How to Calculate Humidity: Every Formula Explained
Eight humidity calculations, one physical starting point. This guide walks each formula from saturation vapor pressure through to enthalpy, shows the same air sample expressed every possible way, and links the calculator for each step.
By Editorial Team · Meteorology · HVAC Engineering · Indoor Air Quality
Everything starts with saturation vapor pressure
There is really only one equation to learn. Once you can compute how much water vapor air could hold at a given temperature — its saturation vapor pressure — every other humidity metric is a short step away. Relative humidity is a ratio of it, dew point is an inversion of it, wet-bulb is a numerical search over it, and the mass-based measures scale it against air pressure.
The table below is the map. Each row links to the calculator that performs that step, and every formula is the one this site actually implements — no simplified variants.
Humidity formulas at a glance
| Metric | Answers | Unit | Calculator |
|---|---|---|---|
| Saturation vapor pressure (eₛ) | How much moisture could this air hold? | hPa | Open → |
| Relative humidity (RH) | What fraction of that capacity is used? | % | Open → |
| Dew point (Td) | At what temperature will this air condense? | °C / °F | Open → |
| Wet-bulb temperature (Tw) | How much can evaporation cool this air? | °C / °F | Open → |
| Absolute humidity (AH) | How many grams of water are in a cubic metre? | g/m³ | Open → |
| Mixing ratio (w) & specific humidity | How much water per kilogram of air? | g/kg · grains/lb | Open → |
| Heat index | What does this actually feel like? | °F | Open → |
| Enthalpy (h) | How much total energy is in this air? | kJ/kg | in main tool |
The eight calculations, step by step
1. Saturation vapor pressure (eₛ)
eₛ = 6.112 · exp(17.625·T / (243.04 + T))
Every humidity calculation starts here. Saturation vapor pressure depends on temperature alone and rises exponentially — which is why warm air can carry so much more moisture than cold air. The coefficients 17.625 and 243.04 come from Alduchov & Eskridge (1996), the refinement of the Magnus-Tetens formula that keeps error under 0.4% from −40°C to +50°C.
2. Relative humidity (RH)
RH = 100 · e / eₛ
Relative humidity is a ratio, not an amount: the actual vapor pressure divided by the saturation value. That is why RH changes when you only change the temperature — the moisture stayed put, but the denominator moved. See the relative humidity formula explained.
3. Dew point (Td)
Td = 243.04·γ / (17.625 − γ), γ = ln(RH/100) + 17.625·T/(243.04+T)
Dew point inverts the Magnus formula: instead of asking how full the air is, it asks what temperature would make it exactly full. Because it is an absolute measure of moisture, it is the number meteorologists compare between days and cities — unlike RH, it does not move when the air simply warms up. Below freezing the same inversion gives the frost point.
4. Wet-bulb temperature (Tw)
solve eₛ(Tw) − A·p·(T − Tw) = e for Tw
Wet-bulb has no closed-form solution, so it is found numerically — this site uses a binary search against the psychrometer equation with constant A = 0.000662. It is the temperature a thermometer wrapped in wet cloth settles at, and the hard physiological limit on evaporative cooling: sustained wet-bulb above ~35°C is survivable by no amount of shade or water.
5. Absolute humidity (AH)
AH = 216.7 · e / (T + 273.15)
Derived from the ideal gas law applied to the vapor alone. Absolute humidity is the number you want when sizing a dehumidifier by volume, or when comparing moisture across temperatures. Compare it with RH in relative vs absolute humidity.
6. Mixing ratio (w) & specific humidity
w = 621.98 · e / (p − e)
Mass-per-mass rather than mass-per-volume, so it does not change when air is heated or cooled — the property HVAC load calculations are built on. Multiply g/kg by 7 for grains per pound, the unit on a North American psychrometric chart. Note the total pressure p in the denominator: altitude matters here.
7. Heat index
Rothfusz regression on T and RH (valid ≥ 80°F)
The US National Weather Service heat index is a multiple-regression fit, not a physical derivation, and it is only defined at or above 80°F (26.7°C) — below that it returns the air temperature itself. It estimates how hot the air feels once high humidity has throttled your ability to shed heat by sweating.
