Relative Humidity Calculator

Q: What is the mixing ratio and why is it shown?

The mixing ratio tells you how many grams of water vapor are in each kilogram of dry air. Unlike relative humidity, the mixing ratio does not change when the air is cooled at constant pressure, as long as no water condenses. 3 It is useful in weather forecasting and HVAC work because it shows the actual amount of moisture in the air, not just a percentage.

Q: How do I measure dry bulb and wet bulb temperatures at home?

You can use a tool called a sling psychrometer . It has two thermometers side by side. One is a regular thermometer (dry bulb). The other has a bulb that is kept wet (wet bulb). You whirl the device by hand so that air flows over the wet bulb and water evaporates from it, then read both thermometers. 7 The dry bulb gives the air temperature, and the wet bulb gives the cooled reading.

Q: Why is 1013 mb used as the default pressure?

The standard pressure at sea level is 1013.25 mb . 9 In inches of mercury, it is 29.92 inHg. 9 Most locations near sea level have pressures close to this value. If you are at a higher elevation or during a storm, your local pressure may differ, and you should enter the actual reading for the most accurate result.

Relative Humidity Calculator

Results

Relative Humidity

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0%30%60%70%100%

Mixing Ratio

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grams of water vapor per kilogram of dry air

Calculated Wet Bulb Temperature

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shown in both °C and °F

Step-by-Step Solution

Vapor Pressure vs. Temperature

Introduction

Relative humidity compares the amount of moisture in the air with the amount that would be there if the air were saturated.¹ It is shown as a percentage. When relative humidity is 100%, the air is saturated.² When it is low, the air feels dry.

This relative humidity calculator lets you find the relative humidity using either the dry bulb and dew point method or the dry bulb and wet bulb method. Just enter your temperatures and atmospheric pressure, and the tool does the rest. It gives you the relative humidity percentage, the mixing ratio, and a full step-by-step breakdown of the math behind the result.

You can switch between Celsius and Fahrenheit or between millibars and inches of mercury with one click. The calculator also shows a comfort rating so you can quickly see if conditions are dry, comfortable, or too humid. A live chart plots the saturation vapor pressure curve and marks where your readings fall on it.

This tool runs the humidity calculation for you and shows a clear result.

How to Use Our Relative Humidity Calculator

Enter your temperature and pressure readings below to calculate relative humidity, mixing ratio, and comfort level. The calculator also shows a step-by-step solution and a vapor pressure chart.

Choose your calculation method. Pick "Dry Bulb + Dew Point" if you know the dew point temperature. Pick "Dry Bulb + Wet Bulb" if you have a wet bulb reading instead.

Select your temperature unit. Click "Celsius (°C)" or "Fahrenheit (°F)" to match the unit your temperatures are in.

Select your pressure unit. Click "Millibars (mb)" or "Inches Hg (inHg)" to match your pressure reading.

Enter the dry bulb temperature. This is the standard air temperature shown on a regular thermometer. It must be between −60 and 60 °C.

Enter the dew point or wet bulb temperature. This value must be equal to or lower than the dry bulb temperature. Which field you see depends on the method you chose.

Enter the atmospheric pressure. Use your local barometric pressure. If you do not know it, leave it at 1013 mb. The standard pressure at sea level is 1013.25 mb.⁹

Set the result decimal places. Choose a number from 0 to 6 to control how many decimal places appear in your results.

Click "Calculate" to see your relative humidity percentage, mixing ratio, and the derived temperature. A color-coded gauge shows whether conditions are dry, comfortable, or humid. Scroll down to view the full step-by-step math and the vapor pressure chart.

What Is Relative Humidity?

Relative humidity tells you how much water vapor is in the air compared to how much the air could hold at that temperature.¹ It is shown as a percentage. If the relative humidity is 100%, the air is fully saturated and cannot hold any more moisture. If it is 50%, the air is holding half of what it could hold.

Why Does Relative Humidity Matter?

