Free absolute humidity calculator
Enter the air temperature and relative humidity to get the absolute humidity — the actual mass of water vapor per cubic metre (g/m³), plus a g/ft³ value — using the Bolton (1980) vapor-pressure formula, updated live, as you type.
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An estimate from temperature and humidity. How accurate is this?
Results are estimates. Consult a professional.
How the absolute humidity calculator works
Absolute humidity is the actual mass of water vapor in a given volume of air, measured in grams per cubic metre (g/m³). This calculator takes two readings you already have — the air (dry-bulb) temperature and the relative humidity — and returns the absolute humidity in g/m³, with the imperial g/ft³ equivalent and the saturation capacity of that air for context.
It works in two standard steps. First it finds how much vapor the air can hold at the current temperature — the saturation vapor pressure — using the August–Roche–Magnus form with Bolton's 1980 coefficients. Multiplying by the relative humidity gives the actual vapor pressure, and the ideal-gas law converts that pressure into a mass per cubic metre.
Absolute humidity vs. relative humidity and dew point
These three numbers all describe moisture, but they answer different questions. Absolute humidity is the one that reports the actual quantity of water present, which is why it is the figure that matters for condensation, drying, and storage.
- Absolute humidity — the real mass of water vapor in each cubic metre of air (g/m³). It tells you how much moisture is actually there, independent of how full the air is.
- Relative humidity — that same moisture expressed as a percentage of the maximum the air could hold at the current temperature. Because warmer air can hold more, relative humidity falls as you heat the same air and rises as you cool it, even though the water never moved.
- Dew point — the temperature to which you would have to cool the air for it to reach saturation (100% RH) and start forming dew. Like absolute humidity, it tracks the actual moisture content rather than a percentage.
If you have a temperature and a dew point instead, use the relative humidity calculator first, then bring the result here. To find the dew point itself, see the dew point calculator.
What determines absolute humidity
Two inputs set the result. Understanding how each one pulls explains why warm air can carry so much more moisture than cold air at the same relative humidity.
Relative humidity
At a fixed temperature, absolute humidity rises in direct proportion to relative humidity. Double the relative humidity and you double the mass of vapor: air at 30 °C holds about 15.2 g/m³ at 50% and 30.4 g/m³ at 100%. This is the lever you usually read off a weather app or hygrometer.
Air temperature
Temperature sets the ceiling. Warm air can hold far more vapor than cold air, so the saturation capacity climbs steeply with temperature — from about 4.8 g/m³ at 0 °C to 17.3 g/m³ at 20 °C and over 30 g/m³ at 30 °C. That is why 80% humidity on a hot day carries vastly more actual water than 80% on a cold one, and why warm, humid air condenses so heavily when it cools.
A worked example using the absolute humidity calculator
Sam runs a greenhouse and reads 30 °C (86 °F) on the thermometer and 60% on the hygrometer. Relative humidity alone does not tell Sam how much water the air actually holds — which is what decides whether moisture will condense on the cooler glass overnight — so the absolute humidity is what matters.
Step 1 — Enter the two readings
Air temperature 30 °C, relative humidity 60%. Both sit well inside the everyday range the formula is built for.
Step 2 — Find the air's capacity, then the actual moisture
At 30 °C the air can hold about 30.4 g/m³ of vapor when saturated. Sixty percent of that capacity is the actual content: 18.2 g/m³, which is 0.516 g/ft³.
Step 3 — Read the result
Each cubic metre of greenhouse air holds about 18.2 grams of water. When that air cools overnight toward its dew point of roughly 21 °C, its capacity falls below 18.2 g/m³ and the surplus condenses on the glass and leaves — exactly the moisture budget a grower watches to head off mildew.
Absolute humidity chart by temperature and humidity
This chart gives the absolute humidity (g/m³) for common combinations of air temperature and relative humidity. Read down to your air temperature, across to your humidity. Notice how each column climbs steeply downward — that is the air's capacity growing with temperature.
| Air temp | 40% RH | 60% RH | 80% RH | 100% RH |
|---|---|---|---|---|
| 0 °C (32 °F) | 1.9 | 2.9 | 3.9 | 4.8 |
| 10 °C (50 °F) | 3.8 | 5.6 | 7.5 | 9.4 |
| 20 °C (68 °F) | 6.9 | 10.4 | 13.8 | 17.3 |
| 25 °C (77 °F) | 9.2 | 13.8 | 18.4 | 23.0 |
| 30 °C (86 °F) | 12.1 | 18.2 | 24.3 | 30.4 |
| 35 °C (95 °F) | 15.8 | 23.8 | 31.7 | 39.6 |
Absolute humidity in g/m³ from the Bolton (1980) saturation-vapor-pressure formula. The 100% column is the saturation capacity — the most water the air can hold at that temperature. Each row scales straight in proportion to relative humidity.
What absolute humidity is used for
Because absolute humidity measures the real quantity of water in the air, it is the figure professionals reach for whenever moisture has to be added, removed, or kept constant. Relative humidity changes the moment the temperature does, which makes it unreliable for these tasks.
- HVAC and dehumidification — sizing a dehumidifier or air conditioner means knowing how many grams of water per cubic metre must be removed, not a percentage that shifts with the thermostat.
- Greenhouses and growing — vapor-pressure-deficit and condensation management depend on the actual moisture in the air, which governs transpiration and the dew that feeds mildew.
- Museums, archives, and storage — paper, wood, and artwork are protected by holding moisture content steady; absolute humidity reveals whether the real water content is constant even as gallery temperatures drift.
- Condensation, damp, and mold — comparing the absolute humidity of indoor air against the saturation capacity at a cold surface (a window, an external wall) tells you whether condensation — and the mold that follows — will form.
- Industrial drying and curing — kilns, food drying, and paint or concrete curing are controlled by the absolute moisture the surrounding air can still absorb.
Absolute humidity definitions
How accurate is this absolute humidity calculator?
The calculator uses the August–Roche–Magnus saturation-vapor-pressure form with Bolton's 1980 coefficients — the standard method in meteorology and HVAC work. Across the everyday range of about −20 to +50 °C, its saturation vapor pressure is accurate to a fraction of a percent, and the resulting absolute humidity matches published psychrometric tables: roughly 4.8 g/m³ at 0 °C, 17.3 at 20 °C, and 30.4 at 30 °C when saturated. For practical moisture budgeting that is well within the precision you need.
Two limits are worth noting. First, the conversion assumes standard atmospheric pressure; at high altitude the actual g/m³ shifts slightly because the air is thinner. Second, the formula is fitted to saturation over liquid water, so it is most accurate above freezing — below about −20 °C, where ice saturation differs, treat the figure as an approximation. For normal indoor and outdoor conditions, the result is a reliable estimate of the real moisture in the air.
Frequently asked questions about the free absolute humidity calculator
About this Absolute Humidity calculator
This calculator runs entirely in your browser. Nothing you type is sent anywhere — the absolute humidity in g/m³ and g/ft³ is computed locally from the Bolton (1980) saturation-vapor-pressure formula as you adjust the air temperature and relative humidity.
It is one of our weather calculators. Browse the full set of free calculators for more moisture, climate, and everyday tools.