Does a sauna burn calories?

It is one of the most common things people want from a sauna app, and the honest answer is that nobody’s watch knows. The heart works hard in the heat. That is not the same thing as burning energy, and the figures in circulation mostly come from treating the two as one.

Why heart rate overstates it

Calorie estimates from a watch rest on an assumption that holds during exercise: when your muscles work, they need oxygen, your heart pumps faster to deliver it, and so heart rate tracks oxygen use, which tracks energy. Measure the heart rate and you can estimate the energy.

Heat breaks the chain at the first link. In a sauna your heart speeds up to move blood to your skin so that you can shed heat, and your muscles are doing almost nothing. A 2003 review of heart rate monitoring sets this out directly: heat raises heart rate without a matching rise in energy use, so heart-rate-based energy estimates fail in the heat.

The cardiac load itself is real. In a study of 19 adults, heart rate and blood pressure in a 93 °C sauna matched those measured while cycling at 60 to 100 watts. The heart was working like it does on a bike. The legs were not, and it is the legs that would have burned the energy.

Where the 413 calorie figure comes from

The number most often quoted traces back to a 2019 study of 45 young, sedentary, overweight men who rarely used a sauna. They did four 10-minute rounds at 90 to 91 °C with five-minute cooling breaks between them. The study reported energy expenditure rising across the rounds, from 73 kcal in the first to 131 kcal in the fourth, and 413 ± 22 kcal for the hour.

The method is the part that rarely travels with the figure. The study’s methods section says energy expenditure was measured indirectly with Suunto heart rate monitors, worn on the wrist with a chest sensor and calibrated to each man’s sex, age, body mass and activity level. So were the oxygen uptake figures, which the paper itself labels as estimated. Its limitations section names the monitors as a potential limitation.

So the best-known sauna calorie figure is a heart rate monitor’s estimate in exactly the conditions where heart rate monitors are known to overestimate. That does not make the study useless, since it carefully records heart rate, blood pressure and fluid loss across the rounds. It does mean the calorie number should not be quoted as a measurement.

The weight you lose is water

People step off the scales lighter after a sauna, and that part is real. In the same study the men lost 0.65 ± 0.24 kg over the hour. The authors attribute that loss mainly to body water, lost as sweat, and recommend matching fluid intake to it.

It comes back when you drink, which you should. Dehydration in the heat is one of the genuine risks of a long sauna session, and a weight drop that looks like progress on the scales is mostly fluid you need to replace.

What Contrast shows instead

Contrast reads your heart rate in each heat round and counts the minutes at or above 64% of your maximum as moderate and at or above 77% as vigorous. Those are the American College of Sports Medicine’s exercise intensity bands. It shows those minutes apart from exercise, and stops there.

That number answers a question the data can support: how long your heart worked at a rate that would count as moderate exercise. It does not pretend to answer how many calories you burned, because the data cannot support that.

One more limit applies to the sauna specifically. Apple advises against wearing any Apple Watch other than Ultra in a sauna, and against wearing Ultra in one above 55 °C (130 °F). A traditional sauna runs hotter, so for most saunas there is no heart rate to count at all, and the round is timed with the watch left outside. Hot baths, which run 38 to 42 °C, are where this measure works for most people.

If weight is the goal

None of this says the sauna is pointless. The long-term Finnish cohort studies associated regular sauna use with lower cardiovascular risk, for reasons that run through the cardiovascular load rather than through energy burned. Those studies are observational, and they do not suggest a sauna as a way to lose weight.

For energy expenditure, the thing that reliably moves the number is moving your body.