How Much Do You Sweat in a Sauna? A Science-Backed Guide

You step out of the sauna dripping and feeling lighter. But how much fluid did you actually lose? Knowing your sweat loss is the first step to rehydrating properly, so you avoid the headache and the washed-out feeling that can follow a hot session.

Here is what the research measured, what makes your number higher or lower, and how to estimate your own.

The Short Answer: About Half a Litre in 20 Minutes

The largest study of sauna sweat loss weighed 674 young Polish adults before and after a standard session: two 10-minute rounds in a 90°C (194°F) Finnish sauna with a short cool-down between them (Podstawski et al. 2014).

Almost all of that is sweat, so for most people a 20-minute traditional session means roughly 0.3 to 0.6 litres. That is less than 1% of body weight for a typical adult. Larger bodies, hotter rooms and longer sessions push it up; smaller bodies, lower benches and short sessions bring it down.

What Changes How Much You Sweat

1. Body Size

Bigger bodies produce more heat and more sweat. In the Podstawski data, men in the overweight range lost about twice as much as men of normal weight, and a later study of 685 students by the same team found body mass was the deciding factor (Podstawski et al. 2016).

Altaflip uses the regression equations from the 2014 study, which predict sweat loss from your body mass index, and scales them by your weight. For a 75 kg man who is 175 cm tall, that gives about 540 ml for 20 minutes at 90°C.

2. Temperature

Hotter air means more sweat. In a 2024 study, young women lost 0.14 kg in 20 minutes at 80°C (176°F) and 0.30 kg at 120°C (248°F), about 2.1 times as much. That study was small (22 women who rarely used a sauna) and three of them fainted at 120°C, which is one reason not to chase extreme heat.

Altaflip turns this into a temperature factor: about 2.2% more sweat for each degree above 90°C in a traditional sauna, and less for each degree below. That straight line through the study's two temperatures reproduces its 2.1-fold difference.

Below about 67°C the app doesn't reduce the estimate any further.

3. Bench Level

Hot air rises, so the top bench is the hottest place in a traditional sauna and the lower benches are cooler. The temperature on the thermometer is closest to what you feel on the top bench.

Altaflip estimates 10°C less for each bench below the top in a traditional sauna (6°C in a steam room, and no difference in an infrared cabin, which heats you directly). Sitting on the lower bench of a two-tier 90°C sauna is treated like the top bench of an 80°C one, so your estimate drops by about 22%.

4. Sauna Type: Dry, Steam or Infrared

Steam rooms feel far wetter, yet you usually lose less fluid in them.

Much of the water on your skin in humid heat isn't sweat at all. In an isotope tracing study in a Finnish sauna, 30 to 54% of the moisture on participants' skin after water was thrown on the stones was condensed steam (Zech et al. 2015).

Infrared cabins run cooler (often 45 to 60°C) and heat you mainly by radiation. Altaflip starts both steam and infrared estimates at half the traditional sauna rate, measured from their own typical temperatures (45°C for steam, 55°C for infrared). Treat these as rough: there is much less research on them.

5. Sex

At the same heat load, women often sweat somewhat less than men. In a lab study of cycling in the heat, women's local sweat rate was about 19 to 25% lower, but only at the highest heat load (Gagnon & Kenny 2012). In a real sauna after exercise, though, trained women and men both lost about 1.2% of body mass (Kirby et al. 2021).

Altaflip takes a middle path: it estimates 15% less sweat for women in a traditional sauna and 25% less in steam and infrared, where heating is slower and closer to lab conditions.

6. Age

Sweating tends to decline with age. Older men in one study sweated less than younger men on parts of the body, mainly in summer (Inoue et al. 1991), and reviews describe a gradual drop in heat loss capacity with age, much of it linked to fitness (Kenney & Munce 2003).

Altaflip uses lower estimates for older users: 0.78 times the baseline for ages 50 to 64 and 0.55 times for 65 and over. These are the app's modelling choices rather than figures from a single study.