Humidity affects how comfortable you feel. When humidity is too high (above 60%), sweat does not evaporate well from your skin, so you feel hot and sticky. This is closely related to the heat index, which measures how hot it actually feels when humidity is factored in. When humidity is too low (below 30%), your skin, eyes, and throat can feel dry and irritated. For homes, the U.S. Environmental Protection Agency advises keeping indoor humidity below 60 percent, ideally between 30 and 50 percent.⁴

Relative humidity also matters for weather forecasting, farming, storing food, protecting buildings, and running heating and cooling systems. Mold grows faster in high humidity. Wood and paint can crack in low humidity.

How Is Relative Humidity Calculated?

This calculator uses the Magnus formula to find the saturation vapor pressure at a given temperature. Saturation vapor pressure is the vapor pressure at which water vapor is in equilibrium with a flat surface of liquid water at the same temperature.⁵ The formula looks like this:

eₛ = 6.112 × e^(17.67 × T / (T + 243.5)).¹¹

Once you know the saturation vapor pressure at the dry bulb temperature and the actual vapor pressure in the air, relative humidity is the actual vapor pressure divided by the saturation vapor pressure, times 100²:

RH = (actual vapor pressure / saturation vapor pressure) × 100

Two Methods to Find Relative Humidity

This calculator gives you two ways to get your answer:

Dry Bulb + Dew Point: The dew point is the temperature to which air must be cooled to reach saturation. If you know the air temperature and the dew point, the calculator finds the vapor pressure at each temperature and divides them to get relative humidity.⁶
Dry Bulb + Wet Bulb: A wet bulb thermometer has a bulb that is kept wet. Evaporation cools it, so it reads lower than a regular (dry bulb) thermometer, and the difference between the two readings is a measure of how dry the air is.⁷ The calculator uses the psychrometric equation to turn this gap into a vapor pressure value, then finds relative humidity from that. It uses a psychrometer coefficient of 0.000662 per °C, the value for ventilated (Assmann type) psychrometers.¹⁰

Key Terms

Dry Bulb Temperature: The regular air temperature measured by a standard thermometer.
Dew Point Temperature: The temperature to which air must be cooled to reach saturation.⁶
Wet Bulb Temperature: The lowest temperature that can be reached by evaporating water into the air.⁸ It is always equal to or lower than the dry bulb temperature.
Mixing Ratio: The weight of water vapor in the air compared to the weight of dry air, measured in grams per kilogram (g/kg).
Atmospheric Pressure: The force the air pushes down with. The standard pressure at sea level is 1013.25 millibars.⁹ In inches of mercury, standard sea-level pressure is 29.92.⁹

Formulas used

Saturation Vapor Pressure (Magnus Formula) ¹¹

e_s(T) = 6.112 \cdot e^{\frac{17.67 \, T}{T + 243.5}}

Relative Humidity ²

RH = \frac{e}{e_s(T_{db})} \times 100

Psychrometric Equation (Actual Vapor Pressure from Wet Bulb) ¹⁰

e = e_s(T_{wb}) - A \cdot P \cdot (T_{db} - T_{wb})

Dew Point from Vapor Pressure (Inverse Magnus) ¹¹

T_{dp} = \frac{243.5 \, \ln\!\left(\frac{e}{6.112}\right)}{17.67 - \ln\!\left(\frac{e}{6.112}\right)}

Mixing Ratio

w = 621.97 \cdot \frac{e}{P - e}

Frequently asked questions

What is a good relative humidity level for indoors?

The U.S. Environmental Protection Agency advises keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, as part of controlling mold.⁴ Below 30%, the air feels too dry and can irritate your skin, eyes, and throat. Above 60%, the air feels sticky.

What is the difference between the dry bulb and dew point method and the dry bulb and wet bulb method?

