How to stay hydrated in the sauna

It's easy to lose more water in a sauna than you realise, and the first you hear about it may be a headache that evening. Staying hydrated is simple once you know roughly how much you lose. This guide covers what to drink, when, and the signs that you've got it wrong.

How much you sweat

One of the clearest measurements comes from a 1976 study of 60 adults who sat in a sauna at 80–90 °C (176–194 °F) for 20 minutes or until they'd had enough. The average sweat loss was 457 g, a little under half a litre. The spread was wide, so plenty of people lost much less and plenty much more.

That's for one round. If you do three rounds over an evening, a litre or more is quite normal. Children are smaller, so it matters more for them: in a Finnish study, children aged 5–10 lost 0.8% of their body weight in 10 minutes.

For comparison, a review of sports research found that sweat rates during exercise typically run from about 0.5 to 2 litres an hour. Those 457 g in 20 minutes work out at nearly one and a half litres an hour, so sitting still in a hot sauna you can sweat about as fast as someone exercising.

What's in sweat

Sweat is almost all water, with some salt. The amount of sodium varies a lot from person to person: from about 10 to 70 millimoles per litre, according to the same sports review. In everyday terms, that's somewhere between a fifth of a gram and one and a half grams of sodium in every litre you sweat.

For a normal sauna visit, your next meal replaces that salt without any effort. It only becomes worth thinking about when you sweat a lot, for example several long rounds or a sauna straight after a hard workout.

Before the sauna

Go in already hydrated. Drink a glass or two of water in the hour before, and check that your pee is pale yellow rather than dark. If you've just done a long or sweaty workout, drink before you go into the heat. A 1988 Finnish review of sauna and body fluids made the same point: after hard exercise, you may need to rehydrate before sauna bathing as well as after. Our guide to using the sauna before or after a workout covers timing.

During the sauna

That same review recommended drinking small amounts often, rather than a lot at once. In practice, that means a few mouthfuls of water between every round. Bring a bottle, and leave it outside the hot room if the venue asks. Rounds of 10–15 minutes with breaks in between are easier on your fluid balance than one long sit. See how long to stay in a sauna for more.

After the sauna

There's an easy way to see how much you lost: weigh yourself before and after. A kilo of weight lost is roughly a litre of sweat, and it's water, not fat. Drink that back over the next hour or two. You'll pee some of it out as you go, so it's fine to drink a little more than you lost.

The simplest check is the one the NHS uses: drink enough that your pee stays pale. If it's dark yellow and strong-smelling a few hours later, you're still short.

What to drink and what to skip

• Water. The best choice for a normal visit.
• Sports drinks and rehydration salts. Useful after a long session, a very sweaty day or a sauna after exercise, because they replace salt as well as water. A salty snack with water does a similar job.
• Tea and coffee. Fine as part of your day, but if you're already dehydrated the NHS suggests avoiding caffeine until you've caught up with water.
• Alcohol. Skip it. The CDC notes that alcohol increases both sweating and urination and dulls your sense of heat. Read sauna and alcohol for why it's a bad mix.
• Huge amounts of plain water. Don't force down litres more than you've lost. Drinking far too much water can dilute the salt in your blood. It's rare, but it's dangerous.

Signs of dehydration

The NHS lists these:

• feeling thirsty, with a dry mouth, lips or tongue
• a headache, dizziness or light-headedness
• feeling tired
• dark yellow, strong-smelling pee, or peeing less than usual

A headache after the sauna is a common sign. Our guide to why you get a headache after the sauna covers the other causes. Get urgent medical advice if dizziness on standing doesn't go away or your heart or breathing is fast, and call an ambulance if someone becomes confused or very drowsy.

Who should be careful

• Older people and children, who are more likely to become dehydrated.
• People on diuretics (water tablets) or other medicines that affect fluid or blood pressure.
• People told to limit fluids, for example with heart or kidney problems. Ask your doctor how a sauna fits with that.
• People with diabetes, which raises the risk of dehydration.
• Anyone ill with vomiting, diarrhoea or a fever. Wait until you're well.

This guide is general information, not medical advice.

Find a sauna

Wherever you go, check whether you can bring your own bottle. A few saunas to try:

Wolf Mother

Tulsa · Outdoor barrel sauna

Sage Sauna

Knoxville · Wood-fired outdoor sauna

Sauna House Raleigh

Raleigh · Finnish sauna

Frequently asked questions