How this instrument works
Relative humidity is a ratio, not an amount: it compares the actual water-vapor pressure in the air to the saturation vapor pressure the air could hold at its current temperature, then multiplies by 100. Warm air can carry far more vapor before it saturates than cold air can, so identical moisture content reads as a low percentage on a hot afternoon and a much higher one after the temperature drops overnight — the water in the air hasn't changed, only the air's capacity to hold it has.
This instrument reaches that ratio without ever measuring vapor pressure directly, because dew point already carries the information. Dew point is defined as the temperature to which air must cool, at constant pressure, for its actual vapor content to become saturated — so the saturation vapor pressure at the dew point equals the air's real, present vapor pressure. Both saturation curves are computed with the same exponential fit, Bolton's 1980 constants of 17.67 and 243.5°C, so the shared reference pressure cancels out and only the difference between the two exponents survives.
That exponential shape isn't decorative — saturation vapor pressure genuinely rises exponentially with temperature, a direct consequence of the Clausius–Clapeyron relation for liquid-vapor equilibrium, and Bolton's fit tracks that curve to within roughly 0.1% across the ordinary weather range. It breaks down over ice and at extremes, which is why forecasters, HVAC engineers sizing dehumidifiers, and agronomists watching for fungal disease risk in stored grain all treat a dew point reported above air temperature as an instrument fault, not a real reading — air cannot be more than fully saturated.
- Enter the Air temperature reading, in °C, or switch to °F using the field's unit menu.
- Enter the Dew point — it should never read higher than the air temperature; if it does, treat the figure as sensor error.
- Read Relative humidity, % directly; the instrument applies the saturation-curve formula for you, with no lookup table needed.
- Adjust either input to see the relationship move: cooling the air toward its dew point pushes relative humidity toward 100%.
Worked example — 25°C air with a 15°C dew point
Take a mild afternoon: the Air temperature reads 25°C and a dew-point hygrometer reports a Dew point of 15°C. The exponents come first: 17.67×15⁄258.5 = 1.02534, and 17.67×25⁄268.5 = 1.64525, using Td+243.5 = 258.5 and T+243.5 = 268.5 as the two denominators. Subtracting gives −0.61991, and 100·exp(−0.61991) = 53.7991%, which is exactly what the Relative humidity, % field displays.
That pairing of numbers isn't arbitrary — 25°C and 15°C are the same inputs that verify this site's dew-point calculator, run in the opposite direction. Feed roughly 53.8% relative humidity back in alongside 25°C air temperature and that instrument returns a 15°C dew point, because both tools share the same Magnus-formula constants published by David Bolton in 1980. A 10°C gap between air and dew point is an unremarkable, comfortable spread — roughly what a dry temperate afternoon feels like before a front moves through.
Questions
Why does the formula use dew point instead of a measured vapor pressure?
Because dew point already defines it: by definition, the saturation vapor pressure at the dew point equals the air's actual, present vapor pressure. Feeding both temperatures through the same saturation-curve formula and subtracting the exponents gives the ratio directly, without ever computing a vapor pressure in hectopascals along the way.
Can relative humidity ever read above 100%?
Not under normal conditions. 100% marks the point at which air is fully saturated, and further cooling produces condensation rather than a higher percentage. A result above 100% almost always means the dew point was entered higher than the air temperature, which is physically impossible for a valid reading and usually points to a faulty sensor or a typo.
How accurate is the Magnus-Tetens formula this calculator uses?
Very, for ordinary weather. Bolton's 1980 constants — 17.67 and 243.5°C — fit the true saturation vapor pressure curve to within roughly 0.1% across the range meteorologists usually work in, about −35°C to 35°C. Outside that band, particularly over ice below freezing, a different constant pair tracks the ice saturation curve more closely.
What is the difference between relative humidity and dew point?
Dew point is an absolute temperature — the point at which air's actual moisture would saturate — so it tracks the amount of water vapor present regardless of the current air temperature. Relative humidity is a percentage that shifts with air temperature even when the water content stays fixed, which is why forecasters often quote dew point as the steadier measure of how muggy the air actually feels.
Who relies on this calculation outside of weather forecasting?
HVAC engineers use it to size dehumidifiers and avoid condensation forming on cold surfaces, agronomists and grain-store managers watch it because many fungal pathogens spread rapidly once relative humidity passes roughly 85%, and pilots use the air-temperature-to-dew-point gap to estimate cumulus cloud-base height when planning a flight.
Does this calculator account for changes in atmospheric pressure?
No — the Magnus-Tetens approximation used here is pressure-independent by design, which is standard practice for surface weather calculations at typical station elevations. At very high altitude, or in engineering work needing precision beyond a few tenths of a percent, a pressure-corrected saturation vapor pressure formula would be the better choice.