Psychrometric Calculator

Reading a Psychrometric State Point

A psychrometric state point is the description of moist air at one moment: temperature, humidity, vapor content, and energy. Given any two independent properties (dry bulb + relative humidity is the most convenient combination), you can derive all the others using the Magnus equation for saturation vapor pressure and ideal-gas mixture equations. This calculator uses dry bulb + RH input and returns dew point, wet bulb, humidity ratio, vapor pressure, and enthalpy.

Why Each Property Matters

Dew point tells you whether condensation will form on cold surfaces — the single best comfort indicator. Wet bulb is the lowest temperature evaporative cooling can reach and is the critical metric for heat stress assessment. Humidity ratio stays constant during sensible heating or cooling, so HVAC engineers use it to track moisture through air handling processes. Enthalpy is total energy per pound of dry air and drives cooling coil capacity calculations. Vapor pressure is the thermodynamic driver for condensation and evaporation rates at every interface.

Comfort Thresholds

ASHRAE comfort zone covers roughly 68-78 °F dry bulb with dew points between 35-65 °F. Dew points below 55 °F feel dry. 55-60 °F is comfortable. 60-65 °F feels slightly humid. 65-70 °F is uncomfortable. Above 70 °F is oppressive. These numbers are independent of temperature — a 75 °F room at 55 °F dew point feels the same as an 85 °F room at 55 °F dew point, just warmer.

The Magnus Formula

Saturation vapor pressure is calculated using the Magnus-Tetens approximation: Pws = 6.112 × exp(17.62 × T / (243.12 + T)) in hPa for T in °C. Actual vapor pressure is Pws × RH/100. Dew point is the temperature at which the actual vapor pressure would be the saturation vapor pressure, solved by inverting Magnus: Td = 243.12 × ln(Pw/6.112) / (17.62 − ln(Pw/6.112)).

Reference Points for Common Indoor & Outdoor Conditions

Computed from the Magnus formula. Use these to ballpark your result or read off whether conditions will be comfortable, produce condensation, or support mold growth.

Comfort & Health Thresholds

• Dew point ≤ 55 °F: feels dry and comfortable. ASHRAE comfort zone lower bound.
• Dew point 55-60 °F: comfortable for most people.
• Dew point 60-65 °F: slightly sticky, noticeable humidity.
• Dew point 65-70 °F: oppressive indoors, uncomfortable outdoors.
• Dew point ≥ 70 °F: extremely humid; mold and dust mites thrive; showers don't dry.
• Wet bulb ≥ 80 °F: heat stress risk during physical work.
• Wet bulb ≥ 88 °F: body can no longer shed heat by sweating — potentially fatal without shade and rest.

Indoor mold prevention: keep relative humidity between 30% and 50%. Below 30% is irritating (dry sinuses, static electricity); above 60% supports mold, dust mites, and musty smells. Dew point above 65 °F on cold interior surfaces (basement walls, bathroom ceilings, closet corners) will condense and grow mold within weeks.

Frequently Asked Questions

Is wet bulb the same as heat index?

No. Heat index is a "feels like" derived metric. Wet bulb is a physical thermodynamic temperature measured by a thermometer with a wet cloth over the bulb in still air.

What is the dew point of 100% humidity?

At 100% RH, dew point equals dry bulb. Condensation is already occurring.

Why does the air feel damp at 70 °F 60% RH?

Because dew point is about 55 °F — near the upper end of comfortable. Above 60 °F dew point most people start to notice stickiness.

How do I find wet bulb without a calculator?

Use a sling psychrometer — two thermometers, one with a wet wick, whirled through the air. The wet one reads lower.

What is the enthalpy of conditioned indoor air?

Typical 75 °F / 50% RH indoor air has enthalpy around 28 BTU/lb (65 kJ/kg). Hot humid outdoor 90 °F / 70% RH reaches 42 BTU/lb (98 kJ/kg).