Relative humidity (RH) tells you how close the air is to being saturated with water vapor at its current temperature — it's the number on every weather app and thermostat.
Choose RH from Dew Point if you have an air temperature and a dew point reading. Choose From Vapor Pressure if you already know the actual and saturation vapor pressure directly. Choose Absolute Humidity if you have a temperature and RH reading and want the water content per cubic meter.
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Enter your values
On the dew point tab, enter air temperature and dew point in °C. On the vapor pressure tab, enter actual and saturation vapor pressure in hPa. On the absolute humidity tab, enter air temperature and relative humidity (%). The calculator updates instantly as you type.
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Read the result
The result card shows the headline percentage (or, on the Absolute Humidity tab, grams per cubic meter), plus a detail line with the supporting vapor pressures. The interpretation line below states the RH plainly and flags whether that reading is inside the typical comfortable indoor range.
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Cross-check across tabs
Because all three tabs use the same underlying Magnus saturation-pressure formula, you can compute RH from a dew point on the first tab, then feed the resulting actual and saturation vapor pressures into the second tab to confirm they agree.
The Magnus-Tetens formula (WMO/CIMO Guide, WMO-No. 8, coefficients 6.112, 17.62, 243.12) estimates the saturation vapor pressure e_s in hectopascals (hPa) for a temperature T in °C. It is evaluated twice on the dew-point tab — once at the actual air temperature, once at the dew point — because the dew point is, by definition, the temperature at which the current actual vapor pressure equals the saturation vapor pressure.
Relative humidity
RH = 100 × e_actual / e_saturation
Relative humidity is the ratio of the actual vapor pressure in the air to the saturation vapor pressure at the current temperature, expressed as a percentage. When RH is derived from a dew point, e_actual = e_s(dew point) and e_saturation = e_s(air temperature).
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General reference
Trust, Methodology & Sources
Written by Calculover Editorial Team · Updated 2026-07-31▸
Editorial accountability
Author: Calculover Editorial Team - Editor
Owner: Calculover Editorial Team - Editorial owner
Last reviewed: 2026-07-31
Last verified: 2026-07-31
Methodology
Limitations & guidance
Calculations strictly follow the World Meteorological Organization (WMO-No. 8) standards for meteorological instruments and psychrometrics. Saturation vapor pressures use the Magnus-Tetens formulation over liquid water ($T \ge 0^\circ\text{C}$) and Sonntag (1990) formulations over ice ($T < 0^\circ\text{C}$).
Professional guidance: Disclaimer: This tool provides precision psychrometric estimates for meteorological, educational, and HVAC planning purposes. For safety-critical cleanroom monitoring or industrial laboratory control, use calibrated chilled-mirror hygrometers.
Primary sources
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Glossary
Key Terms Explained
7 terms▸
Relative humidity ↗The ratio of the actual amount of water vapor in the air to the maximum the air could hold at its current temperature, expressed as a percentage. 100% means the air is fully saturated.
Dew point ↗The temperature to which air would need to cool (at constant pressure and moisture content) for it to become saturated and begin condensing. A higher dew point means more moisture in the air.
Vapor pressure ↗The partial pressure exerted by water vapor within a mixture of air, typically measured in hectopascals (hPa). The 'actual' vapor pressure is how much water vapor is really present; the 'saturation' vapor pressure is the maximum the air could hold at that temperature.
Saturation ↗The point at which air holds the maximum water vapor possible at its current temperature (RH = 100%). Any further cooling or added moisture beyond this point causes condensation.
Absolute humidity ↗The actual mass of water vapor present in a given volume of air, expressed in grams per cubic meter (g/m³). Unlike relative humidity, it does not depend on temperature — it is a direct measure of water content.
Magnus formula ↗The Magnus-Tetens approximation, a widely used empirical equation for estimating the saturation vapor pressure of water as a function of temperature. This calculator uses the WMO/CIMO Guide coefficients.
Condensation ↗The process by which water vapor turns back into liquid water, which happens when air is cooled below its dew point or when its relative humidity reaches 100%.
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Scenarios
Real-World Examples
3 worked examples▸
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Weather enthusiast
RH from a temperature and dew point reading
Air temperature 25 °CDew point 16.7 °C
e_s(25°C) ≈ 31.6 hPa and e_s(16.7°C) ≈ 19.0 hPa, so RH = 100 × 19.0 / 31.6 ≈ 60%. Absolute humidity works out to about 13.8 g/m³ — a comfortable, moderate humidity level for a warm day.
