How long beer stays good once a can or bottle is opened

Two clocks start when you open it

The moment a beer is opened, two things begin. Carbon dioxide escapes, so the beer slowly goes flat. Oxygen gets in, so the beer slowly stales. Warmth speeds up both. A half finished beer left on a table in a 32 °C room loses its sparkle and freshness much faster than one recapped and put straight back in the fridge.

Neither clock makes the beer unsafe. Beer's alcohol, low pH and hop compounds make it a poor home for the bacteria that cause food poisoning. The question is quality. Quality drops off faster than most people think.

The first few hours: carbonation goes

A typical beer holds about 2.5 volumes of carbon dioxide. In an open glass, a noticeable share of that is gone within an hour or two. Within a few hours the beer tastes flat. Flatness changes more than texture. Without the light acidic bite from carbon dioxide, the beer tastes sweeter, heavier and duller. Aroma rises less freely from a flat beer too.

An open bottle loses gas more slowly than a glass because the narrow neck limits the surface exposed to air. An open can sits somewhere between the two. Neither stays lively overnight without a seal.

The next day: oxygen takes over

Beer is very sensitive to oxygen. Breweries work hard to keep dissolved oxygen in packaged beer very low because even small amounts cause staling over weeks. Opening the container lets in far more air than that.

Over a day or two, hop aroma fades first. Hoppy beers like IPAs lose their fresh citrus and pine and can take on a dull, slightly sweet or papery note. Malty beers can develop a stale, bready or wet cardboard character. Our guide to fresh versus stale IPA shows what that looks like in a hoppy beer.

A realistic timeline

• Open glass at room temperature: best within about 30 to 60 minutes. Flat within a few hours.
• Open can or bottle, left uncovered in the fridge: drinkable for the rest of the evening, noticeably flat and dull by the next day.
• Bottle resealed with a tight stopper and kept in the fridge: often still pleasant the next day, though less lively. Past two days it is usually tired.
• Strong, dark beers above about 8% ABV: tend to hold up a little better, because they rely less on fresh hop aroma and bright carbonation.

These are general ranges, not rules. Taste it and decide.

How to slow the decline

Recap it. A reusable bottle stopper with a rubber or silicone seal keeps most of the remaining gas in. A swing top bottle, if your beer came in one, seals well too. For cans, a snap-on can lid helps a little, though nothing matches a sealed bottle.

Get it cold. Cold beer holds carbon dioxide better and stales more slowly. Put it in the main body of the fridge, not the door, where temperature swings every time someone opens it. Keep it upright and out of light. Light can cause a skunky note in beer, especially in clear or green glass.

Better than saving it

The best answer is to open less. Share a 650 ml bottle between two or three people, or choose 330 ml cans when you only want one drink. At a party, open beer as people ask for it rather than pouring a row of glasses in advance.

Leftover flat beer still has uses. It is good in a beer batter, a slow cooked stew or bread dough, where carbonation does not matter and the malt adds flavour. Our guide on how to store beer at home covers unopened beer.

When to pour it away

Pour away any beer that smells sour, vinegary or mouldy, or that has been open at room temperature overnight. It is unlikely to make you ill, but it will not be pleasant. Do the same with any can or bottle that has been open long enough that you are not sure how old it is.

Learning to taste the difference between fresh, a day old and stale is one of the most useful skills for any beer drinker. It is also one of the first faults trained tasters learn to spot.

Try this tonight

Open a bottle of a hoppy beer, pour half and taste it. Recap the rest with a stopper and put it in the fridge. Taste the second half tomorrow at the same temperature and write down what has changed. That one comparison teaches you to recognise staling.

Common questions

Is beer still safe to drink the day after opening?

Yes. Opened beer is unlikely to make you ill, but it will be flatter and less fresh than when you opened it.

How do I keep an opened beer fresh?

Reseal it with a tight stopper, keep it upright in the coldest part of the fridge and drink it within a day.

Why does opened beer taste different the next day?

It has lost carbonation and absorbed oxygen. Flat beer tastes sweeter and heavier, while oxygen fades hop aroma and adds stale notes.

Can I cook with flat beer?

Yes. Flat beer works well in batter, stews and bread, where carbonation does not matter.