The recipe says twenty cups of sugar. The store sells four-pound bags.
Set your batch, say how many Saturday needs, and this works out the whole runs of the pot, the packs and cases to actually buy, what is left standing on the shelf, and an honest cost per jar. Everything stays in your browser.
The batch
A cottage-food kitchen has two shapes: the pot that cooks down, and the tray that just makes a number. Pick yours — everything below follows it.
Market day
You cannot run part of a batch, so the shopping follows whole runs — not the number you wish you needed.
The buy list
What one batch takes, and how the store actually sells it. Each line carries its own pack size, because sugar comes in 4 lb bags where flour comes in 5 — and one figure for both is wrong in half the aisles.
What one costs you
The jars
Your pantry
The numbers here are yours to check. This page does arithmetic on what you type — it cannot see your recipe, your equipment or your batch.
For anything you will eat or sell as food, double-check it against your own process and your local rules. This page does maths, not food safety.
This is one calculator, not five.
The batch feeds the runs, the runs feed the shopping, the packs feed what one jar's ingredients really cost, the case feeds the glass, and the cost feeds what you keep at the table, on Etsy, or cash in hand. Change the fill from 10 oz to 8 and the jars, the runs, the bags, the cases and the price you can defend all move at once.
In your shop it is the same calculator with your own pantry behind it. It answers before you shop: “you can make 32 of the Strawberry Preserve with what is already in the pantry — 18 more would need 9 lb more strawberries,” with the numbers. The plan lands in Make with How many already filled in and the pot helper open on your cook-down. A web calculator cannot do that, because it has never seen your pantry. $19/mo for the whole back office, kept for as long as you stay. The shop stays free for cost a piece and one fair — no product cap. The tax packet, Etsy and Square, and more fairs are the $19.
Free calculator · jam, preserves and the rest of the kitchen
Batch math is mostly folk math — pounds in the pot, a guess at the boil, a shelf of clean jars waiting, and a shopping list written on the back of an envelope. Here is how the calculator handles it, and which numbers are yours to argue with.
How many jars will 12 pounds of strawberries make?
Twelve pounds of prepared berries and twenty cups of sugar is about 21 pounds in the pot, which is 336 ounces. At the default 25% cook-down you ladle out about 252 ounces of finished jam, and filling 10 ounce jars that is 25 jars with 2 ounces left over for the fridge — so wash 25 jars and set out 25 lids before the fruit hits the heat. Your sugar ratio, your pectin and how hard you boil will all move that number, which is why every input on this page is editable.
The recipe says twenty cups of sugar. How many bags is that?
That is the arithmetic this page exists for, and it is the one nothing on the web does end to end. Twenty cups a batch across two runs is forty cups; at the published weight of a cup of granulated sugar that is a little over seventeen and a half pounds, so it is five 4 lb bags with about two and a half pounds standing on the shelf afterwards. The leftover is reported, never charged to this run — it is next Saturday's shopping, already paid for. Where an ingredient is written in one unit and sold in another and there is no weight per cup we can stand behind, the line is left blank and says so rather than inventing a density.
Why is the cook-down percentage a setting instead of a fixed number?
Because it is the most-argued number in preserving, and both camps are right about their own pots. A long open boil to the set point drives off far more water than a quick pectin set, and a soup simmered down for an hour is different again — so the same ingredients can finish at very different volumes. 25% is a seeded middle, not a law, and the answer always says which figure it used. The good news is your own answer is cheap to find: weigh a finished batch against what went into the pot, put that number in, and from then on the page runs on your kitchen instead of anyone's convention.
I already own the jars. What do I still have to buy?
Lids. A flat lid seals once and is not meant to be used again; the bands are reusable as long as they are not rusted. A new case of jars brings its own lids, so the line you re-buy every single batch is one new lid for every jar already washed on your shelf — and that is the line most people forget until the fruit is already cooking. Rings are your call: if jars leave your table band and all, that band is consumed and belongs in the cost.
Can this calculator tell me processing times, headspace, or how much acid to add?
No, and that is deliberate. This tool does yield, packs and money — pounds in, jars out, bags to buy, dollars per jar — and nothing else. Processing time, headspace, acidity and altitude adjustments come from your tested recipe and the canning authorities you already trust, and how much you are allowed to sell comes from your own state's cottage food program. Those change; a stale number on a page like this would be worse than no number at all.