What is absolute humidity?

Absolute humidity is the mass of water vapor contained in a unit volume of air, typically expressed in grams per cubic meter (g/m³). Unlike relative humidity, which is a percentage of the maximum possible moisture, absolute humidity is an absolute measure that does not depend on air temperature.

What formula is used for this calculation?

Saturation vapor pressure is computed with the Magnus formula: e_s = 6.1078 × exp((17.27 × T) / (T + 237.3)) in hPa, where T is in Celsius. Actual vapor pressure is e = RH/100 × e_s. Absolute humidity is then AH = 216.7 × e / (T + 273.15) in g/m³.

How is dew point calculated?

Dew point is derived by inverting the same Magnus approximation used by the calculator: Td = (237.3 × ln(e / 6.1078)) / (17.27 − ln(e / 6.1078)), where e is the actual vapor pressure in hPa.

How does absolute humidity differ from relative humidity?

Relative humidity tells you how saturated the air is as a percentage of its maximum capacity at a given temperature — it changes if you heat or cool the air. Absolute humidity measures the actual amount of water vapor present and stays the same as long as no moisture is added or removed, even when temperature changes.

What is a typical absolute humidity value?

In temperate climates, outdoor absolute humidity ranges roughly from 4 g/m³ (cold, dry winter day) to 20 g/m³ (hot, humid summer day). Tropical conditions can exceed 25 g/m³. Heated indoor winter air at low relative humidity can drop below 3 g/m³.

Why does absolute humidity matter for HVAC systems?

HVAC engineers use absolute humidity (often expressed as mixing ratio or specific humidity) to calculate latent heat loads — the energy needed to condense or evaporate moisture. Relative humidity alone is insufficient for sizing dehumidifiers or calculating moisture removal rates.

Absolute Humidity Calculator

Enter temperature and relative humidity to instantly calculate absolute humidity (g/m³), dew point, and actual vapor pressure using the Magnus formula.

Air Conditions

Temperature Unit

Air Temperature (°C)

Relative Humidity (%)

Enter temperature and relative humidity,
then click Calculate Absolute Humidity.

Absolute Humidity

-- g/m³

Dew Point

°C

Actual Vapor Pressure

hPa

Saturation Vapor Pressure

hPa

Max Absolute Humidity

g/m³ at 100% RH

Absolute Humidity Reference Scale

Very Dry

Below 4 g/m³

Dry

4–8 g/m³

Moderate

8–12 g/m³

Humid

12–18 g/m³

Very Humid

Above 18 g/m³

Formula: AH = 216.7 × e / (T + 273.15), where e is actual vapor pressure in hPa (Magnus approximation).

Summary

The Absolute Humidity Calculator converts temperature and relative humidity into absolute humidity — the actual mass of water vapor present in one cubic meter of air, measured in grams per cubic meter (g/m³). Using the Magnus formula approximation for saturation vapor pressure and the ideal gas law for water vapor, this tool also derives the dew point temperature and actual vapor pressure from your inputs. Both Celsius and Fahrenheit inputs are supported, making it useful for meteorology, HVAC engineering, and scientific research.

How it works

Enter the air temperature in either Celsius or Fahrenheit using the unit toggle.
Enter the relative humidity percentage (0–100%).
The Magnus formula computes saturation vapor pressure at the given temperature.
Actual vapor pressure is calculated as RH% × saturation vapor pressure / 100.
Absolute humidity is derived using the ideal gas law for water vapor: AH = 216.7 × e / (T + 273.15), where e is vapor pressure in hPa.
Dew point is back-calculated from actual vapor pressure using the inverse Magnus formula.

Use cases

Calculate moisture content for HVAC system design and indoor climate control.
Monitor drying conditions in food processing, pharmaceuticals, and manufacturing.
Support meteorological analysis where absolute moisture content matters more than relative humidity.
Assess condensation risk on cold surfaces by comparing dew point to surface temperature.
Calibrate humidity sensors by cross-checking readings against calculated values.
Estimate water vapor for combustion engineering and engine performance tuning.
Plan outdoor activities based on actual moisture load rather than relative humidity alone.
Support agricultural decisions about irrigation, storage, and crop drying conditions.

Frequently Asked Questions

Last updated: 2026-06-13 · Reviewed by Nham Vu