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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 meter
- 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 meter 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.
A worked example
A worked example showing why the same air produces two completely different readings in two rooms.
Start in the living room
72 °F and 45% relative humidity. The air is carrying roughly 51 grains of water per pound of dry air.
Move that air into an unheated bedroom
The same air at 60 °F still carries 51 grains per pound — nothing has been added or removed. But the colder air can hold much less, so the relative humidity reads about 68%.
Now put it against a cold wall
At 55 °F the same air reads about 82% — over the mold germination threshold, with no change whatsoever to the amount of water present.
And the dew point stays put
Around 50 °F throughout, because dew point tracks water content rather than temperature. That is why every serious comparison on this site is made in dew points or grains rather than percentages.
How to read the result
Grains per pound is the unit US heating and cooling work uses, and it is the honest way to compare two rooms or two air masses. Grams per cubic meter is the metric equivalent by volume, which is more intuitive — it answers "how much water is in this room" — but it shifts slightly with temperature because air expands.
The practical use of this converter is diagnostic. If two rooms in a house have the same grains-per-pound figure, they contain the same air and the difference in their readings is entirely temperature. If they differ, one of them has a moisture source or a ventilation path the other does not, and that is a much more useful thing to know than which room reads higher.
It is also the fastest way to see why heating is not drying. Turn a room from 60 to 72 °F and the relative humidity falls sharply while the grains figure does not move at all. Nothing has left the house; the air simply has more room in it.
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 meter 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.
What is a normal grains per pound figure for a house?
Comfortable indoor air usually sits between about 40 and 60 grains per pound. Below 30 is dry enough to notice — static, dry airways, timber moving. Above 75 is the sort of figure a humid summer day produces, and it is why a Gulf Coast house needs mechanical dehumidification while a Denver house does not.
Why do HVAC engineers use grains instead of percentages?
Because the job is removing water, and water is a mass. A cooling coil has to condense a specific number of pounds of water per hour, and that calculation cannot be done in relative humidity, which changes with temperature at constant moisture content. Percentages describe comfort; grains describe the load.
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