Canning Altitude Pressure Adjustment Calculator

Why altitude matters in canning

Water boils at lower temperatures at higher altitudes because atmospheric pressure decreases with elevation. At sea level, water boils at 100°C (212°F). At 1,800 m (6,000 ft), it boils at only 94°C (201°F). Since canning relies on reaching specific temperatures to kill botulism spores (Clostridium botulinum), you must compensate for altitude.

Boiling point at altitude:

Boiling point (°C) ≈ 100 - (Altitude in meters × 0.0034)

Boiling point (°F) ≈ 212 - (Altitude in feet × 0.00184)

Pressure canning adjustments (USDA guidelines):

For pressure canners, increase the pressure to maintain the required 116°C (240°F) internal temperature:

Note: Weighted gauge canners only have 10 and 15 PSI settings. If you are above 300 m (1,000 ft), always use 15 PSI with a weighted gauge.

Where those numbers come from. A canner gauge reads pressure above the surrounding air, and the surrounding air is what thins out as you climb. Sea-level air presses at 14.70 PSI, so an 11 PSI gauge reading is 25.7 PSI of real, absolute pressure, which is what holds water at 116°C. At 1,800 m the air is down to 11.82 PSI, nearly 3 PSI short, and the ladder above hands most of that back. At 3,050 m the shortfall is about 4.6 PSI and the dial goes to 15.
The altitude air pressure calculator gives the ambient figure for any elevation if you want to see the arithmetic. Use the USDA table for actual canning, though: it is a tested schedule with safety margin built in, not a physics exercise.

Water bath canning adjustments:

For acidic foods processed in a boiling water bath, increase processing time instead of pressure. The increment depends on the length of the sea-level process, and this is the part most altitude charts leave out: a process over 20 minutes takes double the addition of a short one.

So a 15-minute jam at 1,500 m becomes 25 minutes, while a 25-minute batch of tomatoes at the same elevation becomes 45. Reading the short column for a long process under-boils by exactly the amount the second column adds.

Worked example: green beans at 1,500 m (4,920 ft)

Green beans are low-acid and require pressure canning. The sea-level recipe says 10 PSI for 25 minutes (quart jars).

At 1,500 m with a dial gauge: increase to 13 PSI, keep the same 25-minute processing time. With a weighted gauge: use 15 PSI, keep 25 minutes.

Why this matters for safety:

Botulism toxin is destroyed at 85°C, but botulism spores survive boiling and can only be killed at 116°C (240°F) sustained for the full processing time. Underpressure at altitude means the internal temperature never reaches 116°C, leaving spores alive. This is the most dangerous food safety mistake in home canning. Always adjust for altitude.

Finding your altitude: Check your city’s elevation on a topographic map or use a GPS/smartphone altitude reading. When in doubt, round up to the next altitude bracket for safety.