Canning Time Altitude Adjustment Calculator Formula

Understand the math behind the canning time altitude adjustment calculator. Each variable explained with a worked example.

Use the Canning Time Altitude Adjustment Calculator

Formulas Used

Additional Time Needed

extra_time = base_time <= 20 ? (altitude > 1000 ? ceil((altitude - 1000) / 1000) * 1 : 0) : (altitude > 1000 ? ceil((altitude - 1000) / 1000) * 2 : 0)

Adjusted Processing Time

adjusted_time = base_time + (base_time <= 20 ? (altitude > 1000 ? ceil((altitude - 1000) / 1000) * 1 : 0) : (altitude > 1000 ? ceil((altitude - 1000) / 1000) * 2 : 0))

Water Boiling Point

boil_temp = 212 - altitude * 0.00184

Variables

VariableDescriptionDefault
base_timeRecipe Processing Time (sea level)(minutes)10
altitudeYour Altitude(feet)3000
alt_bracketDerived value= floor(altitude / 1000)calculated

How It Works

How to Adjust Canning Time for Altitude

Rule

For water-bath canning above 1,000 feet:

  • Processing time 20 min or less: Add 1 minute per 1,000 ft above 1,000 ft
  • Processing time over 20 min: Add 2 minutes per 1,000 ft above 1,000 ft

    • Water boils at a lower temperature at higher elevations (roughly 1.84°F less per 1,000 ft). The longer processing compensates for the reduced heat transfer.

    Important

    This applies to water-bath canning only. For pressure canning, increase the pressure (not the time) at altitude. Always follow USDA-tested recipes.

    Worked Example

    A jam recipe calling for 10 minutes processing at 3,000 feet altitude.

    base_time = 10altitude = 3000
    1. 01Base time = 10 min (under 20, so add 1 min per 1,000 ft above 1,000)
    2. 02Extra = ceil((3000 - 1000) / 1000) x 1 = 2 minutes
    3. 03Adjusted time = 10 + 2 = 12 minutes
    4. 04Boiling point at 3,000 ft = 212 - 5.5 = 206.5°F

    Frequently Asked Questions

    Why does altitude affect canning?

    Water boils at lower temperatures at higher altitudes because atmospheric pressure decreases. At 5,000 feet, water boils at about 203°F instead of 212°F. Lower boiling temperatures require more time to achieve the same level of microbial destruction.

    Does altitude affect pressure canning?

    Yes, but you adjust pressure, not time. Increase the pressure by 1 psi for every 2,000 feet above sea level (for weighted gauge canners, go to the next increment). Processing time stays the same.

    Can I reprocess jars that did not seal?

    Yes, within 24 hours. Remove the lid, check the rim for chips, use a new lid, and reprocess for the full adjusted time. Alternatively, refrigerate the jar and use it within a few weeks.

    Ready to run the numbers?

    Open Canning Time Altitude Adjustment Calculator