Three weights, and only one of them is on the recipe
A recipe that says "8 pounds of tomatoes" means eight pounds of tomatoes after you have taken the stems and the cores out. What you buy at the stand is the weight before that. If you trim 15 per cent away, the eight pounds on the page is 9.4 pounds at the till, and if you are working from a crop with pits and skins it can be 11 or 12.
Then the pot takes another cut, and this one is much larger. Cooking a thin tomato sauce down to something you would put on pasta removes more than half its volume. The yield factor on this page — cups of finished product per pound of trimmed produce — is the number that carries that loss, and it is the one worth measuring rather than looking up.
| Stage | What it is | Who sets it |
|---|---|---|
| Raw weight | What the scale says when you get home | What you picked or bought |
| Trimmed weight | Raw minus stems, cores, pits, peel and culls | The crop, and how hard you trim |
| Finished volume | Cups in the pot at the end | How far the recipe reduces it |
Measuring the yield factor once
Take a container of produce and weigh it. Trim it the way you always trim it and weigh it again — the drop is your trim loss. Cook that trimmed weight exactly as the recipe says, and pour what comes out into a measuring cup. Divide cups by trimmed pounds and you have a number that is correct for your crop, your knife and your stove, which no table can be.
Write it in the front of whatever notebook you keep. It moves a little year to year with the weather, but not by the factor of two that separates one published figure from another. A restaurant kitchen runs the same arithmetic under a different name and turns it into a plate cost — that is the portion yield and trim loss calculator.
The jar size argument
Half the volume of a batch fits neatly in one arrangement of jars and awkwardly in another. Twenty cups goes into five quarts exactly; the same twenty cups in pint-and-a-half jars is six full jars and a third of a seventh. The last-jar remainder is not waste — it goes in the fridge and gets eaten first — but if you are short on jars it decides which size you reach for.
Larger jars are less work per unit of food and less glass to store, and they commit you to opening more at a time than you may want. That is a household question, not an arithmetic one. Once you know the jar count, the jar, lid and case calculator works out what to buy, and the canner load and session calculator turns it into a day.
What this page does not do
It has no process time, no headspace, no acidity or altitude figure and no keeping time, and it makes no judgement about whether anything is safe to eat. Those belong to a tested recipe from the USDA, the National Center for Home Food Preservation or your state extension service. This page counts pounds and jars, and that is all it does.
Questions people ask
Why does the recipe weight not match what I bought?
Because recipes are almost always written in trimmed weight and produce is almost always sold in raw weight. Peel, stems, cores, pits and the fruit you reject at the sink are all gone before the recipe starts counting. Fifteen per cent is a mild case; anything you peel and pit runs far higher.
Where do I get the cups-per-pound figure?
From one test batch of your own. Trim a weighed amount, cook it the way the recipe says, and measure what comes out. Published figures for the same crop differ by a factor of two because they assume different varieties, different ripeness and different degrees of reduction. Yours will be right for your kitchen and it will not change much year to year.
How many pounds is a bushel?
It depends entirely on the crop, and it is a legal weight in some states rather than a volume. The extension service publishes bushel weight tables for exactly this reason. Look yours up and type it into the unit weight field, and the calculator will tell you how many to buy.
Can I run it backward from the jars I have?
That is what the second direction does. Give it a jar count and it works back through the fill volume, the yield factor and the trim loss to the raw weight you need to pick or buy, plus how many boxes or plants that is.
What does this page say about processing?
It gives none, on purpose. This page is quantity and container arithmetic. Process times, pressures, headspace, altitude adjustments and anything about acidity belong to a tested recipe from the USDA, the National Center for Home Food Preservation or your state extension service, and those are the only places to get them. Where the calculator asks for one of those numbers, it is because you are copying it in from the recipe you are already following.