Relative Humidity Calculator
Work out the relative humidity from the air temperature and the dew point, using the Magnus relationship in reverse.
This inverts the Magnus formula to back out humidity from the gap between temperature and dew point. A big gap means dry air; when the two numbers meet, you’re at 100% and fog or dew is likely.
How it works
Relative humidity is the share of moisture the air is holding compared to the most it could hold at that temperature. Weather reports often give the dew point instead, and this tool turns those two numbers back into a percentage.
It runs the Magnus formula in reverse: from the air temperature and the dew point, it works out the saturation vapor pressures and divides them to get the humidity. The wider the gap between temperature and dew point, the drier the air.
This is handy when a station or forecast lists the dew point but not the humidity, or when you're checking a thermostat and hygrometer against each other. It's the inverse of our dew point calculator, so the two always agree.
Frequently asked questions
What relative humidity should I expect indoors?
Most homes are comfortable and healthy between 30% and 50%. Too low and you get dry skin and static; too high and you risk condensation, mold, and dust mites, so many people aim for the low 40s.
Why does relative humidity change even when moisture doesn't?
Warm air can hold more water than cold air. So as temperature rises through the day the same amount of moisture fills a smaller share of the air's capacity, and the relative humidity drops even though nothing was added or removed.
What happens at 100% humidity?
At 100% the air is saturated — the temperature has reached the dew point — and any extra moisture condenses out as dew, fog, or rain. That's why the calculator caps the result at 100% and won't let the dew point exceed the temperature.