Relative Humidity Calculator

Relative humidity is a percentage. It tells you how much water vapor is currently in the air compared to the absolute maximum that air could hold at its current temperature. If your hygrometer reads 50%, the air is exactly half full of moisture. The key word is 'relative' — that percentage shifts dramatically every time the temperature changes, even if no water enters or leaves the room.

Because relative humidity lies to you the moment the temperature changes. Dew point is the temperature at which the air must cool before condensation forms. It reflects the absolute quantity of water in the air and stays stable as temperature fluctuates. If you want to know whether it actually feels sticky outside or whether your basement is at condensation risk, look at the dew point — not the relative humidity percentage.

For your health and your home's structural integrity, aim for the 30–60% range. Below 30%, your sinuses dry out, wood floors shrink and gap, and static electricity spikes. Above 60%, you are creating the perfect breeding environment for mold spores, dust mites, and bacteria. The sweet spot that minimizes all biological threats simultaneously is 40–50%, according to the landmark Sterling chart research.

Simple physics: warm air expands and has a much higher capacity for water vapor. By heating a cold room, you are increasing the air's 'storage capacity' without adding any water. The relative percentage drops immediately, even though the total water in the room is identical. This is why a -5°C winter day pumped through your furnace can drop your indoor humidity to 15% or below.

The Magnus-Tetens formula is the mathematical workhorse used to estimate saturation vapor pressure from temperature. It is the industry standard because it is accurate to about ±0.35°C without requiring advanced calculus. Our calculator uses the Alduchov and Eskridge (1996) coefficients, the same values relied on by the NOAA. We handle all the heavy lifting so you don't have to.

Absolute humidity is the raw measurement: the actual physical mass of water vapor in a cubic meter of air (g/m³). Relative humidity is just a percentage of capacity. You can have two rooms with the exact same absolute humidity but wildly different relative humidities if their temperatures differ. Absolute humidity is what engineers use for closed systems; relative humidity is what your body actually feels.

Mold is not picky, but it typically needs the relative humidity to stay above 70% for a sustained period. The real danger is cold exterior walls or single-pane windows — the air cools near the surface, spikes to 100% relative humidity locally, and deposits condensation directly onto your drywall. You can have 55% average room humidity and still get mold if your wall surfaces drop below the dew point.

Yes. The calculator accepts both °C and °F inputs. Just select your preferred unit before entering values. We convert internally to Celsius for the Magnus formula, then convert all outputs back to your selected unit. The formula itself requires Celsius, but you never need to do that conversion manually.

Wet-bulb temperature is the lowest temperature achievable by evaporating water into the air at constant pressure. It is the critical survival threshold for extreme heat. When the wet-bulb temperature approaches 35°C (95°F), human sweating stops being effective regardless of how much water you drink. This is the physiological limit — the reason certain heatwave conditions are genuinely fatal even for healthy adults.

Vapor Pressure Deficit (VPD) is the difference between how much water vapor the air could hold at a given temperature and how much it is actually holding. It is calculated as VPD = e_s(T) × (1 − RH/100), where e_s(T) is the saturation vapor pressure from the Magnus formula. VPD is the key metric for plant transpiration — high VPD means dry air aggressively pulling moisture from leaves, low VPD means near-saturated air with little evaporative capacity. Optimal VPD for most crops is 0.8–1.6 kPa. The calculator above outputs Saturation Vapor Pressure and Actual Vapor Pressure — subtract the two to get VPD. For full worked examples and a VPD reference table by crop stage, see our guide to calculating VPD from temperature and relative humidity.

Mixing ratio (also called humidity ratio in HVAC) is the mass of water vapor per kilogram of dry air, expressed in g/kg. It is calculated as W = 621.98 × e / (P − e), where e is the actual vapor pressure (e_s(T) × RH/100) and P is atmospheric pressure (1013.25 hPa at sea level). Unlike relative humidity, mixing ratio does not change when you heat or cool the air — this is why HVAC engineers use it instead of RH for system design. At 25°C and 60% RH, the mixing ratio is approximately 11.9 g/kg. The calculator outputs this value directly in the results.

