Step on a scale after a sauna and you are lighter. Check your watch and your heart rate was up for twenty minutes. It is tempting to read that as a workout. Here is what each number means, with the measurements behind it.
Your heart rate climbs, your muscles do not
In a hot sauna the heart rate rises to 100 to 150 beats per minute by the end of a round. A 2019 study put healthy adults through a 25-minute sauna and a session on an exercise bike and found similar heart rate and blood pressure responses in both. That is where the "sauna equals exercise" claim comes from.
The heart is working, but for a different reason. In exercise, it pumps blood to muscles that burn fuel. In the heat, it pumps blood to the skin to dump heat, and the muscles are at rest. The calorie cost of pumping blood is small. The calorie cost of moving your body is what makes exercise expensive.
What a session really costs
No large study has measured energy use in a sauna directly, so any number is an estimate. Using the measured heart rates and what they correspond to in light activity, a session runs at roughly 1.5 to 3 times your resting metabolic rate. For a 70 kg adult that is in the order of 2 to 4 kcal per minute: 30 to 80 kcal for a 20-minute round, 100 to 200 kcal for a full session of three rounds. The hotter the room and the bigger you are, the higher the figure.
That is similar to a slow walk of the same length. It is not nothing, but it is far from the 300 to 600 kcal that circulate online, which would need a running pace.
Sauna Metrics shows a calorie estimate after each session for this reason: it is an estimate built from your heart rate, your weight and the time in the heat, labelled as such, and it will look modest compared with a run. That is the honest number.
Why the scale lies
The weight you lose is water. A Polish study weighed sedentary young adults before and after two 10-minute rounds at 90 °C: the men lost about 0.7 kg on average, the women about 0.4 kg, almost all of it sweat. Drink and it comes straight back. Our hydration guide covers how much to replace.
Losing that water on purpose is a bad idea. Dehydration raises heart rate, lowers blood pressure and is the main reason people feel faint in the heat. Athletes who cut weight in a sauna before a weigh-in do it under supervision and rehydrate immediately afterwards.
What the sauna does do for body composition
There are indirect effects worth knowing. A sauna after training is linked to lower perceived muscle soreness, which can make it easier to train again the next day. Regular use is associated with lower blood pressure and better vascular function, both of which matter more for health than a few dozen calories. And a calm evening session helps many people sleep, and sleep is where appetite hormones are regulated.
So the sauna is not a fat-burning tool. It is a recovery and cardiovascular habit that fits around exercise rather than replacing it. The benefits guide goes through the evidence for each of those effects.
If you track it
Three numbers tell the real story of a session, and none of them is calories. Your peak heart rate in the heat shows how hard the session was. Your heart rate recovery in the ten minutes after the last round shows how well your body handles it. And your weight before and after, if you want to measure it, shows how much to drink. Those are the figures worth logging.
Questions
How many calories does a 20-minute sauna burn?
Measured heart rates in the sauna match light exercise, so a reasonable estimate is 1.5 to 3 times your resting rate: roughly 30 to 80 kcal in 20 minutes for most adults, depending on body size and heat. Claims of 300 to 600 kcal are not supported by measurements.
Does a sauna help you lose weight?
The weight you lose during a session is water, usually around half a litre, and it comes back when you drink. The sauna does not replace a calorie deficit or exercise. Its real benefits are elsewhere: heart, blood pressure, sleep and recovery.
Is the heart rate in a sauna the same as in exercise?
The heart rate is similar to a brisk walk or easy cycling, around 100 to 150 beats per minute at the end of a hot round. The difference is that the muscles are not working: the heart is pumping blood to the skin to shed heat, which uses far less energy than moving your body.
Sources
- Ketelhut S., Ketelhut R. G. The blood pressure and heart rate during sauna bath correspond to cardiac responses during submaximal dynamic exercise. Complementary Therapies in Medicine, 2019.
- Podstawski R. et al. Sauna-induced body mass loss in young sedentary women and men. The Scientific World Journal, 2014.
- Laukkanen J. A., Laukkanen T., Kunutsor S. K. Cardiovascular and other health benefits of sauna bathing: a review of the evidence. Mayo Clinic Proceedings, 2018.
- Hussain J., Cohen M. Clinical effects of regular dry sauna bathing: a systematic review. Evidence-Based Complementary and Alternative Medicine, 2018.
General information, not medical advice.