Air Density Calculator
Find the density and specific volume of dry air from temperature and pressure in metric or imperial units.
What it does
Air density is the mass of air in a given volume. It matters for aircraft lift, engine airflow, wind turbine output, air conditioning loads and any calculation where the weight of the air itself is part of the problem. At ordinary temperatures and pressures, dry air behaves closely enough to an ideal gas that one short equation covers most engineering and everyday questions.
The equation is the ideal gas law rearranged for density: pressure divided by the product of the gas constant and absolute temperature. Pressure must be in pascals, temperature in kelvin, and the gas constant for dry air is 287.05 joules per kilogram per kelvin. The result is in kilograms per cubic meter, and its reciprocal is the specific volume in cubic meters per kilogram.
Temperature has a large effect because it sits in the denominator as an absolute value. Air at 30 C is about 3.5 percent less dense than air at 20 C at the same pressure, which is enough to change an aircraft's takeoff distance or a fan's mass flow. Pressure has a direct effect: a 10 percent drop in pressure gives a 10 percent drop in density at a fixed temperature.
The calculator uses the same physics in both unit systems. In metric, enter degrees Celsius and hectopascals; in imperial, enter degrees Fahrenheit and inches of mercury. The conversion happens inside the tool, so the underlying equation never changes. The output is shown in kg/m³ and m³/kg for metric, or lb/ft³ and ft³/lb for imperial.
The model is for dry air. Water vapor is lighter than dry air, so humid air at the same temperature and pressure is slightly less dense. The difference is usually under one percent at ordinary humidity, but it grows in hot, humid conditions and matters in precision work. Altitude is not an input either; enter the actual absolute pressure at your location if you want the density at altitude.
How to use
- Choose metric or imperial units.
- Enter the air temperature in C or F.
- Enter the absolute pressure in hPa or inHg. Do not use a gauge pressure or a pressure altitude setting.
- Read the air density and the specific volume.
- If you need the other unit system, switch the selector and enter the converted values.
- For humid air or precision work, treat the result as a dry air baseline and apply a humidity correction separately.
Formula
- Pressure
- Absolute pressure in pascals. 1 hPa equals 100 Pa and 1 inHg equals 3,386.389 Pa.
- Temperature
- Absolute temperature in kelvin. Add 273.15 to the Celsius value.
- 287.05
- The specific gas constant for dry air in J/(kg K).
- Density
- Mass per unit volume, in kg/m³ or lb/ft³.
The gas constant 287.05 J/(kg K) comes from the universal gas constant divided by the molar mass of dry air. Using 287 instead of 287.05 changes the result by less than 0.02 percent, which is below the precision shown here. The equation assumes dry air and a pressure low enough that the gas behaves ideally; at everyday temperatures and pressures the error is small, but compressed air and high altitude work may need a real gas correction. Convert gauge pressure to absolute pressure by adding atmospheric pressure before using the formula.
Worked example
Example: 20 C and 1013.25 hPa
Defaults: metric units, air temperature 20 C and absolute pressure 1013.25 hPa.
- Convert the temperature to kelvin: 20 + 273.15 = 293.15 K.
- Convert the pressure to pascals: 1013.25 x 100 = 101,325 Pa.
- Multiply the gas constant by the temperature: 287.05 x 293.15 = 84,148.7 J/kg.
- Divide the pressure by that product: 101,325 / 84,148.7 = 1.2041 kg/m³.
- Take the reciprocal for specific volume: 1 / 1.2041 = 0.8305 m³/kg.
- The tool shows 1.204 kg/m³ and 0.830 m³/kg.
Reference table
| Condition | Pressure | Density |
|---|---|---|
| 0 C | 1013.25 hPa | 1.292 kg/m³ |
| 20 C | 1013.25 hPa | 1.204 kg/m³ |
| 30 C | 1000 hPa | 1.149 kg/m³ |
| 20 C | 900 hPa | 1.070 kg/m³ |
| 68 F | 29.92 inHg | 0.0752 lb/ft³ |
Limits
- Treats air as dry, so humid air will be slightly less dense than the result.
- Assumes ideal gas behavior, which is close enough at everyday conditions but not at high pressure.
- Does not model altitude directly; you must supply the absolute pressure for the elevation.
- Uses a single gas constant, so it does not separate oxygen, nitrogen, argon or carbon dioxide.
Common mistakes
- Entering gauge pressure instead of absolute pressure, which makes the density too low.
- Using degrees Fahrenheit in the Celsius field or forgetting to add 273.15 before dividing.
- Entering pressure in hPa while the formula expects pascals, which gives a density 100 times too small.
- Confusing specific volume with density; they are reciprocals, not the same number.
- Expecting the dry air result to match a humid summer day without a humidity correction.
Try it yourself
Quick checks. Answers sit tucked under each question.
What is the density of dry air at 0 C and 1013.25 hPa?
0 C is 273.15 K and 1013.25 hPa is 101,325 Pa. Then 101,325 / (287.05 x 273.15) = 1.292 kg/m³, with a specific volume of 0.774 m³/kg.
What is the density at 30 C and 1000 hPa?
30 C is 303.15 K and 1000 hPa is 100,000 Pa. Then 100,000 / (287.05 x 303.15) = 1.149 kg/m³, with a specific volume of 0.870 m³/kg.
What is the density at 68 F and 29.92 inHg in imperial units?
68 F is 20 C and 29.92 inHg is about 101,325 Pa, so the density is 1.204 kg/m³. Converted, that is 0.0752 lb/ft³, with a specific volume of 13.30 ft³/lb.
Questions
What is the density of air at room temperature?
At 20 C and standard sea level pressure, dry air is about 1.204 kg/m³, or 0.0752 lb/ft³. Warmer or higher air is less dense.
Why does warm air have a lower density?
Heating a fixed mass of gas makes it expand, so the same mass fills a larger volume. Density is mass divided by volume, so it falls as temperature rises at constant pressure.
Is humid air heavier than dry air?
No. Water vapor molecules are lighter than the average air molecule, so humid air is slightly less dense than dry air at the same temperature and pressure.
What is specific volume?
It is the volume occupied by one unit of mass, the reciprocal of density. In metric it is usually m³/kg; in imperial it is ft³/lb.
Can I use this for altitude?
Yes, if you enter the absolute pressure at that altitude. The calculator does not convert elevation to pressure, because that depends on the weather and the standard atmosphere model.
Does this work for compressed air?
It gives a reasonable estimate at moderate pressures, but real gas effects grow as pressure rises. For high pressure work, use a compressibility factor or a real gas table.
Sources
- NASA Glenn Research Center: Earth atmosphere model, metric units
- NASA Glenn Research Center: Earth atmosphere equation