Enthalpy is the total heat energy content of moist air — both sensible heat (temperature) and latent heat (moisture). It is calculated as h = 1.006 × T + W × (2501 + 1.86 × T), where T is temperature in °C and W is the humidity ratio in kg/kg. At comfortable indoor conditions of 22°C / 50% RH, enthalpy is approximately 43 kJ/kg. On a hot, humid summer day at 35°C / 70% RH, enthalpy reaches 113 kJ/kg — meaning an HVAC system must remove about 70 kJ/kg to condition the air. HVAC engineers size cooling systems using enthalpy differences, not temperature alone, because moisture removal requires energy even when temperature is unchanged.

Grains per pound (GPP) is the traditional US HVAC unit for moisture content — the number of grains (1/7000th of a pound) of water vapor per pound of dry air. It is directly related to mixing ratio: GPP = mixing ratio (g/kg) × 7. At comfortable indoor conditions of 22°C / 45% RH, GPP is approximately 50–55. Above 90 GPP, mold spore activation risk becomes significant. Above 110 GPP, dust mite reproduction accelerates sharply. The calculator outputs this value in the results as 'Specific Humidity (GPP)'. For a full GPP reference table by temperature and RH, see our enthalpy and grains per pound guide.

To convert absolute humidity (g/m³) to relative humidity, you need the temperature. First calculate the saturation absolute humidity at that temperature: AH_sat = 216.7 × e_s(T) / (T + 273.15), where e_s(T) = 6.112 × exp(17.625 × T / (243.04 + T)). Then RH = (AH / AH_sat) × 100. For example, 12.5 g/m³ at 22°C gives RH = 12.5 / 19.37 × 100 ≈ 64.5%. The same 12.5 g/m³ at 30°C would only be 41% RH because warmer air has a higher saturation point. The calculator outputs absolute humidity as a result — enter temperature and any one known value to derive the rest.

Relative humidity from vapor pressure is a direct calculation: RH = (e / e_s(T)) × 100, where e is the actual vapor pressure in hPa and e_s(T) is the saturation vapor pressure at the current temperature. Saturation vapor pressure is found using the Magnus formula: e_s(T) = 6.112 × exp(17.625 × T / (243.04 + T)). For example, at 25°C, e_s = 31.67 hPa. If your weather station records an actual vapor pressure of 19.0 hPa, then RH = (19.0 / 31.67) × 100 ≈ 60%. The calculator above outputs both Saturation Vapor Pressure and Actual Vapor Pressure in hPa — divide them to get RH at any state point.

If you know the mixing ratio (W, in g/kg) and temperature, convert mixing ratio to actual vapor pressure first: e = (W × P) / (621.98 + W), where P is atmospheric pressure (1013.25 hPa at sea level). Then apply: RH = (e / e_s(T)) × 100. At 25°C with W = 12.0 g/kg: e = (12.0 × 1013.25) / 633.98 ≈ 19.18 hPa, and RH = (19.18 / 31.67) × 100 ≈ 60.6%. HVAC systems are designed around mixing ratio (not RH) because it stays constant when air is heated or cooled, making the reverse conversion essential for commissioning and diagnostics.

A digital hygrometer measures and displays relative humidity directly — you read it off the screen. No calculation is needed. However, if you are using a sling psychrometer (which gives dry-bulb and wet-bulb temperatures, not RH directly), you calculate RH from those two readings. Enter your dry-bulb temperature and wet-bulb temperature into this calculator using the 'Dry Bulb + Wet Bulb' mode to get relative humidity instantly — no psychrometric tables required. To verify your hygrometer's accuracy, use the salt calibration test: wet salt in a sealed bag stabilises at exactly 75.5% RH at room temperature.

In geography, relative humidity describes the moisture content of air masses and is central to understanding climate, precipitation patterns, cloud formation, and biome characteristics. It is calculated identically to the meteorological formula: RH = (actual vapor pressure / saturation vapor pressure) × 100. Geographically, RH shapes where deserts form (hot dry air has enormous vapor capacity, so even modest moisture gives low RH), where tropical rainforests develop (warm humid air stays near 80–100% RH), and how altitude affects apparent moisture (air cools as it rises, RH increases even with the same water content, eventually causing orographic rainfall). The Magnus-Tetens formula is the standard calculation used by geographers and climatologists worldwide.