In short: at 20°C (68°F) and 50% relative humidity, air holds about 8.6 g/m³ of water vapor (3.8 grains per cubic foot). Warmer air holds far more of it — by 30°C the same 50% RH carries about 15.1 g/m³ — which is why you need both numbers, not humidity alone, to know how much water is actually in the air.
These figures are planning aids based on the standard Magnus–Tetens approximation. For laboratory-grade psychrometry or regulated environments, confirm with calibrated instruments.
How to calculate water content from humidity
Relative humidity on its own doesn't tell you how much water is actually in the air — it only tells you how close the air is to its own maximum. To get an actual mass of water, you need the temperature too, because warmer air can hold far more vapor before it saturates. This calculator first finds the saturation vapor pressure at your temperature, then scales it by your relative humidity:
Worked example. Take air at 20°C and 50% RH. The saturation vapor pressure at 20°C is 6.112 × exp(17.62×20/263.12) = 23.33 hPa. Scale by 50%: 0.5 × 23.33 = 11.665 hPa of actual vapor pressure. Plug into the formula: 216.7 × 11.665 / 293.15 = 8.62 g/m³. That single calculation — done automatically above — is all it takes once you have the saturation curve.
Same formula, different units. This is the same water-vapor physics behind the water vapor pressure calculator and the dew point calculator — just expressed as a mass per volume instead of a pressure or a temperature.
Why temperature changes the water content of humid air
Air doesn't really "hold" water the way a sponge holds a fixed amount of liquid — water vapor is simply a gas mixed into the air, and how much of it can stay as gas (rather than condense into droplets) depends on temperature. Warmer air molecules move faster and carry more energy, so more water molecules can remain as vapor before the air becomes saturated. That's why saturation water content climbs steeply with temperature: from about 4.8 g/m³ at freezing to over 50 g/m³ at 40°C.
This is also why the same relative-humidity reading can feel very different depending on the season. A muggy 90% RH day at 30°C is carrying roughly five times more actual water than a "damp" 90% RH day at 0°C — the percentage is identical, but the water content is not. If the air cools past its dew point, the excess water condenses out as fog, dew, or condensation on cold surfaces. You can read the full derivation on our methodology page.
Maximum water content of air by temperature
Saturated air (100% RH) at each temperature — the absolute ceiling on how much water vapor that air can carry before it starts condensing.
| Temperature | Sat. vapor pressure (hPa) | Water content (g/m³) |
|---|---|---|
| 0 °C / 32 °F | 6.11 | 4.85 |
| 10 °C / 50 °F | 12.26 | 9.38 |
| 20 °C / 68 °F (room) | 23.33 | 17.25 |
| 25 °C / 77 °F | 31.60 | 22.97 |
| 30 °C / 86 °F | 42.34 | 30.27 |
| 35 °C / 95 °F | 56.13 | 39.48 |
| 40 °C / 104 °F | 73.70 | 51.00 |
Quick reference: water content at 50% relative humidity
A common comfort target. Notice how quickly the water content climbs with temperature at the same 50% RH.
| Temperature | g/m³ | gr/ft³ |
|---|---|---|
| 0 °C / 32 °F | 2.42 | 1.06 |
| 10 °C / 50 °F | 4.69 | 2.05 |
| 20 °C / 68 °F (room) | 8.62 | 3.77 |
| 25 °C / 77 °F | 11.48 | 5.02 |
| 30 °C / 86 °F | 15.13 | 6.61 |
| 35 °C / 95 °F | 19.74 | 8.63 |
| 40 °C / 104 °F | 25.50 | 11.14 |
Worked examples
A 40 m³ bedroom at 20°C, 50% RH
345 g
That's about 0.76 lb of water vapor floating invisibly through the room — less than a can of soda, spread across the whole space.
A 100 m³ greenhouse at 30°C, 80% RH
2.42 kg
About 5.3 lb of water vapor in the air alone — a useful starting point before sizing dehumidification or ventilation.
Drying a 60 m³ basement from 90% to 50% RH
307 g
At 15°C, that's roughly how much water a dehumidifier needs to pull from one full volume of that air to hit the 50% target.
What else affects the result
Barometric pressure. This calculator assumes standard atmospheric pressure at sea level. At high altitude, the physical relationship between vapor pressure and water content shifts slightly because total air pressure is lower — the effect is small below about 1,500 m (5,000 ft) but grows at higher elevations.
Non-ideal behavior at high temperature. The Magnus–Tetens approximation is extremely accurate between 0°C and 50°C, which covers essentially every HVAC, greenhouse, drying and household case. Above roughly 60°C, more exact formulations (like the Arden Buck or Goff–Gratch equations) diverge slightly and would be needed for precision work.
Mixing and stratification. Real rooms rarely have perfectly uniform humidity — air near a cold window or a running shower can be locally much wetter than the room average. Use a reading from the location that matters most for your question.
Frequently asked questions
How much water is in the air at 50% humidity and room temperature?
At 20°C (68°F) and 50% relative humidity, air holds about 8.6 grams of water per cubic meter — roughly 3.8 grains per cubic foot. A 40 m³ bedroom at that condition carries about 345 grams of water vapor, spread invisibly through the whole room.
What's the difference between relative humidity and absolute humidity (water content)?
Relative humidity is a percentage — how full the air is compared with the maximum it could hold at that temperature. Absolute humidity, or water content, is the actual mass of water in a given volume of air, in grams per cubic meter, and it does not depend on temperature the way a percentage does. The same 50% RH means very different amounts of water at 5°C than at 35°C, because warm air can hold far more.
Why can hot air hold so much more water than cold air?
Warmer air molecules move faster and carry more energy, which lets more water stay as vapor instead of condensing into droplets. Saturation water content roughly doubles for roughly every 10–11°C rise in temperature — at 20°C saturated air holds about 17.2 g/m³, while at 30°C it holds about 30.3 g/m³.
What is dew point and how is it related to this calculator?
Dew point is the temperature at which the water vapor already in the air would start condensing into liquid — as fog, dew, or condensation on a cold surface. This calculator computes it from the same relative humidity and temperature using the Magnus formula. At 20°C and 50% RH, the dew point is about 9.3°C (48.7°F).
How accurate is this calculator?
It uses the Magnus–Tetens approximation for saturation vapor pressure, the formula recommended by the World Meteorological Organization for general meteorological and engineering use. It is accurate to within about 0.1–0.2% between 0°C and 50°C, more than sufficient for HVAC, greenhouse, drying and household humidity work.
Sources & references
- Tetens, O. (1930). Über einige meteorologische Begriffe. Zeitschrift für Geophysik, 6, 297–309.
- World Meteorological Organization (WMO). Guide to Instruments and Methods of Observation (WMO-No. 8).
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)