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איך החישוב עובד
Water boils at a lower temperature as altitude increases — roughly 1°F (about 0.5°C) cooler for every 500 feet of elevation gain. Since boiling-water canning relies on maintaining boiling temperature for a set time to destroy spoilage organisms, a process that's perfectly safe at sea level may not get hot enough for long enough once you're above 1,000 feet. USDA and the National Center for Home Food Preservation (NCHFP) solve this by extending the processing TIME rather than trying to raise the temperature, since boiling water can't get hotter than its (altitude-reduced) boiling point no matter how long you leave it on the heat.
Pressure canners work differently: the sealed pot lets you raise the temperature above boiling regardless of altitude, so instead of adding time, you adjust the PSI (pounds of pressure) to compensate for the lower atmospheric pressure at elevation. Weighted-gauge canners only offer a few fixed settings — typically 5, 10 and 15 lb — so their altitude table has just two coarse steps (10 lb up to 1,000 ft, 15 lb above it). Dial-gauge canners can be set to any exact PSI, so their table uses finer 2,000 ft bands (11/12/13/14 lb) to match the exact pressure your elevation requires.
⚠ Safety disclaimer: These are the standard USDA / National Center for Home Food Preservation altitude adjustment tables. Always start from a tested, published recipe's sea-level instructions — from USDA, NCHFP, or your county extension office — never a random blog recipe, and apply this adjustment on top of it. Never reduce processing time or pressure below what a tested recipe specifies. This tool adjusts time and pressure for altitude only; it does not validate whether your recipe, jar size, or process is safe to can at all.