Home canning turns a summer harvest into food for winter, but only when the processing time is right. Too short, and the jar is not safe, however good it looks. Too long, and the food turns soft and dull. This free calculator adjusts the processing time for your food, your jar size and your altitude, so you start from a sensible number instead of a guess.
The rule behind the calculator is based on acidity, jar size and altitude: high-acid foods like fruit, pickles and tomato need about 10 minutes for a small jar, 15 minutes for a medium jar and 20 minutes for a large jar, while low-acid foods like meat, vegetables and beans need about 40, 55 and 75 minutes, with 5 to 15 extra minutes added for altitude. Acidity decides the method, because high-acid foods can be water-bathed while low-acid foods need pressure canning to reach a safe temperature. Jar size decides the time, because heat must reach the centre of the jar, and a large jar takes longer than a small one.
Altitude changes canning time because water boils at a lower temperature high above sea level. At 500 metres the calculator adds 5 minutes, at 1,000 metres it adds 10 minutes, and above 2,000 metres it adds 15 minutes. Skipping this step is the most common home canning error, and it leaves jars under-processed even when everything else looks right. Many home canners miss this step entirely, because sea-level recipes feel universal, and they are not. Altitude tables exist for every common food, and your local extension service publishes them free, so there is no reason to guess.
This calculator is educational guidance, not a tested recipe, and that distinction matters. A tested recipe has been checked in a laboratory for a specific food, jar size and method, while a calculator can only adjust a general time. Always start from a tested recipe for your exact food, and use the calculator to understand how altitude and jar size move the time, not to invent a recipe from scratch.
Jar preparation matters as much as the clock. Use jars made for canning, check rims for chips, use new lids, and leave the headspace your recipe states. Wipe rims clean before sealing, because a single drip of food on a rim can break the seal. After processing, let jars cool undisturbed, check that each lid has sealed, and store sealed jars in a cool, dark place.
After opening, the rules change completely. An opened jar belongs in the fridge and should be used within a few days, just like any other opened food. A jar that did not seal, a jar with a bulging lid, or a jar that spurts or smells strange on opening should be thrown away without tasting. Home canning rewards care, and it punishes shortcuts, so never taste a doubtful jar to check it.
Altitude is only one of the variables that move canning time. Food density matters too: a thick puree heats slower than a thin juice, and packed vegetables heat slower than loose ones. That is why recipes specify a pack style and a headspace, and why changing a recipe by packing jars tighter or adding dense ingredients can break a time that was safe for the original version. Write the food, the date and the processing time on every jar lid, so next winter you know exactly what is inside and when it was sealed.
Save this page as a learning aid for your next canning season. The calculator is free, needs no account, and works in your browser on a phone or a computer. Enter acidity, jar size and altitude, then check the result against a tested recipe before you seal a single jar.