- The 200 rule says sauna temperature in degrees Fahrenheit plus relative humidity in percent should come to about 200. At 170 F, aim for about 30 percent humidity. At 185 F, keep humidity around 15 percent.
- It is a comfort rule of thumb for traditional saunas with steam, popular in North America. It is not a Finnish standard, and we found no heater maker or Finnish sauna body that publishes it.
- It is not a safety limit. A total under 200 does not make a session safe. Leave if you feel unwell.
- It works because steam matters as much as heat. Pouring water on the stones raises humidity, so the same comfort comes at a lower air temperature.
Worked examples.
The common pairings: 190 F with about 10 percent humidity, 180 F with about 20 percent, 170 F with about 30 percent, and 150 F with up to about 50 percent. Each adds up to roughly 200.
The point is the trade-off. A drier room can run hotter. A steamier room feels just as intense at a lower temperature. Going well past 200 on both at once is where Salus Saunas warns it gets hard to take: it calls 190 F with 40 percent humidity physically overwhelming for someone who is not used to it.
Why humidity matters as much as temperature.
In a traditional sauna you pour water on the hot stones. The steam that comes off, called löyly in Finnish, raises the humidity in the room for a few moments and makes the heat feel stronger on your skin.
Research backs the idea that the two act separately. A 2026 study from the University of Jyväskylä put 50 adults through sauna sessions and found that temperature and humidity each raised heart rate and core temperature on their own. The study was supported by the heater maker Harvia. It did not test the 200 rule itself.
A brief spike above 200 when you pour water is normal. The rule describes the room you sit in, not the moment the steam hits.
Where the rule comes from.
It is often described as Finnish, and the evidence does not support that. We found no Finnish Sauna Society publication and no heater maker, including Harvia and HUUM, that states a rule of 200. Haven of Heat, a sauna retailer, says it appears to have been popularized in the North American sauna community. SaunaSpa, a Canadian retailer, says its exact origin is unclear.
Finland measures in Celsius, which is one reason a Fahrenheit rule is unlikely to be Finnish. There is no established Celsius version. Some sites mention a rule of 100 (80 C plus 20 percent), but that is informal, and converting between the scales does not come out clean.
It is a comfort guide, not a safety limit.
SaunaSpa points out there is no established evidence that a total under 200 is automatically safe. Use the rule to tune the room, not to decide how long to stay.
Leave when you feel unwell.
What controls it in your sauna.
The heater and its controller set the temperature. You set the humidity with the ladle. A heater sized to the room is what lets you hold a steady temperature and still get a strong steam when you pour water on the stones.
Common questions.
- What is the 200 rule in a sauna?
- Temperature in degrees Fahrenheit plus relative humidity in percent should add up to about 200. At 170 F that means about 30 percent humidity; at 185 F, about 15 percent. It is a comfort rule of thumb for traditional saunas, not an official standard or a safety limit.
- Is the 200 rule a Finnish rule?
- No evidence says so. We found no Finnish Sauna Society or heater-maker publication that states it. Haven of Heat, a sauna retailer, says it appears to come from the North American sauna community; SaunaSpa says its origin is unclear. Finland also measures in Celsius.
- Is a sauna safe if it stays under 200?
- Not automatically. The rule is about comfort. SaunaSpa points out that no established evidence shows staying under 200 makes a session safe. Leave when you feel unwell.
- Is there a Celsius version of the 200 rule?
- Not an established one. Some sites mention a rule of 100 (80 C plus 20 percent humidity), but it is informal, and the two temperature scales do not convert cleanly.
Sources.
- Haven of Heat: what is the rule of 200 in a sauna
- SaunaSpa: the rule of 200, temperature, humidity and löyly
- Salus Saunas: the relationship between heat and humidity in your sauna
- Laatikainen-Raussi et al., Temperature and humidity independently influence thermoregulatory responses during Finnish sauna bathing, Temperature (2026), University of Jyväskylä
For general information only, not medical advice. Ask a clinician before starting regular sauna use.
Keep reading.
Size the heater to the room.
Send the room dimensions and ceiling height. We spec a name-brand stone heater that holds the temperature and still gives a strong steam.