Relative humidity and dew point calculator

What this calculator does

It converts between three descriptions of the same moist air: air temperature, relative humidity (RH) and dew point. Enter the air temperature and either the RH or the dew point; the calculator returns the other one, together with the water vapour pressure, the humidity ratio and the absolute humidity at the air pressure you set.

With an optional surface temperature it also tells you whether that surface is at or below the dew point of the air, which is the physical condition for water to condense on it. It evaluates one air state that you enter. It does not measure anything, does not predict how long a part will last and does not give a humidity level that is safe for electronics.

Inputs and units

Air temperature is the dry-bulb temperature of the air, from 0.01 to 60 °C (32.02 to 140 °F). Relative humidity is entered in percent, from 1 to 100 %, and is defined with respect to liquid water. Dew point is entered in the same unit as the air temperature and cannot be higher than it.

Air pressure defaults to standard sea-level pressure, 101.325 kPa (29.92 inHg). Change it for a site at altitude or for a measured station pressure. Pressure does not change the relation between temperature, RH and dew point in this calculation; it changes the humidity ratio, because the same water vapour is mixed with less dry air at lower pressure.

The unit switch converts every input and result between SI (°C, kPa, g/kg, g/m³) and US units (°F, inHg, grains per pound, grains per cubic foot). The calculator stores its inputs in SI and only changes how they are displayed, so switching units does not change the air state. Humidity ratio is the mass of water vapour per mass of dry air. Absolute humidity is the mass of water vapour per volume of air. They are different quantities.

Equations and sources

Saturation vapour pressure over liquid water, p_ws(T), comes from the IAPWS vapour-pressure equation (IAPWS SR1-86, eq. 1), which IAPWS endorses from the triple point, 273.16 K, to the critical point. The implementation reproduces the release's check values: 611.657 Pa at 273.16 K and 101 325 Pa at 373.1243 K. Saturation over ice uses the IAPWS sublimation equation (IAPWS R14-08, eq. 6) and reproduces its check value of 8.947 35 × 10⁻⁶ MPa at 230 K.

RH = 100 % × p_w / p_ws(T)

dew point T_d: p_ws(T_d) = p_w

humidity ratio W = 0.621957 × p_w / (p − p_w)

absolute humidity ρ_w = p_w / (R_w × T), R_w = 461.52 J/(kg·K)

Here p_w is the water vapour pressure, p the air pressure and T the absolute temperature. The dew point is found by solving the IAPWS equation numerically rather than with a Magnus-type approximation. 0.621957 is the ratio of the molar masses of water and dry air given in CIPM-2007 (18.015 28 and 28.965 46 g/mol); R_w is the molar gas constant divided by the molar mass of water.

Moist air is treated as an ideal mixture. The enhancement factor, which accounts for real-gas behaviour of water vapour in air, is left out. At 20 °C and 101.325 kPa the CIPM-2007 formula gives a factor of about 1.004, so the humidity ratio and vapour pressure shown here can be about 0.4 % lower than in calculations that include it.

Worked example you can recompute

Air at 22 °C (71.6 °F) and 60 % relative humidity, at standard pressure:

At 80 kPa, roughly the pressure about 2,000 m above sea level, the dew point and RH stay the same but the humidity ratio rises from 9.90 to 12.59 g/kg. A surface at 12 °C in this air is 1.9 °C below the dew point; a surface at 16 °C is 2.1 °C above it.

Surface temperature and condensation

Water condenses on a surface when the surface temperature is at or below the dew point of the surrounding air. What matters is the temperature of the housing, circuit board, heat sink or wall, not the air temperature in the room. A device brought in from a cold vehicle or store can be well below the dew point of warm indoor air even when the room RH looks moderate.

If the air's water vapour pressure is below 611.657 Pa, the air has no dew point above 0.01 °C. The calculator then compares the surface with the frost point, the temperature at which the air is saturated with respect to ice. For a surface below 0.01 °C, deposited water can be frost or water that later freezes.

The result compares two temperatures you entered. It does not account for sensor uncertainty, the thin air layer next to the surface, or temperature differences across a part, so a small positive difference is not a safe margin. How long equipment needs before every relevant surface is above the dew point depends on its mass, packaging and air movement; follow the manufacturer's instructions. Why condensation matters for electronics is covered in How humidity damages electronics.

Reading the psychrometric chart

The chart plots air temperature on the horizontal axis and humidity ratio on the vertical axis, the layout of a classic psychrometric chart. The upper curve is saturation (100 % RH); the thinner curves are constant RH in 10 % steps. All of them depend on air pressure, and the chart is redrawn for the pressure you set.

The circle marks your air. Cooling air without adding or removing water keeps the humidity ratio constant, so the state moves horizontally to the left. Where that dashed line meets the saturation curve is the dew point. A vertical dashed line marks the surface temperature when you enter one. Every value shown in the chart is also listed in the results next to it.

You can drag the circle, or select it and use the arrow keys. The position gives an air temperature and a humidity ratio at the pressure you set; the calculator converts these back to relative humidity or dew point with the same equations as the input fields and writes them into the fields. States the calculator does not support are held at the nearest supported one, with a note: above 100 % RH at saturation, below 1 % RH at 1 %, and in the dew-point mode at a dew point of 0.01 °C.

Limits

Air temperatures below 0.01 °C are not supported. Below freezing, relative humidity can be defined with respect to liquid water or to ice, and the two values differ; this calculator uses liquid water, and the IAPWS liquid-water equation used here starts at the triple point. Supercooled water is therefore outside this calculator, and dew points below 0.01 °C cannot be entered.

Air temperature is limited to 60 °C, air pressure to 50–110 kPa and surface temperature to −40 to 80 °C. These are limits of the tool, chosen inside the validity ranges of the equations. Results are rounded for display; the calculation itself is not rounded.

Use it from scripts and agents

The same calculation is available as a public JSON endpoint with explicit inputs and units, model version and warnings. See the humidity calculation API.

From one air state to exposure at a site

This calculator answers what happens for one temperature and humidity that you enter. It does not know where you are, and its inputs are not site data. Whether a location often reaches such conditions is a different question: how many hours per year the outdoor air is near its dew point, how often RH stays high and how this changes through the year.

Sources

1. IAPWS SR1-86(1992): Revised Supplementary Release on Saturation Properties of Ordinary Water Substance. Equation (1), validity range (10) and check values (Table 1).
2. IAPWS R14-08(2011): Revised Release on the Pressure along the Melting and Sublimation Curves of Ordinary Water Substance. Sublimation equation (6) and check value (Table 3).
3. Picard, A., Davis, R. S., Gläser, M. and Fujii, K. (2008): Revised formula for the density of moist air (CIPM-2007). Metrologia 45, 149–155. Molar masses of dry air and water; enhancement factor (eq. A1.2).
4. NIST Special Publication 811, Appendix B: conversion factors for inch of mercury (conventional), grain, pound and cubic foot.

Equations, constants and check values were checked against these documents on 2026-10-18.