What one ladle of water does to the room.
Löyly is the steam that comes off the stones when you throw water. It feels hotter than the dry air before it, even though the thermometer barely moves. The reason is the dew point. Once the air's dew point rises past your skin temperature, steam condenses on you and gives up the heat it took to boil it.
The short answer
96°F → 121°F
That's the dew point in a 6-foot barrel at 185°F after one 150 ml ladle. Your skin sits near 104°F in a hot sauna. Before the ladle, the air can't condense on you. After it, it can, by about 17°F.
Physics, not a survey · see the method below
Throw water, watch the dew point.
Set your sauna's temperature and size, then how big your ladle is and how many you throw. The ember dot is the dew point after the steam spreads. Past the skin line, it condenses on you.
Steam condenses on your skin, 17°F past it
Room average, right after the steam spreads, with no venting. Near the ceiling the wave runs hotter, and the vent clears it within minutes. Skin is taken as 104°F. Method and assumptions below.
Sweat cools you. Steam does the reverse.
In dry sauna air, sweat evaporates off your skin, and every gram that evaporates carries about 2,260 joules of heat away with it. That's why 195°F dry air is bearable.
Throw a ladle and the air suddenly holds far more water. When its dew point climbs past your skin temperature, the process runs backwards. Water vapor condenses on your skin, and each gram that condenses releases the same 2,260 joules into you. You feel it as the soft wave rolling down from the ceiling a second or two after the hiss.
The numbers are larger than most people expect. In the example above, one ladle takes the air from 35 to 72 grams of water per cubic meter. A steam room at 113°F and 100% humidity holds about 65. For a minute, the top bench of a barrel sauna holds more water per breath than a steam room does, at 70°F hotter.
Infrared cabins don't do any of this. There are no stones to throw water on, and the air runs 110–150°F, so the dew point stays far below your skin. That's the main difference you feel between the two. See infrared vs. traditional saunas and sauna vs. steam room for the full comparisons.
| Finnish sauna, dry | Finnish sauna, one ladle | Steam room | Infrared cabin | |
|---|---|---|---|---|
| Air | 185°F | 185°F | 113°F | 140°F |
| Relative humidity | 10% | ~20% | ~100% | ~10% |
| Water per m³ of air | 35 g | 72 g | 65 g | 13 g |
| Dew point | 96°F | 121°F | 113°F | 63°F |
| Condenses on 104°F skin? | No | Yes, until it vents | Yes, the whole time | No |
Calculated with the method below for a 6-foot barrel and a 150 ml ladle. The infrared and dry-sauna humidity of 10% is a typical starting value, not a measurement.
How much water to throw.
The calculator says what the air can do. The stones decide whether it happens.
One ladle, then wait
Each 150 ml ladle takes about 390 kJ out of the stones to turn into steam. A 6 kW heater puts that back in roughly a minute. Throw again before then and the water lands on cooler stones.
Listen to the stones
A sharp hiss and no dripping means the water flashed to steam. Water running through to the bottom of the heater means the stones are too cool, and you get a wet floor instead of löyly.
Small rooms need less
The same ladle does about twice as much in a 170 cubic foot barrel as in a 340 cubic foot cabin. Try a smaller ladle before you turn the heat down.
Only on stones made for it
Throw water only on a sauna heater or stove built to take it. Never on infrared panels, and never on an electric heater's elements where they show.
The vent matters
Without a working intake and exhaust, the room stays humid after each throw and the air goes stale. With them, each wave clears in a few minutes and the next one feels just as good.
Leave when you're done
A higher dew point means more heat reaches you, faster. Nobody's counting ladles. Get out when your body says so.
How the numbers are worked out.
Room air. Interior volume is π × radius² × length for a barrel, or width × length × height for a square room. We count 85% of it as air, because benches and bodies take up the rest.
Before the ladle. Saturation vapor pressure at the air temperature comes from the Antoine equation for water. Multiplying it by your starting humidity gives the vapor pressure, and the ideal gas law (water vapor's gas constant is 461.5 J/kg·K) turns that into grams of water per cubic meter.
After the ladle. All the water is assumed to boil off the stones and spread evenly through the air. Adding its mass and converting back gives the new vapor pressure, relative humidity and dew point. If the result passes 100%, the extra condenses as fog.
Skin. Skin temperature in a hot sauna climbs toward about 104°F (40°C). Condensation happens on any surface colder than the dew point.
The stones. Heating water from about 68°F to boiling, then boiling it, takes about 2,590 kJ per liter. A heater's kW rating is kJ per second, which gives the recovery time.
What it leaves out. Venting and leakage, which clear the steam over a few minutes. Uneven mixing, which makes the top bench more humid than the average. Steam that condenses on the walls and benches before it reaches you. Treat the result as the room-wide peak, not the reading at your face.
About löyly.
What is löyly?
Löyly (said roughly "LEU-lu") is the Finnish word for the steam that rises when water is thrown on hot sauna stones. Finns also use it for the feel of a particular sauna's heat. It's older than the word "sauna" and comes from a root meaning breath or spirit.
Why does throwing water make a sauna feel hotter?
Because the steam raises the air's dew point past your skin temperature, about 104°F. Water vapor then condenses on your skin and releases about 2,260 joules per gram, the same heat it took to boil it. In dry air the opposite happens: your sweat evaporates and cools you.
How humid should a sauna be?
A traditional sauna runs dry, around 5–15% relative humidity, between throws. One ladle in a small barrel roughly doubles the water in the air for a minute or two, and the vent clears it. If the room stays humid with nobody throwing water, the ventilation isn't keeping up.
How much water should I put on sauna stones?
One ladle at a time, then wait about a minute for the stones to recover. A 150 ml ladle takes about 390 kJ out of the stones, which a 6 kW heater replaces in roughly 65 seconds. If water drips through to the bottom of the heater, the stones are too cool.
Does an infrared sauna have löyly?
No. Infrared cabins have no stones and run at 110–150°F air with low humidity, so the dew point stays far below skin temperature. Never put water on infrared panels.
Is a steam room more humid than a sauna after löyly?
Not always. For a minute or two after a ladle, a sauna can hold more water per cubic meter than a steam room. A steam room at 113°F and 100% humidity holds about 65 grams of water per cubic meter. One 150 ml ladle in a 6-foot barrel at 185°F reaches about 72 grams for a minute or two. The steam room stays there the whole time, which is why it feels wet throughout.