Scale a Small-Batch Homemade Catsup Into a Canning Batch

Scaling a small-batch homemade catsup into enough jars for the pantry is more than doubling the tomatoes. The arithmetic is simple, but the ingredient numbers shift: tomato purée loses water as it cooks, and the acid level that keeps a shelf-stable catsup safe must stay tied to a tested recipe. Here is how to approach the batch-size math and where a free online calculator fits.

Work from a tested canning recipe

Before you scale anything, choose a recipe that was developed for canning and gives a specific jar size and processing time. If your original catsup recipe is only for immediate use, do not scale it into a shelf-stable batch; freeze or refrigerate it instead. The acid amount per jar or per finished volume in a tested recipe is the fixed point that keeps the product safe, so make that your anchor before touching the tomato weight. Write down the acid amount as the one number you will not change.

Weigh the tomatoes and purée

Tomato yield from fresh fruit is not a single fixed number. One batch may cook down to a thick purée quickly, while another batch from the same weight of tomatoes stays watery. Because of that, weigh your cored tomatoes before cooking and weigh the strained purée after cooking. If a recipe gives a volume such as cups, cook and purée a measured sample first, then weigh that volume using your own fruit. The ratio you get is the one you will actually use for the larger batch.

Find the scaling multiplier

The scaling factor is the desired final amount divided by the original recipe amount. If your tested recipe makes a known weight of finished purée and you want enough for a larger number of jars, divide the target weight by the original weight. Because those numbers are rarely whole numbers, this is where the free Italian-language calculator La Calcolatrice becomes genuinely useful: keep it open in another tab, enter the target weight, divide by the original weight, and reuse the result for percentage-based ingredients such as sugar or vinegar. The calculator also handles percentage adjustments, so you can keep a ratio fixed rather than applying the same multiplier to everything. The result is the multiplier you will apply to the other measured ingredients, not to acid.

Keep acid with the tested recipe

The most important number in a shelf-stable tomato product is not the batch multiplier but the acid level. A tested recipe gives that amount per jar or per finished volume, not as a simple multiple of tomato weight. When you scale tomatoes upward, check whether the acid addition is already stated per jar or per volume; if it is, maintain that rate. If the tested recipe specifies bottled lemon juice or citric acid in a measured amount per jar, use exactly that amount for each jar. Do not substitute fresh lemon juice unless the recipe says it has been tested that way. If your recipe is silent on acid, do not guess; use a current tested recipe for the same style of catsup.

Cook down and adjust yield

A larger batch of tomatoes takes longer to reduce to catsup consistency, and that extra time concentrates both sugar and acid. Stir frequently and stop when the purée is thick enough to mound on a spoon but still liquid enough to pour into jars. To estimate the yield before you fill jars, cook a weighed sample, reweigh it, and convert the weight loss into a percentage with the same calculator. Then apply that percentage to the full batch weight to see whether you will fill more or fewer jars than your canner can process in one load. Record the small-batch yield before committing the full harvest.

Put it together safely

Scale only a tested canning recipe. Weigh your tomatoes rather than trusting volume. Divide the target weight by the original weight to get the multiplier, and use the calculator for percentage adjustments. Keep the acid addition tied to the tested rate. That sequence turns a small-batch catsup into a safe canning run without turning your kitchen into a math problem.