Weather calculators

Absolute Humidity Calculator

Updated Aug 9, 2026 By Jehan Wadia
Rate Formulas
Inputs
Switching units converts the value automatically. Must be above absolute zero.
%
Value between 0 and 100.
Standard sea level = 101.325 kPa. Use the altitude table to auto-fill.
Provide either Relative Humidity or Actual Vapor Pressure — not both. If both are filled, Actual Vapor Pressure takes precedence.
Controls the number of decimals in every result below.
Altitude → Pressure
Select a row to auto-fill the pressure field (converted to your selected unit).
Altitude (m) Altitude (ft) Pressure (kPa)

Absolute Humidity Results

Absolute Humidity — g/m³
Absolute Humidity — kg/m³
Humidity Ratio — g/kg dry air
Humidity Ratio — gr/lb dry air
Supporting Values
Saturation Vapor Pressure (Psat) —
Actual Vapor Pressure (Pa) —
Absolute Humidity vs. Air Temperature
Step-by-Step Solution

Introduction

Absolute humidity tells you how much water vapor is in the air. It is the mass of water in each cubic meter of air, usually shown as grams per cubic meter (g/m³). Unlike relative humidity, it does not change when the air gets warmer or cooler — it only changes when water is added or removed.

This Absolute Humidity Calculator does the math for you. Type in the air temperature, the relative humidity, and the air pressure. The tool then gives you the absolute humidity in g/m³ and kg/m³, plus the humidity ratio in g/kg and gr/lb of dry air. If you already know the actual vapor pressure, you can enter that instead of relative humidity.

There is also a second tab that works backward. Give it the temperature and the absolute humidity, and it will find the relative humidity as a percent. For a dedicated tool, see our Relative Humidity Calculator or the general-purpose Humidity Calculator.

The calculator uses the Wagner and Pruß equation to find saturation vapor pressure, so the results are accurate. You can pick your own units, set how many decimal places you want, and choose your altitude from a table to fill in air pressure fast. A chart shows how humidity changes with temperature, and the step-by-step section shows every part of the math.

How to use our Absolute Humidity Calculator

Type in the air temperature, the humidity, and the air pressure. The calculator gives you the absolute humidity in g/m³ and kg/m³, the humidity ratio in g/kg and gr/lb, the saturation vapor pressure, and the actual vapor pressure. Use the second tab to go the other way and find relative humidity.

Air Temperature: Enter how warm the air is. Pick °C, °F, or K from the drop-down. If you change the unit, the number changes with it. Need a quick conversion first? Try the Celsius to Fahrenheit Calculator or the Fahrenheit to Celsius Calculator.

Relative Humidity: Enter the relative humidity as a percent, from 0 to 100. This is the most common way humidity is reported in weather apps.

Atmospheric Pressure: Enter the air pressure and pick kPa, hPa, Pa, mmHg, or inHg. At sea level it is 101.325 kPa. You can also click a row in the altitude table to fill this in for you. Pilots can cross-check with the Pressure Altitude Calculator and the Density Altitude Calculator.

Actual Vapor Pressure (optional): If you know the vapor pressure, enter it here instead of the relative humidity. If you fill in both, this one is used.

Decimal Places: Slide it to choose how many decimals show in the answers, from 0 to 6. For rounding rules in lab work, see the Sig Fig Calculator.

Vapor pressure output unit: Choose Pa, kPa, hPa, mmHg, or psi for the two vapor pressure results.

Chart humidity unit: Pick the unit shown on the graph of absolute humidity versus air temperature.

Absolute Humidity (second tab): To find relative humidity, enter the water vapor amount in g/m³, kg/m³, g/kg, or gr/lb, along with the temperature and pressure.

Click Calculate to see your results, open Show Calculation Steps to see the full math, or click Reset to Defaults to start over.

What Is Absolute Humidity?

Absolute humidity is the amount of water vapor in the air. It tells you the mass of water floating in a set volume of air. It is usually written in grams per cubic meter (g/m³). If air holds 12 g/m³, then every cubic meter of that air carries 12 grams of water vapor.

Absolute Humidity vs. Relative Humidity

Relative humidity is a percent. It compares how much water the air holds to the most it could hold at that temperature. Absolute humidity is not a percent. It is a real weight of water, and it does not change when the air gets warmer or cooler (as long as no water is added or removed).

That is why 50% relative humidity feels very different in winter than in summer. Warm air can hold much more water than cold air. So 50% on a hot day means a lot more water vapor than 50% on a cold day. The temperature where that air would reach 100% is given by the Dew Point Calculator, and how hot it actually feels is shown by the Heat Index Calculator and the Humidex Calculator.

How It Is Calculated

The math has three main steps:

  1. Saturation vapor pressure (Psat): the most water pressure the air can hold at that temperature. This tool uses the Wagner & Pruß (2002) equation, which is very accurate.
  2. Actual vapor pressure (Pa): multiply Psat by the relative humidity. So Pa = (RH ÷ 100) × Psat.
  3. Absolute humidity: use the ideal gas law, AH = Pa ÷ (Rw × T), where Rw = 461.5 J/(kg·K) and T is the temperature in kelvin. The same relationship powers our Ideal Gas Law Calculator and Density Calculator.

