Wet Bulb Calculator

Enter the dry bulb and relative humidity to get the wet bulb temperature. It also gives the dew point, humidity ratio and enthalpy, using the ASHRAE Handbook equations.

Units

Wet bulb

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Fill in the boxes above to see the answer.

Dew point
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Humidity ratioMoisture per pound (kg) of dry air
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EnthalpyPer pound (kg) of dry air
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Leave empty for sea level. Thinner air at elevation changes wet bulb and humidity ratio.

What is wet bulb?

Wet bulb, or wet-bulb temperature, is the temperature a thermometer reads when its bulb is wrapped in a wet wick and air moves across it. Water evaporating off the wick cools it, and the drier the air, the faster it evaporates and the lower the reading. In air at 100% humidity nothing evaporates, so wet bulb equals dry bulb.

Because it depends on both temperature and moisture, wet bulb closely tracks how much total heat the air holds. That's why charging charts and cooling capacity tables use the indoor wet bulb. A sling psychrometer measures it directly, and digital probes work it out from temperature and humidity, the same way this calculator does.

How it's calculated

Saturation pressure: ASHRAE eqns 5 (over ice) and 6 (over water)
Humidity ratio W = 0.621945 × pw ÷ (p − pw)
Wet bulb: ASHRAE eqns 33 and 35, solved by bisection
Dew point: eqns 5 and 6 solved for the temperature where pws = pw
Enthalpy h = 0.240 t + W (1061 + 0.444 t) Btu/lb, or 1.006 t + W (2501 + 1.86 t) kJ/kg
Pressure at elevation z: p = 14.696 (1 − 6.8754×10⁻⁶ z)^5.2559 psia

Sources

If a number doesn't match your chart, email me what you entered and what you expected, and I'll check it against the source. You can also read how the numbers are checked.

Checked by Paul Williams · Updated

Questions

What is the wet bulb at 75°F and 50% humidity?

At 75°F and 50% relative humidity the wet bulb is 62.6°F at sea level. At 80°F and 50% it's 66.7°F. In metric, 24°C and 50% gives a 17.1°C wet bulb.

What's the difference between wet bulb and dew point?

Wet bulb is the temperature a wet thermometer cools to as water evaporates off it. Dew point is the temperature the air would have to cool to before moisture condenses. Wet bulb always sits between the dew point and the dry bulb; all three are equal at 100% humidity.

Why do techs need wet bulb?

Indoor wet bulb at the return is one of the two readings a manufacturer's target superheat chart uses for fixed orifice systems, along with outdoor dry bulb. It also gives the air's enthalpy, which is how coil capacity is worked out.

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