 minutes.

Those are the shopping totals. In the pot, make it as three separate batches of 4 cups of fruit and 4 cups of sugar, which is the size the recipe is written and tested for.

Why the fruit is measured crushed

The quantities are in cups of crushed fruit, not whole fruit and not pounds. A cup of whole strawberries is partly air and a cup of crushed strawberries is not, so the two are different amounts of fruit.

Prepare the fruit first, crush it, and then measure. If you are buying for a batch, buy more than the cup count suggests and measure what you get after crushing.

Why the sugar is not optional

These recipes have no added pectin, so the set comes from the fruit, the sugar and the cooking. The amounts were tested together, along with the gel temperature and the processing time that go with them. Change the sugar and you are no longer making the recipe that was tested.

That is why the calculator has no slider for sweetness. If you want a lower sugar jam, use a recipe that has been developed and tested as one. Do not reduce the sugar in these.

Testing for the gel point

The source gives two tests.

The temperature test. Boil the mixture until it reaches the temperature for your elevation. At sea level that is 220 °F. The full table:

• Sea level: 220 °F
• 1,000 feet: 218 °F
• 2,000 feet: 216 °F
• 3,000 feet: 214 °F
• 4,000 feet: 212 °F
• 5,000 feet: 211 °F
• 6,000 feet: 209 °F
• 7,000 feet: 207 °F
• 8,000 feet: 205 °F

The target falls by about 2 °F for each 1,000 feet, because water boils at a lower temperature the higher you are. At 5,000 feet the target is 211 °F, which is 9 °F below the sea level figure, so a recipe that only says “cook to 220 °F” is wrong for a kitchen at that height.

The refrigerator test. Take the pot off the heat. Put a small spoonful of the mixture on a cold plate and set it in the freezer compartment for a few minutes. If it gels, the jam is ready to go into jars. If not, return the pot to the heat and test again.

Processing times

For half-pint or pint jars, hot pack, in a boiling-water canner:

• 0 to 1,000 feet: 5 minutes
• 1,001 to 6,000 feet: 10 minutes
• Above 6,000 feet: 15 minutes

The calculator shows the time for the elevation you choose. If you are between two rows of the gel table, it uses the row below you. Check your exact elevation, since the processing time changes at 1,001 and 6,001 feet.

Tips and common mistakes

Measure the fruit after crushing, every time. The recipe counts cups of crushed fruit, and whole fruit measured by the cup comes up short.

Use a thermometer you trust. Check it in boiling water first: it should read the boiling point for your elevation, and if it is a degree or two out, allow for that.

Take the pot off the heat while you run the refrigerator test. Jam keeps cooking while you wait for the plate, and it can pass the gel point in that time.

Use a large pot. A full batch of fruit and sugar boils up high.

Do not stretch a batch with extra fruit. The fruit and sugar ranges in the recipe are the limits.

Keep the yield in mind when you count jars. The calculator shows the range, so have the larger number of jars ready.