How to Sterilise Glass Jars: The Safe Method Step by Step

The reliable way to sterilise glass jars is a 10-minute boil in a deep pot of water, adding one extra minute per 300 m of altitude, then keeping the jars hot until you fill them. It costs nothing, works for jams, pickles, sauces and fermentation, and avoids the two classic accidents: recontaminating a cooled jar, and cracking one with thermal shock. Here is the full method, the alternatives, and what actually matters microbiologically.

The boiling-water method in detail

1. Inspect. Run a finger around each rim. Chips, cracks or a rough sealing surface mean the jar will not seal — recycle it. New jars need this check too.
2. Wash. Hot soapy water, inside and out, then a thorough rinse. Sterilising is not cleaning: residue survives boiling.
3. Submerge. Stand jars upright in a deep pot on a rack or folded tea towel (glass rattling on bare metal can crack), cover by at least 3 cm of water, and bring to a rolling boil from warm — starting jars in cold water and heating together avoids thermal shock.
4. Boil 10 minutes. At sea level, 10 minutes at 100 °C kills the organisms that matter for high-acid preserving. Water boils cooler at altitude, so add 1 minute per 300 m (1,000 ft).
5. Keep hot, fill hot. Leave the jars in the hot water until the moment of filling, then lift them out with jar tongs, drain, and fill while still hot. A hot preserve into a hot jar keeps every temperature step small — this is the whole trick against both germs and thermal shock.

Dishwasher and other alternatives

Dishwasher: a hot cycle (65 °C+) with a sanitise setting is acceptable for jars that will be processed again after filling (water-bath canning), and convenient for large batches — time the cycle so jars are still hot when you fill. Oven: widely described online (jars at 110–140 °C for 15–20 min) but not recommended by canning authorities: dry heat is a poorer steriliser than boiling water, heats unevenly, and ovens and jars disagree about sudden temperature changes. Microwave: no — dry glass heats unpredictably and metal lids cannot go in at all. Sterilising tablets/solutions (as used in brewing) work chemically at room temperature and suit fermentation vessels, but leave rinse requirements — follow the product’s food-contact instructions.

Lids, rings and rubber seals

Metal two-part canning lids should be simmered (not boiled) at about 82 °C — a hard boil can damage the sealing compound. Modern lids from major makers often need only hot soapy washing; follow the box. Rubber gasket rings for clip-top jars are simmered a few minutes and used hot. Never reuse single-use flat lids for canning: the compound seals reliably once. The jar formats themselves — and which mouth sizes take which lids — are covered in our mason jar size guide.

When sterilising is (and isn’t) enough

Boiling sterilisation is designed for high-acid preserving — jams, jellies, fruit, pickles, chutneys (pH below 4.6) — and for fermentation and dry storage. It is not sufficient for low-acid canning: vegetables, meats, stocks and most sauces can harbour Clostridium botulinum spores that survive 100 °C and require pressure canning at 116 °C+. That is a hard safety line, not a preference. If your recipe is low-acid, use a pressure canner or refrigerate/freeze instead — with the headspace rules for freezing in glass.

Frequently asked questions

How long do you boil jars to sterilise them?

10 minutes at a rolling boil at sea level, plus 1 minute per 300 m (1,000 ft) of altitude. Keep them hot until filling.

Can I sterilise jars in the oven?

It is common advice but not recommended by canning authorities: dry heat sterilises less effectively at these temperatures and raises thermal-shock risk. The boiling-water method is safer and better validated.

Can I sterilise jars in the microwave?

No. Dry glass heats unevenly in a microwave, metal lids cannot go in, and superheated pockets are a burn risk. See our guide to glass and appliances.

Do new jars need sterilising?

Yes, for preserving. New jars are clean, not sterile — wash and boil them like any other. Inspect rims too; transit chips happen.

Why did my jar crack during sterilising or filling?

Thermal shock: a big, fast temperature change — cold jars into boiling water, or hot preserve into a cool jar. Standard jars tolerate only about a 40 °C sudden swing. Heat jars gradually and fill hot-into-hot.

Is boiling enough for canning vegetables or sauces?

No. Low-acid foods require pressure canning at 116 °C or higher to address botulism spores. Boiling covers only high-acid preserves (pH < 4.6), fermentation and storage.

Preserving jars by GlassRock