Sauna Hydration Guide: How Much Water Should You Drink?

You've just finished a hot session and you're soaked. How much should you drink now, and does it matter what?

Get it wrong and you can be left with a headache, dizziness or a flat, tired feeling for the rest of the day. Here is what sports medicine guidelines say, applied to the sauna.

How Much Fluid a Sauna Costs You

In the largest study of sauna sweat loss, 674 young adults spent two 10-minute rounds in a 90°C (194°F) sauna. Men lost about 0.5 kg and women about 0.37 kg, nearly all of it sweat (Podstawski et al. 2014). For most people, then, a 20-minute session costs roughly 0.3 to 0.6 litres. Larger bodies, hotter rooms and longer or repeated rounds cost more. Our guide to how much you sweat in a sauna covers what changes the number.

Unlike exercise, where you can sip as you go, most of that loss happens in a short stretch with nothing to drink. You come out with a small deficit to repay. Sweat also carries sodium and a little potassium, which is why water alone is not always the whole answer after long sessions.

Signs You're Under-Hydrated

Common signs of dehydration include (McDermott et al. 2017):

• Thirst and a dry mouth
• Headache
• Dizziness or light-headedness, especially when you stand up
• Unusual tiredness or irritability
• Muscle cramps
• Dark yellow urine, or not needing to go for hours

If you feel dizzy, sick or confused during a session, get out, sit or lie down somewhere cool and drink. These can be early signs of heat illness.

Before, During and After

Before: Start Topped Up

Don't go in already dry. For activity in the heat, the American College of Sports Medicine suggests drinking about 5 to 7 ml per kg of body weight at least 4 hours before, slowly (Sawka et al. 2007). For a 70 kg adult that is about 350 to 500 ml, or two glasses. Drinking it early gives your body time to pass any excess before you sit down.

During: Between Rounds, Not On the Bench

Traditional sauna practice is to drink between rounds rather than on the bench. For a single session of 15 to 20 minutes, most people don't need to drink during it. If you're doing several rounds, a few sips during each break is sensible.

After: Drink More Than You Lost

Replacing exactly what you sweated out still leaves you short, because some of what you drink leaves as urine. The National Athletic Trainers' Association recommends replacing 100 to 150% of the fluid lost, towards the top of that range if you need to recover within a few hours. The American College of Sports Medicine puts it as about 1.5 litres for every kilogram lost (McDermott et al. 2017; Sawka et al. 2007).

Altaflip uses 150% of your estimated sweat loss as your target:

If you want precision, weigh yourself without clothes before and after the session, after towelling off. Each kilogram lost is about a litre.

Spread it out. Drink a large volume in one go and much of it comes straight back out as urine. Sipping steadily over the next 2 to 4 hours, ideally with some food or a little salt, helps your body hold on to it (Shirreffs & Sawka 2011).

When Do You Need Electrolytes?

For most sessions, plain water is enough. NATA's guidance is that activity lasting under an hour generally needs only water (McDermott et al. 2017).

Electrolytes or a salty snack start to make sense when:

• A session is long and you sweat heavily
• You do several rounds or sessions in a day
• You get muscle cramps, or you eat very little salt

Altaflip suggests considering electrolytes once a session runs past 30 minutes or you lose more than half a litre, and recommends them when a session runs past 45 minutes and you lose a litre or more. Those thresholds are the app's own conservative choice for intense passive heat, not a guideline.

What's in Your Sweat?

Sodium is the one that matters. At an average concentration, 500 ml of sweat carries roughly 400 mg of sodium, but saltier sweaters can lose more than twice that.

Simple options: a salty snack with your water (crackers, olives, pickles), a small pinch of salt in a glass of water, or an electrolyte drink. Sodium helps your body keep the fluid you drink rather than passing it straight out (Shirreffs & Sawka 2011).

One Public Example: Bryan Johnson

Bryan Johnson, who publishes his "Blueprint" longevity routine, describes a daily 20-minute sauna at 200°F (93°C), followed by about 36 oz (roughly 1 litre) of mineral-supplemented water (Johnson, Blueprint). That is more than most people lose in 20 minutes and in line with the 150% principle. He has no connection to Altaflip; it is just a well-known routine that takes rehydration seriously.

Your Daily Water Goal on Sauna Days

On sauna days, add your rehydration target on top of what you normally drink.

Altaflip starts your daily goal at 33 ml per kg of body weight (you can set your own) and adds each session's rehydration target to it. For a 75 kg person that is about 2.5 L, plus 750 ml after a session that cost 500 ml of sweat: about 3.25 L for the day. For comparison, Europe's reference values for total water intake, from drinks and food combined, are 2.0 L a day for women and 2.5 L for men (EFSA 2010), so the app's default leans generous.

Common Mistakes

Chugging It All at Once

Down a litre straight after the session and much of it is passed as urine rather than kept. Sip steadily instead.

Drinking Alcohol Before or During

Alcohol and sauna heat both lower blood pressure, and together they raise the risk of fainting, falls and heart rhythm problems (Ylikahri et al. 1988). In a review of sauna deaths in Finland, about half of the people who died had alcohol in their blood (Kenttämies & Karkola 2008).

After the sauna, a small, low-alcohol drink once you've had water is a different matter. In one study, drinks up to 2% alcohol made no difference to rehydration, while a 4% drink, about regular beer strength, tended to slow it (Shirreffs & Maughan 1997), so have your water first.

Forgetting Salt on Long, Multi-Round Days

If you do several rounds or a long contrast session, water alone may leave you feeling washed out. Add a salty snack or an electrolyte drink.

Relying Only on Urine Colour

Dark urine is a useful warning sign, but pale urine straight after drinking a lot doesn't mean you're fully topped up. Look at the whole picture: thirst, headache, energy, and how you feel standing up.

A Simple Post-Sauna Routine

Right after (first 30 minutes):

• 250 to 500 ml of water, sipped
• Add a salty snack or electrolytes if it was a long, heavy-sweat session

Over the next 2 to 4 hours:

• Keep sipping until you reach about 1.5 times what you lost
• Eat a normal meal, which brings salt and more water with it

For the rest of the day:

• Back to your normal drinking
• Pale yellow urine and no headache are good signs

Stay Safe

If you have a heart condition, low or high blood pressure, kidney problems, are pregnant, or take medication that affects fluid balance (such as diuretics), talk to your doctor about sauna use and how much to drink.

The Bottom Line

1. Start topped up: a glass or two of water a few hours before
2. Replace up to 150% of what you lose, because some of it leaves as urine
3. Sip it over 2 to 4 hours, not all at once
4. Add salt or electrolytes after long, heavy-sweat sessions or several rounds
5. Skip alcohol before and during, and have water before any drink after

Getting this right means fewer post-sauna headaches and feeling better for the rest of the day.