Running out of tomato paste in the middle of cooking is annoying, but it rarely ruins a dish. Tomato paste is simply tomatoes cooked down until most of the water is gone, so almost anything made from tomatoes can stand in for it if you adjust the amount. This free calculator works out exactly how much tomato sauce, crushed tomatoes, or ketchup you need for the amount of paste in your recipe, so you can keep cooking without guessing.
The key idea is concentration. One tablespoon of tomato paste holds about as much tomato flavour as three tablespoons of tomato sauce, because sauce still contains most of its water. Crushed tomatoes sit between the two: they are thicker than sauce but still much wetter than paste, so you need about twice as much. Ketchup is already thick and seasoned, so it swaps spoon for spoon, but it brings extra sugar and vinegar that you must keep in mind.
To use the tool, enter the amount of tomato paste your recipe asks for and pick the unit you have: tablespoons, cups, or grams. The calculator then shows all three substitutes at once, in tablespoons, with a cup conversion when the amount is large. Pick whichever substitute you actually have in the cupboard, measure it out, and carry on with the recipe as written.
Tomato sauce is usually the best substitute because its flavour is plain and close to paste. Use three times the paste amount, then simmer the dish a little longer so the extra water cooks off. If the sauce is already seasoned with herbs or garlic, taste before you add extra seasoning later, since the dish can easily end up too salty or too herby when a seasoned sauce replaces plain paste.
Crushed tomatoes work well in soups, stews, and braises where a chunkier texture is welcome. Use twice the paste amount and give the dish extra cooking time to thicken. Because crushed tomatoes vary a lot between brands, check the tin: some are mostly juice, while others are thick and pulpy. Thicker crushed tomatoes need less extra simmering, and watery ones need more.
Ketchup is the emergency option, and it works better than many cooks expect, but only in small amounts. Use the same amount as the paste, then cut back on any sugar in the recipe and go easy on vinegar or lemon juice, because ketchup already contains both. It suits barbecue sauces, meatloaf, and chilli, but it can taste out of place in a delicate Italian tomato sauce.
Whatever substitute you choose, remember the extra liquid. Paste adds almost no water, while sauce and crushed tomatoes add a lot, so a dish can turn thin and watery if you do not adjust. The simplest fix is time: simmer uncovered for five to fifteen extra minutes until the sauce coats a spoon again. If you are in a hurry, leave out a splash of stock or water elsewhere in the recipe.
A common mistake is swapping the amounts one for one. Using one tablespoon of sauce where a recipe wants one tablespoon of paste gives only a third of the tomato flavour, and the dish tastes flat and pale. Another mistake is forgetting the sugar in ketchup, which can make a savoury dish taste sweet. Measure the substitute properly, taste as you go, and adjust salt, sugar, and acid at the end.
If you often have paste left over, freeze it instead of throwing the tube or tin away. Spoon tablespoon-sized blobs onto a tray, freeze them solid, then store them in a bag. Next time a recipe needs a spoonful, you can take exactly what you need. And if you are out of paste entirely, this calculator plus a tin of tomatoes in the cupboard means dinner is never more than a few minutes away.