Water Bath Canning Timer
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Common uses for a water bath canning timer
- Jams and jellies
- Pickles and relishes
- Acidified tomatoes
- Canned fruit in syrup
Water bath canning processes high-acid foods — fruits, jams, pickles, tomatoes with added acid — in boiling water for a set time. Only high-acid foods are safe by this method. Low-acid vegetables, meats and soups require a pressure canner, because boiling water cannot reach the temperature needed to destroy Clostridium botulinum spores[1].
Processing time is counted from the moment the water reaches a full rolling boil with the jars already submerged under at least 2.5 cm of water, not from when the jars go in[1]. If the boil stops at any point, bring it back and start the full time again — a partially processed jar is not partially safe. Times come from tested recipes, and are not interchangeable between recipes even for the same food, because acidity, jar size and how tightly the food is packed all change how heat penetrates.
Altitude changes the answer, not just the margin
Water boils cooler as altitude rises, so the same time delivers less lethality. Processing times must be increased accordingly, and the adjustment is not optional[1]. Someone at 1,800 m following a sea-level recipe is under-processing every jar, with no visible sign that anything is wrong.
Find your elevation before you start, and apply the increase in the table below to the time your tested recipe specifies.
Why a tested recipe matters more than a timer
This timer counts the processing time your recipe gives you. It cannot tell you what that time should be, because it depends on the acidity of the specific preparation, the jar size, and the density of the pack. Use recipes from a tested source — the USDA Complete Guide to Home Canning, your national extension service, or the instructions supplied with your equipment — and do not scale a recipe up or substitute ingredients that change its acidity.
Add the time below to whatever your tested recipe specifies at sea level[1].
| Altitude | Recipe time 20 min or less | Recipe time over 20 min |
|---|---|---|
| 0–300 m (0–1,000 ft) | no change | no change |
| 300–900 m (1,001–3,000 ft) | +5 min | +10 min |
| 900–1,800 m (3,001–6,000 ft) | +10 min | +15 min |
| 1,800–2,400 m (6,001–8,000 ft) | +15 min | +20 min |
| 2,400–3,000 m (8,001–10,000 ft) | +20 min | +25 min |
- Water must cover the jar tops by at least 2.5 cm (1 inch) for the whole processing time.
- If the boil stops, return it to a full boil and begin the entire processing time again.
Source: Complete Guide to Home Canning — Principles of home canning and boiling water processing
Sources
- Complete Guide to Home Canning — Principles of home canning and boiling water processing , National Center for Home Food Preservation / USDA — accessed 2026-07-28