Absolute Humidity Calculator
Find absolute humidity — the actual mass of water vapor in the air — from the temperature (°F or °C) and relative humidity. The tool returns grams of water per cubic meter and HVAC-style grains per cubic foot.
Example: with Air temperature 77 · Relative humidity (%) 50 · Temperature unit Fahrenheit (°F) → Absolute humidity: 11.49 g/m³.
- In grains per cubic foot5.02 gr/ft³
- Moisture levelModerate moisture
Computed by the calculator below using its default values. Change any input to see your own numbers.
Absolute humidity is the real water content of the air, in grams per cubic meter — unlike relative humidity, it does not change just because the temperature does.
Absolute versus relative humidity
Absolute humidity is a straight count of water: how many grams of vapor sit in each cubic meter of air. Relative humidity, by contrast, is a percentage of the maximum the air could hold at its current temperature. That is why a heated winter room can read 40% RH yet hold almost no water — the absolute humidity is tiny because cold outdoor air brought in little moisture to begin with.
To get the mass, this tool first finds the saturation vapor pressure from temperature, scales it by the relative humidity to get the actual vapor pressure, then applies the ideal-gas relation for water vapor. The result is what matters for condensation, mold, static electricity, and how muggy a space actually feels, none of which track RH alone.
How it’s calculated
Actual vapor pressure e = (RH/100) · es, with es = 611.2 · exp(17.625·T/(243.04+T)) in Pa (Magnus form), T in °C. Absolute humidity = 2.16679 · e / T(K) in g/m³, from the water-vapor gas law (Rv = 461.5 J/kg·K). Grains per cubic foot = g/m³ × 0.43706.
Uses the ideal-gas law for water vapor and standard Magnus coefficients; accurate to a fraction of a percent in ordinary indoor and outdoor conditions. Assumes sea-level total pressure.
Absolute humidity at 50% RH by temperature
| Temperature | Saturation g/m³ | At 50% RH |
|---|---|---|
| 32 °F (0 °C) | 4.8 | 2.4 |
| 50 °F (10 °C) | 9.4 | 4.7 |
| 68 °F (20 °C) | 17.3 | 8.6 |
| 77 °F (25 °C) | 23.0 | 11.5 |
| 86 °F (30 °C) | 30.4 | 15.2 |
| 95 °F (35 °C) | 39.6 | 19.8 |
Computed with AH = 2.16679·e/T from the Magnus saturation curve; rounded.
Common mistakes
- Treating relative humidity as if it measured water content — 50% RH holds very different amounts of water at 30 °F and 90 °F.
- Entering temperature in Fahrenheit while leaving the unit set to Celsius.
- Forgetting to convert to Kelvin in the gas law; the T in the denominator is absolute temperature.
- Expecting grams per cubic meter to match grains per pound — grains per cubic foot is the volume-based HVAC unit used here.
Frequently asked questions
What is the absolute humidity formula?
AH = 2.16679 × e / T, where e is the actual vapor pressure in pascals and T is temperature in kelvin. The result is grams of water vapor per cubic meter.
What is the difference from relative humidity?
Absolute humidity is the real mass of water per cubic meter; relative humidity is that amount as a percentage of the maximum the air could hold at its temperature. Warm and cold air at the same RH hold very different absolute amounts.
What is a comfortable absolute humidity indoors?
Roughly 7 to 12 g/m³ feels comfortable. Below about 5 g/m³ air feels dry and static builds; above 15 g/m³ it feels muggy and condensation risk rises.
How do I convert g/m³ to grains per cubic foot?
Multiply grams per cubic meter by 0.43706. Grains per cubic foot is the moisture unit common in North American HVAC work.