Sauna Humidity Calculator: How Much Water to Throw

A Finnish sauna at 80°C is far drier than most people expect: before any water goes on the stones, the relative humidity is usually only a few per cent. Löyly is what changes that, and the amount of water it takes depends on the size of the room. Enter your cabin size, the temperature and how much you plan to throw, and this calculator shows the relative humidity you reach, the dew point of the air, and how many ladles it takes to hit a target. The dew point is the number that explains the sting: once it climbs above the temperature of your skin, steam starts condensing on you and dumping its heat as it does.

How the calculator works

Warm air can hold far more water vapour than cold air. At 80°C a cubic metre of air can carry around 290 grams of vapour before it is saturated, nearly 23 times what it can hold at 15°C. So when cool outdoor air is drawn in through the inlet vent and heated to sauna temperature, the water it carries barely registers as a percentage. That is why a dry sauna reads a few per cent relative humidity at most.

The calculator works out how much vapour the incoming air brings, adds the weight of the water you throw (a millilitre of water weighs a gram), spreads it across the room's volume and compares the result with what the air can hold at your temperature. Saturation figures use the Buck equation for the vapour pressure of water, which stays accurate up to 100°C and slightly beyond, so it holds at real sauna temperatures where simpler formulas drift.

Why the dew point matters more than the percentage

Relative humidity figures in a sauna look small and can mislead. Twenty-five per cent sounds dry, yet at 80°C it means more water in every cubic metre than a steam room running at 45°C and 100%. The useful number is the dew point: the temperature at which that vapour turns back into liquid.

In a hot sauna your skin is much cooler than the air around it. When the dew point rises above its temperature, vapour condenses on you, and condensing steam gives up a lot of heat as it does. That is the wave of heat you feel a second after the hiss from the stones, even though the thermometer on the wall hardly moves. Sweat also stops evaporating so easily, so your body loses its main way of cooling down. If a round feels too fierce, use smaller ladles or wait longer between them rather than dropping the temperature. Our sauna temperature guide covers how heat and humidity trade off.

Getting good löyly at home

• Measure your ladle. Fill it, tip it into a measuring jug and use that figure. Ladle sizes vary a lot, and the result scales directly with it.
• Throw little and often. Several small ladles spaced out give a softer, longer steam than one big pour, and they give the stones time to recover.
• Stone mass matters. A heater with too few stones cools quickly and stops turning water into steam. Check yours with the sauna stones calculator and the heater size calculator.
• Let the room dry out. All that water has to leave afterwards. A properly sized inlet and outlet clear it and stop the timber staying damp; the sauna ventilation calculator sizes them.
• Infrared cabins are different. Never pour water on an infrared panel. This calculator is for traditional electric and wood-fired heaters with an open stone basket.