Humidity Ratio

The humidity ratio (also called mixing ratio) is a close cousin. It measures water vapor per kilogram of dry air, not per cubic meter. It is shown in g/kg or grains per pound (gr/lb). HVAC engineers use this a lot, because it stays the same when air is heated or cooled in a duct. The formula is w = 0.621945 × Pa ÷ (P − Pa), where P is the total air pressure. If you are sizing equipment, pair it with the AC Tonnage Calculator, the CFM Calculator, and the BTU Calculator.

Why Air Pressure Matters

Pressure drops as you go higher up a mountain. Lower pressure changes the humidity ratio in g/kg and gr/lb, so entering the right pressure gives better numbers. Standard sea level pressure is 101.325 kPa. At 2,000 meters it falls to about 79.5 kPa. See the Air Density Calculator to learn how pressure and moisture together set the weight of a cubic meter of air.

Where Absolute Humidity Is Used

  • HVAC and drying: sizing dehumidifiers, air dryers, and cooling coils — see also the Heat Loss Calculator and Duct Size Calculator.
  • Greenhouses and farming: plants lose water based on how much vapor the air already holds; plan beds with the Plant Spacing Calculator.
  • Museums and storage: paper, wood, and art need steady moisture levels.
  • Health and comfort: very dry indoor air can irritate skin, eyes, and lungs; heat stress outdoors is tracked with the Wet Bulb Calculator and WBGT Calculator.
  • Weather: humid air is lighter than dry air and helps fuel storms; cold-season risk is covered by the Wind Chill Calculator.

Quick Facts

  • Air at 25 °C and 60% relative humidity holds about 13.8 g/m³ of water vapor.
  • Saturated air at 30 °C holds roughly 30 g/m³; at 0 °C it holds only about 4.8 g/m³.
  • When relative humidity hits 100%, the air is saturated and dew, fog, or rain can form.
  • 1 g/kg equals 7 grains per pound of dry air. Convert other mass units with the Grams to Ounces Calculator or the Kg to Lbs Calculator.

Formulas used

Reduced temperature variable (Wagner & Pruss)
\tau = 1 - \frac{T}{T_c}, \quad T_c = 647.096\;\mathrm{K}
Saturation vapor pressure (Wagner & Pruss, 2002)
P_{sat} = P_c \cdot \exp\!\left[\frac{T_c}{T}\left(a_1\tau + a_2\tau^{1.5} + a_3\tau^{3} + a_4\tau^{3.5} + a_5\tau^{4} + a_6\tau^{7.5}\right)\right], \quad P_c = 22.064 \times 10^{6}\;\mathrm{Pa}
Actual vapor pressure from relative humidity
P_a = \frac{RH}{100} \times P_{sat}
Absolute humidity (ideal gas law)
AH = \rho_v = \frac{P_a}{R_w\,T}, \quad R_w = 461.5\;\mathrm{J/(kg\cdot K)}
Humidity ratio (mass of vapor per mass of dry air)
w = \frac{0.621945\,P_a}{P - P_a}
Vapor pressure from humidity ratio (inverse)
P_a = \frac{P\,w}{0.621945 + w}
Relative humidity from absolute humidity
RH = \frac{P_a}{P_{sat}} \times 100 = \frac{\rho_v R_w T}{P_{sat}} \times 100
Temperature unit conversion to kelvin
T_K = T_{^\circ C} + 273.15 = (T_{^\circ F} - 32)\times\tfrac{5}{9} + 273.15

Frequently asked questions

Does absolute humidity change when I heat or cool the air?

No. Absolute humidity in g/m³ stays close to the same unless water is added or taken out. Heating air only lowers the relative humidity, because warm air can hold more water.

One small note: if air is heated in an open room it expands, so g/m³ can drop a little. The humidity ratio in g/kg does not change at all.

Why does the calculator ignore relative humidity when I type a vapor pressure?

Both fields describe the same thing, so only one can be used. The tool picks actual vapor pressure because it is a direct measurement.

Clear the vapor pressure box if you want the calculator to use your relative humidity value again.

Why do I need air pressure if absolute humidity is per cubic meter?

You do not need it for the g/m³ and kg/m³ answers. Those only use temperature and vapor pressure.

Pressure is needed for the humidity ratio in g/kg and gr/lb, because that compares water to dry air. Lower pressure means less dry air in the same space, so the ratio goes up.

What is the highest absolute humidity air can reach?

It depends on temperature. At saturation (100% RH), air holds about:

  • 0 °C — about 4.8 g/m³
  • 10 °C — about 9.4 g/m³
  • 20 °C — about 17.3 g/m³
  • 30 °C — about 30.4 g/m³
  • 40 °C — about 51.1 g/m³

Set the relative humidity to 100 to see the limit at any temperature.

