Weather calculators

Humidity Calculator

Updated Sep 1, 2026 By Infinity Calculator
Rate Formulas Sources
Calculation Method
Dew Point Method
Temperature Unit

Fill in exactly two fields and leave one blank — the calculator finds the missing value.

°F
°F
%
Relative Humidity
—
0%20%40%60%80%100%
Step-by-Step Solution
Vapor Pressure vs. Temperature

Introduction

Humidity is the amount of water vapor in the air. When humidity is high, the air feels sticky and warm. When it is low, the air feels dry. Knowing the humidity level helps you stay comfortable, protect your home from mold, and plan for weather changes.

This humidity calculator lets you find relative humidity, dew point, or air temperature using two simple methods. The dew point method works when you know any two of the three values: air temperature, dew point, and relative humidity. Just fill in two fields, leave one blank, and the calculator solves for the missing value. The wet-bulb/dry-bulb method uses readings from a psychrometer to calculate relative humidity and dew point for you.

Along with your results, the tool shows a comfort rating, a step-by-step solution with full math, and a vapor pressure chart. All calculations use the Magnus formula, a trusted equation used by meteorologists around the world. You can switch between °F and °C at any time, and your values will convert automatically.

How to Use Our Humidity Calculator

Enter your known temperature and humidity values below, and this calculator will find the missing value along with relative humidity, dew point, humidity deficit, comfort level, and a step-by-step solution.

Choose a Calculation Method: Pick either the Dew Point Method or the Wet-Bulb / Dry-Bulb Method based on the data you have.

Dew Point Method

Temperature Unit: Select °F or °C to set which unit you want to use for your temperatures.

Air Temperature (T): Enter the current air temperature. Leave this field blank if it is the value you want to solve for.

Dew Point Temperature (Td): Enter the dew point temperature. Leave this field blank if it is the value you want to solve for.

Relative Humidity (RH): Enter the relative humidity as a percentage from 0 to 100. Leave this field blank if it is the value you want to solve for.

Fill in exactly two of the three fields above and leave one blank. The calculator will find the missing value for you.

Wet-Bulb / Dry-Bulb Method

Temperature Unit: Select °F or °C to set which unit you want to use.

Dry-Bulb Temperature: Enter the standard air temperature reading from a regular thermometer.

Wet-Bulb Temperature: Enter the temperature reading from a thermometer wrapped in a wet cloth. This value must be equal to or lower than the dry-bulb temperature.

Press Calculate to see your results, or press Reset to clear all fields and start over.

What Is Humidity?

Humidity is the amount of water vapor in the air. You can't see it, but you can feel it. On a sticky summer day, the air holds a lot of moisture. On a dry winter day, it holds very little. This calculator helps you measure and understand that moisture.

Relative Humidity

Relative humidity (RH) tells you how full the air is with water vapor compared to how much it could hold at the same temperature. It is shown as a percentage. At 100% RH, the air is completely full and can't hold any more moisture. This is when fog, dew, or rain can form. At 50% RH, the air is only half full.

Warm air can hold more water vapor than cold air. This is why the same amount of moisture in the air gives a lower RH on a hot day and a higher RH on a cool night.

What Is Dew Point?

The dew point is the temperature at which the air becomes fully saturated and water starts to condense. Think of the water droplets on a cold glass on a warm day. The glass cooled the air around it below its dew point. A higher dew point means more moisture in the air. A dew point above 65°F (18°C) feels muggy to most people.

Wet-Bulb and Dry-Bulb Temperatures

A dry-bulb temperature is just the regular air temperature you read on a thermometer. A wet-bulb temperature comes from a thermometer wrapped in a damp cloth. As water evaporates from the cloth, it cools the thermometer down. Drier air causes more evaporation and a bigger temperature drop. By comparing the two readings, you can figure out the relative humidity.

Comfort Levels

Humidity affects how comfortable you feel. Air below 30% RH feels dry and can irritate your skin, eyes, and throat. Air between 30% and 50% RH feels the most comfortable for most people. Above 60% RH, the air starts to feel sticky and heavy. Very high humidity also encourages mold growth indoors.

