“A sauna session burns hundreds of calories.”

The research found this

The cleanest measurement comes from a randomized crossover study of 14 men. Each spent an hour sitting in 40°C water and, on another day, an hour cycling at a moderate pace. The two sessions were matched for the rise in core temperature.1Source 1 · Crossover trialThe effect of passive heating on heat shock protein 70 and interleukin-6: A possible treatment tool for metabolic diseases?Faulkner SH, Jackson S, Fatania G, et al.. Temperature, 2017.Read the paper ↗PubMed ↗ Indirect calorimetry showed the hot water raised energy expenditure by 61 kcal an hour over rest, which is a 79.5% increase. The cycling added 556 kcal an hour.

Body size made little difference. The hour of heating burned an extra 63.4 kcal in lean men and 58.5 kcal in overweight men.1Source 1 · Crossover trialThe effect of passive heating on heat shock protein 70 and interleukin-6: A possible treatment tool for metabolic diseases?Faulkner SH, Jackson S, Fatania G, et al.. Temperature, 2017.Read the paper ↗PubMed ↗ For a dry sauna, a 1986 study of 10 men who sat in a Finnish sauna at 80°C for an hour twice a day reported metabolic rate up 25–33% after the first day, and its abstract doesn't say how or when that was measured.2Source 2 · Clinical trialSome cardiovascular and metabolic effects of repeated sauna bathingLeppäluoto J, Tuominen M, Väänänen A, et al.. Acta Physiologica Scandinavica, 1986.Read the paper ↗PubMed ↗

The big numbers come from wrist devices. In 60 inactive young men, heart-rate monitors estimated 66.80 kcal for a 10-minute session at 90°C in the normal-weight group and 92.37 kcal in men with class 1 obesity.3Source 3 · Clinical trialPhysiological Response to Thermal Stress in Obese vs. Non-Obese Physically Inactive MenPodstawski R, Borysławski K, Pomianowski A, et al.. Biology, 2022.Read the paper ↗PubMed ↗ Those algorithms assume heart rate tracks oxygen use, and in the heat much of the rise in heart rate goes to cooling the body. The authors listed the monitors as a limitation.

Sweat does carry heat away. By the Institute of Medicine's figure of 0.58 kcal per gram, evaporating 500 g of sweat takes about 290 kcal of heat with it.4Source 4 · BookDietary Reference Intakes for Water, Potassium, Sodium, Chloride, and SulfateInstitute of Medicine. National Academies Press, 2005.Read the paper ↗ In a sauna most of that heat came from the room in the first place, and sweat that drips off never evaporates. The fuel the body burns is a separate quantity, and calorimetry is how you measure it.