How much a batch makes
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Work it from the fruit, not from the recipe
A recipe tells you what one batch makes. It cannot tell you what a flat of strawberries makes, because that depends on how much of the flat is usable after hulling and how much of it was already going over when you bought it. Working forward from the fruit you actually have is the only version of this arithmetic that survives contact with a real delivery.
The step everyone skips is the second weighing. Hulls, pits, stems and the fruit you throw away are not a rounding error on soft fruit, and a yield calculated from the number on the invoice will be optimistic by whatever proportion of that flat went in the bin.
- Weigh what you actually bought, then weigh it again after hulling, pitting and discarding. The second number is your usable fruit.
- Apply the ratio. At 45 parts fruit to 55 parts sweetener, the finished batch before cooking is roughly your fruit weight divided by 0.45.
- Subtract what boils off. Without added pectin that is substantial and it is the step people leave out; with added pectin it is small.
- Divide by the net weight your jar actually holds, which you got by weighing jam into it rather than from the fluid-ounce figure.
- Round down to whole jars, then round down again to what a case holds.
The two benchmarks worth having
Benchmarks matter here because yield is one of the few numbers in this craft with a published floor. If you are below it, the problem is upstream of the arithmetic: usually the fruit, sometimes the extraction, occasionally a pan that is boiling off more than you think. A low yield is a symptom worth chasing rather than a figure to accept and plan around.
- For jelly, the guide gives a floor on extraction: one pound of fruit should produce at least one cup of clear juice. Less than that and something upstream is wrong.
- Your own loss from prep to pot, measured once per fruit. It does not change much year to year and it is the number that makes a purchase decision possible.
What a case does to the arithmetic
This is the step that turns a yield figure into a purchasing decision, and it is where a tidy calculation meets the fact that nobody sells you forty jars. Plan the batch to the case rather than the case to the batch: buy fruit in the quantity that fills whole cases, and the per-jar cost stops drifting batch to batch for reasons that have nothing to do with the jam.
Jars come in cases, and a case is a case whether you fill it or not. If a batch makes 40 jars and the case is 48, the per-jar packaging cost on that batch is the full case divided by 40.
