Relative Humidity Calculator

Q: What is a good relative humidity level for indoors?

A relative humidity between 30% and 60% is best for most homes. Below 30%, the air feels too dry and can irritate your skin, eyes, and throat. Above 60%, the air feels sticky and mold can start to grow. Aim for around 40% to 50% for the best comfort.

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 temperature changes. 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: What formula does this calculator use?

It uses the Magnus formula to find saturation vapor pressure: eₛ = 6.112 × e^(17.67 × T / (T + 243.5)) . For the wet bulb method, it also uses the psychrometric equation with a coefficient of 0.000662 per °C. These are widely accepted formulas used in meteorology.

Q: How accurate is this relative humidity calculator?

The Magnus formula is accurate to within about ±0.1% for temperatures between −40 °C and 50 °C. This is close enough for weather, HVAC, and most scientific uses. For extreme temperatures outside the −60 to 60 °C range, the calculator will ask you to enter a valid value.

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 wet cloth wrapped around its tip (wet bulb). You spin the device in the air for about a minute, then read both thermometers. The dry bulb gives the air temperature, and the wet bulb gives the cooled reading.

Relative Humidity Calculator

Updated Jun 30, 2026 By Jehan Wadia

Results

Relative Humidity

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0%25%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 tells you how much moisture is in the air compared to how much the air can hold at that temperature. It is shown as a percentage. When relative humidity is 100%, the air is fully saturated and cannot hold more water vapor. When it is low, the air feels dry.

This relative humidity calculator lets you find the exact 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.

Whether you are a student learning about weather, a technician checking HVAC systems, or just curious about the humidity outside, this tool makes the calculation fast and clear.

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, which is standard sea-level pressure.

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. 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. A relative humidity between 30% and 60% is considered the most comfortable range for most people.

Relative humidity also matters for weather forecasting, farming, storing food, protecting buildings, and running heating and cooling systems. Understanding humidity is essential when sizing HVAC equipment using tools like a BTU calculator or an AC tonnage calculator. 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 maximum pressure that water vapor can reach before it turns into liquid. The formula looks like this:

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

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

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 at which moisture in the air starts to condense into water droplets. 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.
Dry Bulb + Wet Bulb: A wet bulb thermometer has a damp cloth wrapped around its tip. Evaporation cools it down, so it reads lower than a regular (dry bulb) thermometer. The bigger the gap between the two readings, the drier the air. The calculator uses the psychrometric equation to turn this gap into a vapor pressure value, then finds relative humidity from that.

Key Terms

Dry Bulb Temperature: The regular air temperature measured by a standard thermometer.
Dew Point Temperature: The temperature at which the air becomes fully saturated and dew or fog begins to form.
Wet Bulb Temperature: The lowest temperature a damp thermometer can reach through evaporation alone. 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. Standard sea-level pressure is 1013 millibars (29.92 inches of mercury). Pressure also plays a key role in calculations like those performed by the ideal gas law calculator.

Formulas used

Saturation Vapor Pressure (Magnus Formula)

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

Relative Humidity

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

Psychrometric Equation (Actual Vapor Pressure from Wet Bulb)

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

Dew Point from Vapor Pressure (Inverse Magnus)

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?

A relative humidity between 30% and 60% is best for most homes. Below 30%, the air feels too dry and can irritate your skin, eyes, and throat. Above 60%, the air feels sticky and mold can start to grow. Aim for around 40% to 50% for the best comfort.

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. 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 where moisture starts to condense out of the air. 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. 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 temperature changes. 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.

What formula does this calculator use?

It uses the Magnus formula to find saturation vapor pressure: eₛ = 6.112 × e^(17.67 × T / (T + 243.5)). For the wet bulb method, it also uses the psychrometric equation with a coefficient of 0.000662 per °C. These are widely accepted formulas used in meteorology.

How accurate is this relative humidity calculator?

The Magnus formula is accurate to within about ±0.1% for temperatures between −40 °C and 50 °C. This is close enough for weather, HVAC, and most scientific uses. For extreme temperatures outside the −60 to 60 °C range, the calculator will ask you to enter a valid value.

What is the comfort gauge at the bottom of the result?

The color-coded gauge gives you a quick comfort rating based on the humidity level:

Below 25% — Very Dry (orange)
25% to 29% — Dry (yellow)
30% to 60% — Comfortable (green)
61% to 70% — Humid (teal)
Above 70% — Very Humid / Uncomfortable (red)

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 wet cloth wrapped around its tip (wet bulb). You spin the device in the air for about a minute, then read both thermometers. The dry bulb gives the air temperature, and the wet bulb gives the cooled reading.

Why does the calculator show a wet bulb temperature when I enter a dew point?

The calculator works both ways. When you enter a dew point, it solves for the wet bulb temperature so you have the full picture. When you enter a wet bulb, it solves for the dew point. This way, you always get all three key temperatures no matter which method you use.

What does the vapor pressure chart show?

The chart plots the saturation vapor pressure curve, which shows the maximum vapor pressure the air can hold at each temperature. Two dots are marked on the chart: one at the dry bulb temperature (saturation point) and one at the dew point (actual vapor pressure). The gap between them shows how far the air is from being fully saturated.

Can I use this calculator for weather forecasting?

Yes. Relative humidity helps predict fog, dew, frost, and cloud formation. When humidity nears 100%, condensation is likely. Meteorologists use humidity data along with temperature and pressure to forecast precipitation and visibility. This calculator gives the same results professionals use.

What happens if I set decimal places to 0?

Setting decimal places to 0 shows the raw output without rounding to a fixed number of decimals. This gives you the full precision of the calculation. If you want a cleaner result, set it to 1 or 2 decimal places.

Why is 1013 mb used as the default pressure?

1013.25 mb (or 29.92 inHg) is the internationally accepted standard sea-level atmospheric pressure. 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.