Canning Calculator

Home canning preserves food by heating jars hot enough and long enough to destroy microorganisms that cause spoilage and foodborne illness. The two main enemies are bacteria (particularly Clostridium botulinum) and mold spores. Botulism is rare but deadly, making proper canning technique absolutely essential. Following tested recipes and processing times from reliable sources prevents these dangers while preserving food safely for months or years.

Food acidity determines which canning method you must use. High-acid foods (fruits, pickles, tomatoes with added acid, jams) can be safely processed in a boiling water bath canner at 212°F. The acid environment prevents botulism bacteria from growing even if a few spores survive. Low-acid foods (vegetables, meats, soups, beans) require pressure canning at 240-250°F because only these higher temperatures kill botulism spores reliably in low-acid environments.

Never process low-acid foods in a water bath canner regardless of how long you boil them. Botulism spores survive boiling water temperatures. Pressure canning uses steam under pressure to reach temperatures high enough to destroy these dangerous spores. Attempting to can green beans, corn, or meat in a water bath canner creates potentially lethal jars that look and smell fine but harbor botulism toxin. Only pressure canning makes these foods safe.

Altitude affects boiling temperature and processing times. Water boils at 212°F at sea level but only 202°F at 5,000 feet elevation. This lower temperature means longer processing times are needed to achieve the same sterilization. The calculator automatically adjusts processing times based on your altitude. Pressure canners need pressure adjustments rather than time adjustments - higher altitudes require higher pressure to reach the necessary temperature.

Always use tested recipes from reliable sources like the USDA, university extension services, or reputable canning books. Grandma's recipe might taste wonderful but may not follow current safety standards developed through scientific research. Recipe developers test processing times to ensure safety while maintaining food quality. Modifying recipes by changing acidity, adding thickeners, or adjusting proportions can make them unsafe even if you follow processing times exactly.

Start by inspecting your jars, lids, and rings carefully. Run your finger around jar rims checking for chips or cracks - even tiny nicks prevent proper sealing. Discard damaged jars. Wash jars in hot soapy water and keep them hot until filling to prevent thermal shock when adding hot food. New canning lids work only once since the sealing compound degrades after the first use. Rings can be reused indefinitely if they're not bent or rusted.

Prepare your recipe following tested instructions exactly. Measure ingredients precisely - adding extra vegetables or reducing acid changes the safety profile. Fill hot jars with hot food, leaving the specified headspace (usually half inch for most foods, quarter inch for jams). Headspace matters because food expands during processing, and too little space prevents proper venting and sealing. Too much headspace leaves excess air that can cause food discoloration.

Remove air bubbles by sliding a non-metallic spatula between food and jar sides, gently pressing to release trapped air. Metal tools can scratch jars, creating weak spots that break during processing. Wipe jar rims thoroughly with a clean, damp cloth - any food residue, especially sticky jam or greasy soup, prevents the lid from sealing. Center the lid on the jar and screw on the ring finger-tight - over-tightening prevents air from escaping during processing.

Load jars into your canner without letting them touch each other or the pot sides. For water bath canning, ensure water covers jars by at least one inch and bring to a rolling boil before starting your timer. For pressure canning, vent steam for ten minutes before closing the vent and building pressure. Process for the full time specified - never reduce processing time even if jars seal early. Sealing indicates vacuum formation but doesn't guarantee adequate heat penetration to kill all microorganisms.

After processing completes, turn off heat and let pressure canners depressurize naturally - forcing pressure release can cause jar breakage and liquid loss. Remove jars carefully and set on a towel-covered counter away from drafts. Don't disturb or retighten rings for 12-24 hours while jars cool and seal. You'll hear the distinctive ping as each jar seals. Press the center of each lid once cool - properly sealed lids don't flex when pressed. Any unsealed jars should be refrigerated and used within a few days.

Atmospheric pressure decreases at higher elevations, which lowers water's boiling point. At sea level, water boils at 212°F. At 5,000 feet, water boils at only 203°F. At 10,000 feet, boiling temperature drops to 194°F. These lower temperatures mean longer processing times are necessary to achieve adequate sterilization. A recipe developed at sea level becomes unsafe at high altitude if you don't adjust processing time or pressure.

Water bath canning requires time adjustments for altitude. Add five minutes to processing time for altitudes between 1,001 and 3,000 feet. Add ten minutes for 3,001 to 6,000 feet. Add fifteen minutes for altitudes above 6,000 feet. These adjustments compensate for lower boiling temperatures by extending the time food stays at that temperature. The calculator automatically applies these adjustments when you enter your altitude.

Pressure canning uses pressure adjustments instead of time adjustments. Increase pressure to raise the temperature back to the required 240-250°F regardless of altitude. For dial gauge pressure canners at 1,001 to 2,000 feet, use 11 PSI instead of 10 PSI. For 2,001 to 4,000 feet, use 12 PSI. Continue adding one PSI for each 2,000 foot increase in altitude. Weighted gauge canners simply use 15 PSI for all altitudes above 1,000 feet.

Know your exact elevation for safe canning. Topographic maps, GPS devices, or online altitude finders provide accurate measurements. Using your city's average elevation works if you live in relatively flat areas, but mountain residents should get specific elevation for their property. When in doubt, round up to the next altitude category - slightly over-processing is safe while under-processing is dangerous. Store elevation information with your canning supplies for quick reference.