8. Enthalpy (h)
h = 1.006·T + w·(2501 + 1.86·T)
Sensible heat plus the latent heat carried by the water vapor. Enthalpy is what an air handler actually has to move, and the reason dehumidifying is so much more expensive than cooling alone: most of the energy is hiding in that second term. See how to calculate enthalpy from humidity.
One air sample, every metric
Here is a single state point — 85°F air at 60% relative humidity, at sea level — expressed every way described above. These figures are generated at build time by the same engine behind every calculator on this site, so they match its output exactly.
| Property | Value |
|---|---|
| Relative Humidity | 60.0% |
| Dew Point | 69.6°F |
| Wet-Bulb Temperature | 74.1°F |
| Air Temperature | 85.0°F |
| Heat Index (Feels Like) | 89.3°F |
| Dew Point Depression | 15.4°F (temp − dew point) |
| Atmospheric Pressure | 1013.25 hPa |
| Saturation Vapor Pressure | 41.05 hPa |
| Actual Vapor Pressure | 24.63 hPa |
| Absolute Humidity | 17.64 g/m³ |
| Mixing Ratio | 15.50 g/kg |
| Specific Humidity | 108.5 GPP (grains/lb) |
| Enthalpy | 69.22 kJ/kg |
Notice how differently the same air reads depending on the metric: 60% sounds moderate, but a dew point near 70°F is the threshold most people describe as oppressive, and the heat index lands several degrees above the actual temperature. That is exactly why one number is never enough.
Which calculation do you need?
- Comfort, mold and condensation indoors → relative humidity, then check the dew point against your coldest surface. See why pipes sweat.
- Comparing muggy days or cities → dew point, never RH. Or read it off the dew point chart.
- Heat stress and worker safety → wet-bulb and heat index.
- Sizing HVAC or dehumidification → mixing ratio in grains per pound and enthalpy.
- Greenhouses and plant transpiration → vapor pressure and its deficit (VPD).
- Drying, materials and lab work → absolute humidity in g/m³.
Every guide on this site
23 in-depth articles covering the physics, the instruments and the practical thresholds behind these calculations.
relative humidity
specific humidity
air conditioning
absolute humidity
vapor pressure
indoor air quality
psychrometric chart
sleep quality
Frequently Asked Questions
- How do you calculate relative humidity?
- Divide the actual vapor pressure by the saturation vapor pressure at that temperature and multiply by 100: RH = 100 × e / eₛ. In practice you measure two of temperature, dew point, or wet-bulb, compute both pressures with the Magnus formula, and take the ratio.
- How do you work out relative humidity from temperature and dew point?
- Apply the Magnus formula twice — once at the air temperature for the saturation vapor pressure, once at the dew point for the actual vapor pressure — then divide: RH = 100 × eₛ(Td) / eₛ(T). This is the most reliable method because dew point is measured directly by chilled-mirror instruments.
- Can you calculate relative humidity without a hygrometer?
- Yes. A sling psychrometer gives you dry-bulb and wet-bulb temperatures, and those two values determine relative humidity through the psychrometer equation. This was the standard method for a century before electronic sensors, and it is still the calibration reference.
- What is the difference between relative and absolute humidity?
- Relative humidity is a percentage of the air's current capacity, so it changes with temperature even when no moisture is added or removed. Absolute humidity is the actual mass of water vapor per cubic metre of air (g/m³) and only changes when moisture is genuinely added or removed.
- Which humidity calculation should I use?
- Use relative humidity for comfort and mold risk, dew point to compare moisture between days or to predict condensation, wet-bulb for heat stress and evaporative cooling, and mixing ratio or specific humidity in grains per pound for HVAC load calculations.
- How accurate are these formulas?
- The Alduchov & Eskridge Magnus coefficients hold saturation vapor pressure to within about 0.4% between −40°C and +50°C, which is far tighter than the accuracy of typical consumer humidity sensors (±2–5% RH). The heat index regression is coarser and only valid at or above 80°F.
Sources & references
- Discussion of Humidity — US National Weather Service
- Mold and Moisture — US EPA
- Alduchov & Eskridge (1996) — Magnus Formula Coefficients — Journal of Applied Meteorology