Wet Bulb to Relative Humidity Calculator
This simple approximation estimates relative humidity from dry bulb and wet bulb temperatures using the '5° rule': for each 1°F difference between dry bulb and wet bulb, RH drops...
Formula
Source: Engineering Toolbox, ASHRAE Fundamentals | Last reviewed: June 8, 2026
Examples
0 °F
= 100 °F
- T_db = 75
- T_wb = 75
T_db = T_wb → 100% RH (saturated air)
0 °F
= 50 °F
- T_db = 75
- T_wb = 65
10°F depression → ~50% RH typical comfort condition
0 °F
= 0 °F
- T_db = 90
- T_wb = 70
20°F depression → very low RH (desert conditions)
Where is this used?
HVAC commissioning: quick humidity checks during system startup.
Process air monitoring: verifying that dryer or humidifier performance is in expected range.
Educational demonstrations of psychrometric principles.
Frequently Asked Questions
How accurate is this approximation?
This is a rough field approximation accurate within ±15% RH for typical comfort conditions (60-80°F). At extreme temperatures and very low or very high humidity, the error increases significantly. For precise work, use a psychrometric chart, ASHRAE psychrometric equations, or a digital psychrometer.
What is wet bulb temperature?
Wet bulb temperature is the temperature a parcel of air reaches when water evaporates into it adiabatically. It is measured with a thermometer whose bulb is covered by a wetted wick exposed to moving air. The evaporation cools the bulb; the depression below dry bulb indicates humidity.
Why does the '5° rule' work?
It approximates the slope of the psychrometric chart in the comfort range. At 75°F and sea level, the saturation curve and lines of constant RH have a slope of approximately 5% RH per °F depression. The relationship is nonlinear and varies with temperature, but this linear approximation works for quick field checks.
Reviewed for accuracy
· Last reviewed: June 8, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.