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Lab technician
RH directly from measured vapor pressures
Actual vapor pressure 19.0 hPaSaturation vapor pressure 31.6 hPa
RH = 100 × 19.0 / 31.6 ≈ 60.1% — essentially the same result as the dew-point example, since these are the same underlying vapor pressures. Useful when a psychrometer or sensor reports vapor pressure directly instead of dew point.
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Homeowner
Absolute humidity from a thermostat reading
Air temperature 25 °CRelative humidity 60%
With T = 25°C and RH = 60%, the actual vapor pressure is 0.60 × 31.6 ≈ 19.0 hPa, giving an absolute humidity of about 13.8 g/m³ — the same physical air as the two examples above, just entered from a typical smart-thermostat readout (temperature + RH%).
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Reference
Cite This Calculator
APA & MLA▸
Use either format to cite this calculator in a paper, report, or resource list.
Relative humidity (RH) tells you how close the air is to being saturated with water vapor at its current temperature — it's the number on every weather app and thermostat. This calculator computes RH three ways depending on what data you have (temperature and dew point, direct vapor pressures, or temperature and RH), and also converts to absolute humidity, the actual mass of water vapor per cubic meter of air.
How the Relative Humidity Calculator works
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All three tabs rely on the same building block: the Magnus-Tetens approximation for saturation vapor pressure, e_s(T) = 6.112 × e^(17.62T / (243.12 + T)), using the WMO/CIMO Guide coefficients. On the dew-point tab, the calculator evaluates this formula at both the air temperature and the dew point — the dew point is, by definition, the temperature at which the current actual vapor pressure would be the saturation vapor pressure, so e_s(dew point) gives the actual vapor pressure directly. Relative humidity is then the ratio of actual to saturation vapor pressure, RH = 100 × e_actual / e_saturation. Absolute humidity converts a vapor pressure into a mass concentration via the ideal gas law for water vapor: AH = 216.7 × e (hPa) / T (Kelvin), in grams per cubic meter.
Inputs and what they mean
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Air temperature and dew point are entered in °C — dew point is always less than or equal to air temperature (they're equal only at 100% RH). Vapor pressures are entered in hectopascals (hPa); at sea level, saturation vapor pressure ranges from under 1 hPa at very cold temperatures to over 100 hPa on a hot, humid day. Relative humidity is a percentage from 0% (bone dry) to 100% (saturated) — values above 100% are physically possible in supersaturated conditions like fog formation, and the calculator flags this rather than treating it as an error.
Limits and edge cases
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The Magnus-Tetens approximation is accurate to within about 0.1% over its typical validity range, roughly −45°C to 60°C, and assumes standard atmospheric pressure — it does not apply a separate correction for high-altitude locations where total air pressure is lower. Dew point can never exceed air temperature in real conditions; if you enter a dew point above the air temperature, the calculator still computes a result (RH above 100%) but flags it as a supersaturated reading rather than a normal atmospheric state. This calculator is a meteorological approximation, not a substitute for a calibrated hygrometer when precise measurement matters (e.g. for lab or industrial humidity control).
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Questions
Frequently Asked Questions
6 questions▸
What is the formula for relative humidity?+
Relative humidity equals 100 times the actual vapor pressure divided by the saturation vapor pressure at the current air temperature: RH = 100 × e_actual / e_saturation. Both vapor pressures are typically measured in hectopascals (hPa).
How do you calculate relative humidity from dew point?+
Compute the saturation vapor pressure at the air temperature and again at the dew point, using the Magnus-Tetens formula. The vapor pressure at the dew point equals the actual (current) vapor pressure, so RH = 100 × e_s(dew point) / e_s(air temperature).
What does absolute humidity mean?+
Absolute humidity is the actual mass of water vapor in a given volume of air, expressed in grams per cubic meter (g/m³). Unlike relative humidity, it doesn't change with temperature alone — it's a direct measure of how much water vapor is physically present.
What does 100% relative humidity mean?+
100% relative humidity means the air is fully saturated with water vapor at its current temperature — any further cooling or added moisture will cause condensation, fog, dew, or rain.
What units does the Relative Humidity Calculator use?+
Relative humidity and absolute-humidity-tab RH input are shown as a percentage (%). Vapor pressures are entered and displayed in hectopascals (hPa). Absolute humidity is shown in grams per cubic meter (g/m³). Temperatures are entered in degrees Celsius (°C).
What relative humidity is comfortable indoors?+
Most health and building-science guidance recommends keeping indoor relative humidity between 30% and 50%. Below that range, air feels dry and can irritate skin and airways; above it, air feels muggy and increases the risk of mold and dust-mite growth.
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