Both methods give you the same result, but they use different inputs. The dry bulb + dew point method needs the air temperature and the dew point, which weather stations often report. The dry bulb + wet bulb method needs the air temperature and a wet bulb reading, which you get from a sling psychrometer.⁷ Use whichever method matches the data you have.

Why does my dew point have to be lower than or equal to the dry bulb temperature?

The dew point is the temperature to which air must be cooled to reach saturation.⁶ Air cannot have a dew point higher than its current temperature. If the dew point equals the dry bulb temperature, the air is fully saturated at 100% relative humidity.³ A dew point above the dry bulb is physically impossible.

What is the mixing ratio and why is it shown?

The mixing ratio tells you how many grams of water vapor are in each kilogram of dry air. Unlike relative humidity, the mixing ratio does not change when the air is cooled at constant pressure, as long as no water condenses.³ It is useful in weather forecasting and HVAC work because it shows the actual amount of moisture in the air, not just a percentage.

Does atmospheric pressure affect relative humidity?

Yes, but only slightly. Atmospheric pressure matters most when you use the wet bulb method because the psychrometric equation uses pressure directly. For the dew point method, pressure affects the mixing ratio but not the relative humidity percentage itself. If you do not know your local pressure, the default value of 1013 mb works fine for most situations.

Can relative humidity be over 100%?

In normal conditions, no. The calculator caps the result at 100%. In rare real-world cases, air can become slightly supersaturated (just above 100%) for a short time before water droplets form, but standard instruments and formulas treat 100% as the maximum.

How do I measure dry bulb and wet bulb temperatures at home?

You can use a tool called a sling psychrometer. It has two thermometers side by side. One is a regular thermometer (dry bulb). The other has a bulb that is kept wet (wet bulb). You whirl the device by hand so that air flows over the wet bulb and water evaporates from it, then read both thermometers.⁷ The dry bulb gives the air temperature, and the wet bulb gives the cooled reading.

Why is 1013 mb used as the default pressure?

The standard pressure at sea level is 1013.25 mb.⁹ In inches of mercury, it is 29.92 inHg.⁹ Most locations near sea level have pressures close to this value. If you are at a higher elevation or during a storm, your local pressure may differ, and you should enter the actual reading for the most accurate result.

Sources

Glossary: Relative Humidity. NOAA National Weather Service. Accessed September 29, 2026.

Stull R. Practical Meteorology: An Algebra-based Survey of Atmospheric Science. Chapter 4: Water Vapor. University of British Columbia. 2017;pp. 90-92, Eqs. 4.4, 4.14a, 4.14d. Accessed September 29, 2026.

Lawrence MG. The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications. Bulletin of the American Meteorological Society. 2005;86(2):225-233. doi:10.1175/BAMS-86-2-225. Accessed September 29, 2026.

A Brief Guide to Mold, Moisture and Your Home. U.S. Environmental Protection Agency. Mold Cleanup in Your Home; Moisture and Mold Prevention and Control Tips. Accessed September 29, 2026.

Glossary: Vapor Pressure. NOAA National Weather Service. Accessed September 29, 2026.

Glossary: Dew Point. NOAA National Weather Service. Accessed September 29, 2026.

Glossary: Psychrometer. NOAA National Weather Service. Psychrometer; Sling Psychrometer. Accessed September 29, 2026.

Glossary: Wet-Bulb Temperature. NOAA National Weather Service. Accessed September 29, 2026.

Air pressure. NOAA National Weather Service. JetStream. Accessed September 29, 2026.

Allen RG, Pereira LS, Raes D, Smith M. Crop evapotranspiration - Guidelines for computing crop water requirements (FAO Irrigation and Drainage Paper 56), Chapter 3: Meteorological data. Food and Agriculture Organization of the United Nations. 1998;Eqs. 15, 16. Accessed September 29, 2026.

Relative Humidity and Dewpoint Temperature from Temperature and Wet Bulb Temperature. NOAA National Weather Service, El Paso. saturated vapor pressure and dewpoint equations. Accessed October 7, 2026.