Can absolute humidity be zero?

Yes. Enter 0% relative humidity and you get 0 g/m³. That means perfectly dry air with no water vapor at all.

Real outdoor air is almost never fully dry, but very cold or desert air can get close.

Does this calculator work below freezing?

Yes. You can enter temperatures well below 0 °C and get valid answers.

The formula uses saturation over liquid water. Over ice or frost the true saturation value is a little lower, so very cold results may read slightly high in relative humidity terms.

Why does my answer differ a little from another humidity calculator?

Different tools use different saturation formulas. Many use simple ones like Magnus or Tetens.

This tool uses the Wagner and Pruß equation, which is the accepted standard for water. Gaps are usually less than 0.5%, so both answers are fine for everyday use.

What are grains per pound (gr/lb)?

It is a US unit used in HVAC work. One pound has 7,000 grains, so 1 g/kg = 7 gr/lb.

It measures water vapor per pound of dry air. The calculator shows it next to the g/kg value.

Why does the chart show two lines?

The blue line shows absolute humidity at your relative humidity as temperature changes. The green line shows the saturation limit at 100% RH.

The orange dot marks your exact air. The gap between the lines shows how much more water the air could still hold.

What does the warning about supersaturated air mean?

It means the vapor pressure you entered is higher than the air can hold at that temperature. That is above 100% relative humidity.

In real life the extra water would turn into fog, dew, or rain. Check your input values if you see this message.

Can I use the dew point instead of relative humidity?

Not directly, but there is an easy trick. Saturation vapor pressure at the dew point is the actual vapor pressure of the air.

So enter the dew point as the temperature with 100% RH, read the saturation vapor pressure, then put that number in the vapor pressure box with your real air temperature.

What is a good indoor absolute humidity?

Most homes feel best between about 7 and 12 g/m³. That matches roughly 40–60% relative humidity at normal room temperature.

Below 5 g/m³ air feels dry and can bother skin and throats. Above 14 g/m³ rooms feel sticky and mold risk goes up.

How do I use this to size a dehumidifier?

Find the absolute humidity of your current air, then find it again at your target temperature and humidity. Subtract the two to get grams of water per cubic meter you must remove.

Multiply that by your room volume in cubic meters, then by how many air changes happen per day. That gives grams of water per day.

Why does the calculator convert temperature to kelvin?

The ideal gas law only works with an absolute temperature scale. Kelvin starts at true zero, so it never goes negative.

You can still type °C or °F. The tool converts for you and shows the step.

What happens when I switch units in a drop-down?

The number in the box is converted for you. If you type 25 °C and switch to °F, it becomes 77.

This works for temperature, pressure, vapor pressure, and absolute humidity, so your air stays the same.

Where do the pressure values in the altitude table come from?

They come from the standard atmosphere model, which gives average pressure at each height.

Click any row to fill the pressure box in your chosen unit. If you have a real barometer reading, use that instead for better accuracy.

Can I get the humidity ratio without knowing exact pressure?

Yes, but the answer is only as good as your pressure guess. Sea level 101.325 kPa works for most low places.

At high altitude the error grows fast. At 2,000 m, using sea level pressure makes the g/kg answer about 25% too low.

Why is my humidity ratio blank or showing a dash?

That happens when the vapor pressure is equal to or larger than the total air pressure. The formula divides by (P − Pa), so it breaks.

Check that your pressure unit is right. A common mistake is entering 101.325 while the unit is set to Pa instead of kPa.

What is saturation vapor pressure in simple words?

It is the push water vapor makes when air is completely full of moisture at that temperature.

It rises fast as air gets warmer. That is why hot air can hold so much more water than cold air.

Does the second tab need the same pressure as the first?

Only if you use g/kg or gr/lb. Those units need pressure to work out vapor pressure.

If you enter g/m³ or kg/m³, the pressure value does not change the relative humidity answer.

Is humid air heavier than dry air?

No, it is lighter. A water molecule weighs less than the nitrogen and oxygen molecules it replaces.

So air with more water vapor is less dense. This is one reason humid air helps storms grow.

Related Weather calculators

All Weather calculators
Weather

Dew Point Calculator

Calculate dew point temperature instantly using our free dew point calculator. Enter humidity and temperature to determine moisture levels and comfort.

Open Weather

Humidity Calculator

Compute relative humidity from dew point or wet-bulb/dry-bulb temps. Solves for any missing value. Shows deficit, comfort level, and worked math. °F/°C.

Open Weather

Relative Humidity Calculator

Calculate relative humidity from dry bulb plus dew point or wet bulb. Outputs mixing ratio, comfort rating, vapor pressure chart, and step-by-step math.

Open Weather

Wet Bulb Calculator

Compute wet-bulb and dewpoint in °C, °F, K from temperature and humidity. Shows heat stress bands, step-by-step solution, and an interactive RH chart.

Open