Look up adjusted water bath canning processing times for jams, pickles, tomatoes, and fruit by altitude and jar size. Times are based on USDA guidelines — enter your food type, jar size, and elevation to get your safe processing time.
Water boils at 212°F (100°C) at sea level. As altitude increases, atmospheric pressure drops, and water boils at a lower temperature. At 5,000 feet, water boils at about 202°F. At 10,000 feet, it boils at around 194°F. That 18°F difference is significant — the heat from boiling water is what kills spoilage organisms and creates the vacuum seal in water bath canning. A lower boiling point means less heat penetration into the jar, which means you need more time to achieve the same level of safety.
This is not a minor adjustment. At 6,001–8,000 feet, you add 15 minutes to every processing time. At 8,001–10,000 feet, you add 20 minutes. Skipping the altitude adjustment at high elevation is one of the most dangerous mistakes in home canning — the food may look and smell fine while still harboring pathogens that weren't fully destroyed during processing. If you live in Colorado, Wyoming, Montana, Utah, or any elevated state, know your elevation before you start. The USDA recommends checking your local extension office if you're unsure of your precise altitude.
Water bath canning is only safe for high-acid foods with a pH of 4.6 or lower. The acid environment prevents the growth of Clostridium botulinum, the bacteria responsible for botulism, even without reaching temperatures above 212°F. High-acid foods that are safe for water bath canning include all jams, jellies, and preserves; most pickles (the vinegar provides the acid); tomatoes with added acid (lemon juice or citric acid — plain tomatoes are borderline and must have acid added); fruit juices; and most whole fruits. These are the categories in this calculator.
Low-acid foods — green beans, corn, carrots, meat, poultry, fish, and plain tomatoes without acid — must be processed in a pressure canner. A pressure canner reaches 240°F at 10 PSI, which is the only reliable way to destroy C. botulinum spores in low-acid environments. No amount of water bath canning time makes low-acid foods safe. This is non-negotiable. If someone tells you to water bath can green beans for 3 hours, that advice is wrong and dangerous. Use a pressure canner for all low-acid foods. See our general canning calculator and canning altitude calculator for pressure canning guidance.
Headspace is the empty space between the top of the food or liquid and the underside of the lid. The standard headspace for most water bath canned products is 1/4 inch for jams and jellies, and 1/2 inch for most fruits, tomatoes, and pickles. Headspace is not a suggestion — it determines whether a seal forms and holds.
Too little headspace means the food expands during processing and pushes against the lid, preventing a proper seal or even forcing the lid off entirely. Too much headspace means there's too much air trapped above the food, which may not be fully evacuated during processing — the residual air allows oxidation and can interfere with seal formation. At very high headspace (more than 1 inch), you also risk the jar not sealing at all. Use a canning funnel to fill jars consistently, and a headspace tool or ruler to verify before wiping the rim and applying the lid.
A properly sealed jar will have a lid that is concave (curved inward) and does not flex when pressed. You should hear a high-pitched ring when the lid is tapped with a metal spoon, not a dull thud. After the jars cool completely (12–24 hours), check each lid individually: press the center — it should not move at all. Remove the band and try to lift the lid off with your fingers; a good seal cannot be lifted without a tool. Any lid that pops up when pressed, makes a hollow sound, or lifts easily has not sealed.
Unsealed jars should be refrigerated and used within a few days, or reprocessed within 24 hours with a new lid. Do not simply re-tighten the band and put the jar on the shelf. Other signs of failed preservation include bubbling, spurting liquid when opened, off smells, mold growth, or a spurting sound when the lid is removed. When in doubt, throw it out — no home-canned food is worth a botulism risk. Botulism toxin is odorless and colorless, so appearance and smell are not reliable safety indicators for low-acid foods.
The USDA and NCHFP recommend consuming water bath canned food within 1–2 years for best quality, though properly sealed jars that have been stored in a cool, dark, dry location can remain safe beyond that. "Safe" and "best quality" are different things — flavor, color, texture, and nutritional value degrade over time even in sealed jars. Vitamin C content, for example, drops significantly after the first year. Jams and jellies may crystallize or lose their bright color after 18–24 months. Pickles can soften and lose their crunch. Most home canners rotate stock annually: can this year's harvest, use up last year's, and discard anything more than 2 years old if quality has noticeably declined. Always label jars with the food type and date before storing.