7. Regular Use

Your body adapts to repeated heat. In runners who added 30-minute sauna sessions after training for three weeks, more sweat glands were active on the forearm and heart rate in the heat was lower (Kirby et al. 2021), although their whole-body sweat loss during a running test in the heat didn't change. In an 8-week Finnish sauna trial, sweat rate in the sauna rose by 0.3 litres per hour (Debray et al. 2023).

Altaflip raises its estimate by up to 1.3 times if you have been using the sauna regularly over the past four weeks.

Why Short Sessions Produce Little Sweat

Sweating doesn't start the moment you sit down. Your body first has to warm up enough to need it, which takes a few minutes in a hot sauna and longer at lower temperatures or if you rarely use one. After that, sweat output builds gradually rather than switching on at full rate.

Altaflip models both effects. It estimates the delay before sweating starts:

A session shorter than the delay counts as minimal sweat. For sessions under 20 minutes, the app also scales the estimate down to reflect the slow build-up: a 10-minute session gets about two thirds of a straight-line estimate, a 15-minute session about four fifths. These timings are modelled from research on sweating during passive heating, and individual bodies vary.

How to Estimate Your Own Sweat Loss

The most accurate method is simple: weigh yourself without clothes before and after, after towelling off. Every kilogram lost is about a litre of fluid.

Without a scale, here is a simplified version of the calculation Altaflip uses for a traditional sauna:

Sweat (ml) = 0.32 x body weight (kg) x minutes x temperature factor x your adjustments

• 0.32 ml per kg per minute is the average rate in the Podstawski study at 90°C (0.5 kg in 20 minutes for men averaging 76 kg).
• Temperature factor from the table above, using the temperature where you sit (10°C less per bench below the top).
• Adjustments: women 0.85, ages 50 to 64 0.78, ages 65+ 0.55, regular users up to 1.3.

Example: a 75 kg man, 20 minutes, top bench at 90°C.

0.32 x 75 x 20 x 1.00 = 480 ml

Add about 20% either way for individual variation: about 400 to 600 ml. Move to the lower bench of a two-tier sauna and it becomes 480 x 0.78 = 374 ml.

The app goes further: when it knows your height it uses the body mass index equations instead (about 540 ml for a 175 cm man in the example), and it applies the onset and build-up corrections above. For contrast sessions it uses 77% of the sauna-only figure, based on a study where cold breaks reduced sweat loss compared with sauna rounds broken up by rests at room temperature (Kauppinen 1989).

How Much Should You Drink?

Drink more than you lost, because some of what you drink leaves as urine. The National Athletic Trainers' Association suggests replacing 100 to 150% of the fluid lost, towards the top of that range if you need to recover quickly, and the American College of Sports Medicine suggests about 1.5 litres for each kilogram lost (McDermott et al. 2017; Sawka et al. 2007).

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

Sip it over the next few hours rather than all at once. For timing, electrolytes and common mistakes, see the sauna hydration guide.

Do You Need Electrolytes?

For a typical session, plain water is enough. NATA's position is that activity under an hour generally needs only water. Altaflip suggests considering electrolytes or a salty snack 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.

Stay Safe

Sweat loss adds up quickly in long or repeated rounds. Get out if you feel dizzy, sick, confused or get a pounding headache, and cool down slowly. Skip alcohol before and during a session. If you have a heart condition, low or high blood pressure, are pregnant or take regular medication, talk to your doctor before using a sauna.

The Bottom Line

A typical 20-minute session at 90°C costs most people about 0.3 to 0.6 litres of sweat. What moves your number most:

1. Body size: larger bodies sweat more
2. Temperature and bench level: each bench down is a cooler room
3. Sauna type: humid heat and infrared usually mean less fluid loss than a hot dry sauna
4. Session length: sweating takes a few minutes to start
5. Rehydration: drink up to 1.5 times what you lost, over a few hours

Weighing yourself before and after is the most accurate way to know. Otherwise, an estimate built on these factors gets you close enough to drink the right amount.