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Moved to 40 °F this air would be at 97%
Cooling air raises its relative humidity without adding any water.
- Grains per pound
- 55 gr/lb
- Grams per cubic metre
- 9.4 g/m³
- Dew point
- 50.5 °F
- Vapor pressure
- 12.5 hPa
Why the percentage is not the amount
Warm air can hold far more water than cold air. Relative humidity tells you what fraction of that capacity is in use, so the same percentage means completely different quantities at different temperatures.
Air at 85 °F and 50% carries roughly twice the water of air at 60 °F and 50%. That is why comparing the two percentages tells you nothing useful, and why the third slider above — the same air moved to another temperature — is the interesting one.
What each unit is for
Grains per pound is the US HVAC convention: grains of water per pound of dry air, 7,000 grains to the pound. It is the right unit for comparing indoor and outdoor air, because it does not change when air is heated or cooled.
Grams per cubic metre is the metric equivalent by volume. It is intuitive — it answers "how much water is in this room" — but it shifts slightly with temperature because air expands, so it is less clean for comparisons.
Dew point is the same information expressed as a temperature, and it is the most directly useful of the three: any surface below it is collecting water.
Vapor pressure is what actually drives moisture through materials, from higher to lower. It is the quantity behind why damp moves through a wall at all.
The comparison this makes possible
Set the temperature and humidity to your indoor conditions, note the grains per pound, then do the same for the outdoor forecast. Whichever is lower is the drier air, whatever the percentages say.
That single comparison decides whether ventilating helps or hurts, and the window calculator does it for you in one step. It is also why a basement at 68 °F and 65% and an upstairs room at 72 °F and 55% can be carrying almost identical amounts of water.
Common questions
How do I convert relative humidity to absolute humidity?
You need the temperature as well. The calculator above uses the Magnus-Tetens saturation vapor pressure to get the actual water content in grains per pound or grams per cubic metre from a temperature and a relative humidity.
What are grains per pound?
The US convention for moisture content: grains of water per pound of dry air, with 7,000 grains to a pound. Unlike relative humidity it does not change when air is heated or cooled, which makes it the right unit for comparing two places.
Why does cooling air raise its relative humidity?
Cooling removes capacity, not water. The same amount of moisture becomes a larger share of a smaller maximum, so the percentage climbs — and at the dew point it reaches 100% and starts condensing.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions