Relative Humidity Calculator
Fast, accurate, and free online Relative Humidity Calculator tool that runs directly in your browser.
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Relative Humidity Calculator
RH = 100·e/e_s(T). e_s from Magnus. From Td: e = e_s(Td). From AH: e = (AH/1000)·R_v·T. From w and p: e = p·w/(0.622+w). Td from T and RH as a check.
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Other tools you may find usefulRelative humidity calculator - from temperature, dew point or vapor pressure
This calculator allows you to precisely determinerelative humidity (RH)based on various sets of input data:temperature (T_in), dew point (Td_in), water vapor pressure (e_in), absolute humidity (AH_in)ormixing ratio (w_in). You select the mode (mode:dewpoint, e, AHorw, and the calculator converts all related thermodynamic quantities.
Input data
- Air temperature T_in[°C or K]
- Calculation mode (mode): dewpoint | e | AH | in
- Dew point Td_in[°C]
- Vapor pressure e_in[Pa, hPa, kPa, bar, atm]
- Absolute humidity AH_in[g/m³]
- Mixing factor w_in[g/kg]
- Total pressure p_in[hPa]
- Result precision (precision)
Each mode uses a different set of psychrometric formulas. Results include: RH, dew point (Td), saturation pressure (es), vapor pressure (e), absolute humidity (AH), and mixing ratio (w).
Physical formulas
1. Saturated vapor pressure (Magnus–Tetens)
e_s = 6.112 · exp[(17.62 · T) / (243.12 + T)]
2. Real vapor pressure (from dew point)
e = 6.112 · exp[(17.62 · Td) / (243.12 + Td)]
3. Relative humidity
RH = 100 · e / e_s
4. Absolute humidity
AH = (216.7 · e) / (T + 273.15)
5. Mixing factor
w = 621.97 · e / (p − e)
The calculator converts all values between units and allows you to analyze the influence of temperature and pressure on RH.
Units
| Quantity | Symbol | Units |
|---|---|---|
| Temperature | T, Td | °C / K |
| Pressure | p, e, e_s | Pa / hPa / kPa / bar / atm |
| Relative humidity | RH | % |
| Absolute humidity | AH | g/m³ |
| Mixing factor | w | g/kg |
Calculation examples
Example 1 – dewpoint mode
- T = 25°C
- Td = 15°C
- p = 1013.25 hPa
e = 6.112 × exp[(17.62×15)/(243.12+15)] = 17.0 hPa e_s = 31.7 hPa RH = 100×17.0/31.7 = 53.6%
Result:RH = 53.6%, AH = 10.8 g/m³, w = 7.7 g/kg
Example 2 – steam pressure mode
- T = 20°C
- e = 12.5 hPa
e_s(20°C) = 23.4 hPa → RH = 100×12.5/23.4 = 53.4%
Result:RH = 53.4%, Td = 9.9°C, AH = 10.4 g/m³.
Application
- Room microclimate analysis
- Control of storage conditions
- Modeling of meteorological processes
- Design of HVAC and air dehumidification systems
FAQ
What is relative humidity?
It is the ratio of the current amount of water vapor in the air to the maximum amount it can hold at a given temperature.
What are typical RH values?
Comfortable conditions for humans are 40-60%. Above 70%, the risk of condensation and mold growth increases.
What is the difference between absolute humidity and relative humidity?
Absolute gives the amount of water in grams per cubic meter of air, relative - the percentage of water vapor saturation.
Why does pressure affect RH?
Pressure changes the ability of air to hold water vapor. As the pressure (height) decreases, the saturation point also decreases.
Summary
The relative humidity calculator converts RH from various combinations of data - dew point, vapor pressure, absolute humidity or mixing ratio - using the Magnus equations and the basic laws of thermodynamics. It enables the analysis of air conditions in meteorology, laboratories, buildings and HVAC systems.