Humidity Deficit

Humidity deficit is the difference between how much moisture the air could hold and how much it actually holds right now. It is measured in grams per cubic meter (g/m³). A large deficit means the air is dry and can absorb more water. Farmers and greenhouse growers use this number to decide when to water plants, because a high deficit means water evaporates faster.

Formulas used

Saturation Vapor Pressure (Magnus Formula)
e_s = 6.112 \times e^{\frac{17.67 \times T}{T + 243.5}}
Relative Humidity from Air Temperature and Dew Point
RH = 100 \times e^{\left(\frac{17.625 \times T_d}{243.04 + T_d} - \frac{17.625 \times T}{243.04 + T}\right)}
Dew Point from Air Temperature and Relative Humidity (Inverse Magnus)
T_d = \frac{243.04 \times \gamma}{17.625 - \gamma}, \quad \gamma = \ln\!\left(\frac{RH}{100}\right) + \frac{17.625 \times T}{243.04 + T}
Actual Vapor Pressure (Psychrometric Equation)
e_{act} = e_{wet} - 0.00066\,(1 + 0.00115 \times T_{wet})(T_{dry} - T_{wet}) \times 1013.25
Relative Humidity from Vapor Pressures
RH = 100 \times \frac{e_{act}}{e_{dry}}
Absolute Humidity at Saturation
AH_{sat} = \frac{e_s \times 216.7}{T + 273.15}

Frequently asked questions

What is a psychrometer?

A psychrometer is a tool with two thermometers side by side. One has a dry bulb that reads the regular air temperature. The other has a wet bulb wrapped in a damp cloth. The difference between the two readings tells you how much moisture is in the air. This calculator's wet-bulb/dry-bulb method uses those two readings to find relative humidity.

Why can't the wet-bulb temperature be higher than the dry-bulb temperature?

The wet bulb cools down because water evaporates from the cloth around it. Evaporation always takes heat away, so the wet-bulb reading must be the same as or lower than the dry-bulb reading. If it were higher, it would mean evaporation is adding heat, which does not happen in normal conditions.

Can the dew point be higher than the air temperature?

No. The dew point can equal the air temperature at most, which means the air is fully saturated at 100% relative humidity. It cannot go above the air temperature. If you enter a dew point higher than the air temperature, the calculator will show an error.

What is a good indoor relative humidity level?

The EPA's guidance is to keep indoor humidity below 60%, ideally between 30% and 50%.1 Below 30%, the air feels dry and can cause dry skin, nosebleeds, and static electricity. Above 50%, moisture can build up on windows and walls, which encourages mold and dust mites.

How do I measure the air temperature and dew point at home?

You can use a digital hygrometer or a home weather station. Many of these devices show air temperature, relative humidity, and dew point all at once. If you have any two of those values, you can enter them into this calculator to find the third or to see the full step-by-step solution.

Why does the same humidity feel worse on a hot day?

Your body cools itself by sweating. When humidity is high, sweat does not evaporate as fast, so you feel hotter. On a cooler day with the same humidity percentage, your body does not need to cool down as much, so the moisture in the air bothers you less.

What is absolute humidity?

Absolute humidity is the actual mass of water vapor in a given volume of air, measured in grams per cubic meter (g/m³). Unlike relative humidity, it does not change with temperature. This calculator uses absolute humidity to compute the humidity deficit shown in the results.

How is humidity deficit useful for gardening?

Humidity deficit tells you how much more moisture the air can absorb. A high deficit means water evaporates quickly from soil and plant leaves, so plants may need more watering. A low deficit means the air is nearly full of moisture, and plants lose less water. Greenhouse growers watch this number closely to keep plants healthy.

Does altitude or air pressure affect the results?

The dew point method uses only temperature and is not affected by altitude. The wet-bulb/dry-bulb method in this calculator uses standard sea-level pressure (1013.25 hPa) for the psychrometric correction. If you are at a very high altitude, the wet-bulb result may be slightly less accurate, but for most locations the difference is small.

Sources

  1. A Brief Guide to Mold, Moisture and Your Home. U.S. Environmental Protection Agency. Accessed September 1, 2026.

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