Percent Humidity Calculator: Formula & How to Use

This calculator works out relative humidity from air temperature and dew point, or from wet-bulb and dry-bulb temperatures, and returns the percentage you see on weather reports. You enter two measurements, choose the method, and get the relative humidity as a percent, plus the dew point if you used the wet-bulb method. The formulas below are the standard ones used in meteorology and HVAC work, so you can check the math yourself.

Formula

Relative humidity (RH) is the ratio of the actual water vapor pressure in the air to the saturation vapor pressure at the same temperature, expressed as a percent. The most common way to calculate it uses the dew point and air temperature with the Magnus formula:

RH = 100 × exp( (17.625 × T_d) / (243.04 + T_d) − (17.625 × T) / (243.04 + T) )

• T = air temperature in °C
• T_d = dew point temperature in °C
• exp = the exponential function (e raised to the power in parentheses)

If you only have wet-bulb and dry-bulb temperatures, the calculator uses a psychrometric approximation that accounts for the cooling effect of evaporation. That formula is more involved, but it is built into the calculator—you just enter the two temperatures and the air pressure (default 1013 hPa, standard sea-level pressure).

Worked Example

Using the default values: air temperature 25°C, dew point 15°C.

1. Calculate the saturation vapor pressure at the dew point: 17.625 × 15 / (243.04 + 15) = 264.375 / 258.04 ≈ 1.0246
2. Calculate the saturation vapor pressure at the air temperature: 17.625 × 25 / (243.04 + 25) = 440.625 / 268.04 ≈ 1.6438
3. Subtract: 1.0246 − 1.6438 = −0.6192
4. Take the exponential: e^−0.6192 ≈ 0.538
5. Multiply by 100: 0.538 × 100 = 53.8%

The calculator rounds to one decimal place, so it shows 53.8%. That matches the default output.

Why Relative Humidity Matters

Relative humidity tells you how close the air is to saturation, which affects how comfortable you feel, how quickly sweat evaporates, and how likely condensation or fog will form. At 100% RH, the air cannot hold any more water vapor, so dew, fog, or rain occurs. At 30% RH, the air feels dry, and at 70% or higher, it feels muggy. Indoor humidity is often kept between 30% and 50% for comfort and to avoid mold or dry skin.

Note that relative humidity depends on temperature—warm air can hold more water vapor than cold air. So a RH of 50% at 25°C represents much more actual water vapor than 50% at 10°C. For that reason, meteorologists also use dew point as a more absolute measure of moisture.

How to Measure the Inputs

You can get the air temperature and dew point from a weather station, a smartphone weather app, or a simple digital thermometer with a humidity sensor. If you are using the wet-bulb method, you need a psychrometer: two thermometers side by side, one with a wet cloth wick over its bulb. You swing the psychrometer (or fan the wet bulb) until the wet-bulb temperature stops dropping—that reading is the wet-bulb temperature. The dry-bulb temperature is just the regular air temperature.

For accurate results, make sure the wet-bulb wick is clean and saturated with distilled water, and that the air is moving across it. A common error is reading the wet-bulb before it reaches its lowest point, which gives too high a value and leads to an overestimate of humidity.

Common Mistakes and Limits

• Mixing units: The formulas require temperatures in Celsius. If you have Fahrenheit, convert first: °C = (°F − 32) × 5/9.
• Using the wrong pressure: The wet-bulb formula assumes sea-level pressure (1013 hPa). At high altitudes, the lower pressure changes the result—by about 1–2% RH per 1000 m elevation.
• Rounding too early: If you round intermediate values to 2 decimals, the final RH can be off by a few percent. Keep full precision until the end.
• Dew point above air temperature: If you enter a dew point higher than the air temperature, the formula gives RH above 100%, which is physically impossible in normal conditions—it means the air is supersaturated or you made a measurement error.

The Magnus formula is accurate to about 0.5% RH for temperatures between −40°C and +50°C, which covers most practical situations. For extremely cold or hot conditions, more complex equations exist, but they are rarely needed outside of lab work.

Alternative Ways to Find Humidity

If you do not have a dew point or wet-bulb reading, you can use a simple hygrometer—a device that measures RH directly. Many digital thermometers and weather stations include one. You can also estimate RH by comparing the temperature to the dew point: the closer they are, the higher the humidity. A rule of thumb: every 1°C difference between air temperature and dew point corresponds to roughly 5% RH, but that is only an approximation and becomes less accurate at high temperatures.

Another method is the sling psychrometer, which uses the same wet-bulb/dry-bulb principle but with a built-in chart. The calculator here removes the need for the chart.

Frequently Asked Questions

What is the difference between relative humidity and absolute humidity?

Relative humidity is a percentage comparing the actual vapor pressure to the maximum possible at that temperature. Absolute humidity is the actual mass of water vapor per volume of air (grams per cubic meter). RH changes with temperature even if the amount of water vapor stays the same; absolute humidity does not.

Why does humidity feel worse on hot days?

When the air is humid, sweat evaporates more slowly because the air already holds a lot of water vapor. Your body relies on evaporation to cool down, so high humidity makes you feel hotter and stickier. The heat index accounts for this by combining temperature and RH.

Can relative humidity be over 100%?

In normal atmospheric conditions, no—when RH reaches 100%, condensation occurs. However, in controlled lab settings, air can become supersaturated (RH above 100%) if there are no surfaces for water to condense on, but this is rare and unstable.

What is a comfortable humidity level for a home?

Most people find 30% to 50% RH comfortable. Below 30% can cause dry skin and static electricity; above 60% can promote mold growth and make the air feel stuffy. The exact comfort range varies with temperature and personal preference.

How do I convert Fahrenheit to Celsius for this calculator?

Use the formula °C = (°F − 32) × 5/9. For example, 68°F is (68 − 32) × 5/9 = 20°C. Make sure to convert both the air temperature and the dew point or wet-bulb temperature before entering them.

Why does my weather app show a different humidity than my thermometer?

Weather apps report conditions from a nearby weather station, which may be several kilometers away. Local humidity can vary significantly due to vegetation, water bodies, and urban heat. Your thermometer measures the air right where you are, so differences are normal.

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sel method | Method | Dew point=1, Wet-bulb=2
in t | Air temperature | °C | 25 | -50
in td | Dew point temperature | °C | 15 | -50
in tw | Wet-bulb temperature | °C | 18 | -50
out rh | Relative humidity | % | method == 1 ? 100 * exp((17.625 * td) / (243.04 + td) - (17.625 * t) / (243.04 + t)) : 100 * ((1013 * (tw - t) + 6.112 * exp((17.67 * tw) / (tw + 243.5)) * (tw + 243.5) - 6.112 * exp((17.67 * t) / (t + 243.5)) * (t + 243.5)) / (1013 * (tw + 243.5) - 6.112 * exp((17.67 * tw) / (tw + 243.5)) * (tw + 243.5) + 6.112 * exp((17.67 * t) / (t + 243.5)) * (t + 243.5))) | 1
out td_out | Dew point | °C | method == 1 ? td : (243.04 * (log(rh / 100) + (17.625 * t) / (243.04 + t))) / (17.625 - log(rh / 100) - (17.625 * t) / (243.04 + t)) | 1