Dew Point vs Relative Humidity Calculator

Dew point and relative humidity describe the same moisture state of the air from two different angles, and the two calculators are inverses of each other. Dew point is the temperature at which the current air would become saturated if cooled at constant moisture — a direct measure of how much water vapor is actually present. Relative humidity is the ratio of the actual vapor pressure to the saturation vapor pressure at the current air temperature — a measure of how close the air is to saturation. A warm afternoon and a cool morning can have very different relative humidity but nearly the same dew point, which is why the choice between the two tools comes down to which quantity you already have and which question you are answering.

What each calculator does

The dew point calculator takes air temperature and relative humidity, converts Fahrenheit inputs to Celsius, and applies the Magnus approximation with constants a = 17.27 and b = 237.7:

$\gamma = \frac{a \times T}{b + T} + \ln\left(\frac{RH}{100}\right), \qquad T_{\text{d}} = \frac{b \times \gamma}{a - \gamma}$

The result appears in the selected unit and also lists the dew point in Celsius, rounded to two decimals. It is useful for weather interpretation, HVAC troubleshooting, greenhouses, storage rooms, and any situation where moisture becomes a problem only after a surface gets cold enough: condensation can occur when a surface is at or below the dew point.

The relative humidity calculator takes air temperature and dew point (rejecting a dew point above the air temperature), converts to Celsius, and applies the Magnus-Tetens relationship with constants 17.625 and 243.04:

$RH = 100 \times \frac{\exp\left(\frac{17.625 \times T_{\text{d}}}{243.04 + T_{\text{d}}}\right)}{\exp\left(\frac{17.625 \times T}{243.04 + T}\right)}$

The displayed result is clamped between 0 percent and 100 percent, rounded to two decimals, and labeled positive only in the 30 percent through 60 percent comfort band used by the page — the range commonly cited for indoor comfort and moisture control.

Side-by-side

When to use which

Use the dew point calculator when you have temperature and relative humidity and want the dew point — for example, to anticipate condensation on a window, pipe, wall, or stored material, or to compare how muggy two days are with a number that does not shift every time the temperature changes. Use the relative humidity calculator when you have temperature and dew point and want the percentage — for example, to check a room against the 30 percent to 60 percent indoor comfort band or to understand how close the air is to saturation.

The two tools describe the same air, so use both when comfort, condensation, and heat stress all matter. In hot conditions, pair either result with the heat index calculator for heat-stress context. Do not mix observations from different times or places: both formulas assume the temperature and the moisture reading describe the same parcel of air.

Limits and disclaimer

Both pages are educational tools. The comfort figures are rules of thumb, not official medical limits — dew point comfort depends on acclimation, wind, sun, and activity, and mold risk depends on surface temperature, wetting duration, leaks, ventilation, and materials, not the room-air percentage alone. The Magnus approximations are accurate for common weather and indoor conditions but not for industrial drying, cryogenic temperatures, compressed-air systems, or precision psychrometrics, which may need full thermodynamic calculations and calibrated instruments.

Try them

• Dew point calculator
• Relative humidity calculator