Relative Humidity Calculator
Find relative humidity from air temperature and dew point using the Tetens saturation vapor pressure formula.
🌫️ What is Relative Humidity?
Relative humidity (RH) is the ratio of the actual water vapor pressure in the air to the saturation vapor pressure at the current air temperature, expressed as a percentage. It tells you how close the air is to being fully saturated with moisture, not how much water vapor is physically present, which is why the same absolute moisture content can read very differently on a hot afternoon versus a cool morning.
Meteorologists, HVAC engineers, and agronomists use relative humidity constantly, to forecast fog and dew formation as air cools overnight toward its dew point, to compute the heat index and judge outdoor comfort and heat stress risk, to control greenhouse and storage conditions where crops and materials are sensitive to moisture, and to size dehumidifiers and air conditioning systems for a target indoor comfort level.
A common point of confusion is assuming relative humidity is a direct measure of "how much water is in the air." It isn't, RH is temperature-dependent, the exact same air mass can show 90 percent RH at dawn and drop to 40 percent RH by mid-afternoon purely because the temperature rose, with no water vapor added or removed. Dew point, not relative humidity, is the metric that tracks actual moisture content.
This calculator computes relative humidity directly from air temperature and dew point using the Tetens saturation vapor pressure formula, the same approximation used in most operational meteorology, and shows every intermediate value used to derive it.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Warm Afternoon
T = 25°C, Td = 16.68°C
Example 2 — Humid Tropical Evening
T = 30°C, Td = 26.16°C
Example 3 — Dry Winter Air
T = 15°C, Td = 1.51°C
❓ Frequently Asked Questions
🔗 Related Calculators
What is relative humidity?
Relative humidity (RH) is the ratio of the actual water vapor pressure in the air to the saturation vapor pressure at the current air temperature, expressed as a percentage. It measures how close the air is to being fully saturated with moisture at its current temperature, not the absolute amount of water vapor present.
How do you calculate relative humidity from temperature and dew point?
RH = 100 times es(Td) / es(T), where es(T) is the Tetens saturation vapor pressure at air temperature T, and es(Td) is the same formula evaluated at the dew point Td. Both es values use es(x) = 6.112 times exp(17.67*x / (x + 243.5)) in hectopascals, with x in Celsius.
What is the relationship between relative humidity and dew point?
Dew point is the temperature at which the air's actual water vapor content would saturate it, so it directly determines the numerator of the RH ratio. When dew point equals air temperature, RH is 100 percent, the wider the gap between them, the lower the relative humidity.
Why does relative humidity change during the day even without new moisture?
Relative humidity depends on temperature because warmer air can hold more water vapor before saturating. If the actual moisture content (dew point) stays constant while air temperature rises through the afternoon, the saturation vapor pressure in the denominator grows, so relative humidity falls, even though nothing changed about the actual water vapor in the air.
What relative humidity feels comfortable?
Comfort depends on both RH and temperature together, but as a general guide, 30 to 50 percent RH feels comfortable at typical room temperatures, below 30 percent feels dry (and can cause static and irritation), while above 60 percent starts to feel humid or muggy, especially at warmer temperatures.
Can relative humidity exceed 100 percent?
In practice, no, once the air reaches 100 percent RH it is saturated and any excess water vapor condenses out as dew, fog, or cloud droplets, capping RH at 100 percent under normal equilibrium conditions. Supersaturation above 100 percent can briefly occur in clean air with no condensation nuclei, but this is a special case, not typical weather.
What is the Tetens formula used for saturation vapor pressure?
The Tetens formula estimates saturation vapor pressure as es(T) = 6.112 times exp(17.67*T / (T + 243.5)) in hectopascals, with T in Celsius. It's a widely used empirical approximation, accurate to about 1 percent across typical atmospheric temperatures, and this calculator applies it at both the air temperature and the dew point.
How is this different from the dew point calculator?
This calculator solves the inverse problem, given air temperature and dew point, it finds relative humidity. A dew point calculator instead takes air temperature and relative humidity to find the dew point. Both use the same underlying saturation vapor pressure physics, just solved for a different unknown.
What is dew point depression and how does it relate to RH?
Dew point depression is air temperature minus dew point (T minus Td). A small depression, a few degrees or less, corresponds to high relative humidity and air close to saturation, useful for forecasting fog. A large depression corresponds to low relative humidity and comparatively dry air.
Does relative humidity depend on air pressure?
The Tetens saturation vapor pressure formula used here is derived from temperature alone and does not depend directly on atmospheric pressure, this is the standard approach for calculating RH from temperature and dew point at typical surface conditions. Pressure effects on saturation are a small secondary correction not needed for everyday RH calculations.
What input range does this calculator accept?
This calculator accepts air temperatures from -20 to 60 degrees Celsius and dew points from -30 to 60 degrees Celsius, with the requirement that dew point cannot exceed air temperature, since that combination is not physically possible.