Calculate relative humidity from dry-bulb + wet-bulb, dew point, or absolute humidity. Find RH, humidity ratio, dew point, and enthalpy.
The **Relative Humidity Calculator** determines relative humidity using three selectable input methods: dry-bulb/wet-bulb temperatures, dry-bulb/dew point, or dry-bulb/absolute humidity. Relative humidity (RH) is the ratio of actual water vapor pressure to saturation vapor pressure at the same temperature, expressed as a percentage.
Relative humidity affects human comfort, building integrity, electronics reliability, and industrial processes. At 50% RH, the air holds half the maximum possible moisture at that temperature. But because warm air can hold much more moisture than cold air, 50% RH at 30°C contains far more water vapor than 50% RH at 10°C.
This calculator uses the August-Roche-Magnus approximation for saturation vapor pressure and provides comprehensive results including humidity ratio, dew point, absolute humidity, vapor pressure, and enthalpy — along with a comfort assessment and reference table. Check the example with realistic values before reporting. Use the steps shown to verify rounding and units. Cross-check this output using a known reference case. Use the example pattern when troubleshooting unexpected results.
Accurate humidity measurement is critical for HVAC commissioning, weather monitoring, food storage, museum conservation, and electronics manufacturing. This multi-mode calculator handles any available measurement pair. Keep these notes focused on your current workflow. Tie the context to real calculations your team runs. Use this clarification to avoid ambiguous interpretation. Align the note with how outputs are reviewed.
Pws = 0.61078 × exp(17.27 × T / (T + 237.3)) [Magnus formula] From wet-bulb: Pw = Pws(Twb) − 0.000662 × P × (Tdb − Twb) From dew point: Pw = Pws(Tdp) RH = (Pw / Pws(Tdb)) × 100%
Result: 53.6% RH
Pws(22°C) = 2.643 kPa, Pws(16°C) = 1.818 kPa. Pw = 1.818 − 0.000662 × 101.325 × 6 = 1.416 kPa. RH = 1.416/2.643 × 100 = 53.6%. The 6°C wet-bulb depression indicates moderate humidity in the comfortable range.
Relative humidity is expressed as a percentage, but its meaning is often misunderstood. RH = 50% means the air contains half the maximum vapor it could hold at the current temperature — not half of some absolute scale. Because warm air can hold exponentially more moisture than cold air, the same RH at different temperatures represents vastly different amounts of actual water.
At 30°C, saturated air (100% RH) holds about 30.4 g/m³ of water vapor. At 10°C, saturated air holds only 9.4 g/m³. So 50% RH at 30°C (15.2 g/m³) contains more than 100% RH at 10°C — which is why tropical air feels so much more humid than maritime temperate air even at the same RH reading.
**Sling Psychrometer:** Two thermometers (dry and wet) whirled through air. Simple, reliable, and self-calibrating. Still used for HVAC commissioning and weather station checks.
**Capacitive Sensors:** Modern electronic humidity sensors use a polymer film whose capacitance changes with moisture absorption. Accurate to ±2-3% RH, these dominate in commercial HVAC controllers and weather stations.
**Chilled Mirror Hygrometer:** The gold standard for laboratory accuracy (±0.1°C dew point). A mirror is cooled until condensation forms; the mirror temperature at the onset of condensation equals the dew point. Used for calibrating other instruments.
Food storage requires precise humidity control — too dry causes shrinkage and weight loss, too humid promotes mold and bacterial growth. Grain storage typically targets 60-65% RH. Cheese aging rooms maintain 85-95% RH. Pharmaceutical clean rooms require 30-50% RH to prevent both microbial growth and electrostatic discharge.
The comfortable range for most people is 30-60% RH. Below 30% causes dry skin, static electricity, and respiratory irritation. Above 60% promotes mold growth, dust mites, and a feeling of stuffiness.
At 30°C and 80% RH, there is much more absolute moisture (about 24 g/m³) than at 10°C and 80% RH (about 7.5 g/m³). The heat index combines temperature and moisture to reflect how humid heat actually feels to humans.
Technically, supersaturation can occur briefly (RH slightly above 100%) before condensation begins. In practice, weather instruments cap at 100%. Fog and clouds form at or very near 100% RH.
RH is a percentage (actual/maximum moisture at that temperature). Absolute humidity (g/m³) is the actual mass of water vapor per volume. RH depends on temperature; absolute humidity does not.
Lower atmospheric pressure at altitude affects vapor pressure calculations. The psychrometric coefficient changes, and wet-bulb measurements require barometric pressure correction. Enter the local pressure for accurate results.
The difference between dry-bulb and wet-bulb temperatures. Greater depression = drier air. At 0 depression (Tdb = Twb), the air is saturated at 100% RH.