Air Density Calculator
Compute the density of air from barometric pressure, temperature, and optional humidity. Enter pressure in hPa, inHg, kPa, or psi and temperature in °F or °C — the tool returns density in kg/m³ and lb/ft³ against the 1.225 kg/m³ sea-level standard.
Example: with Air pressure 1013.25 · Pressure unit hPa / mb · Temperature 15 · Temperature unit Celsius (°C) · Relative humidity (%, optional) 0 → Air density: 1.2250 kg/m³.
- In pounds per cubic foot0.07648 lb/ft³
- Versus sea-level standard100.0% of the 1.225 kg/m³ standard
Computed by the calculator below using its default values. Change any input to see your own numbers.
Air density comes straight from the ideal gas law: ρ = P/(R·T). Warm, low-pressure, humid air is thinner — which is why engines and wings lose lift on hot days at altitude.
Why air density moves with weather
Air is a gas, so it obeys ρ = P/(R·T): density rises with pressure and falls as temperature climbs. Drop the barometer or heat the air and each cubic meter holds fewer molecules. That is the whole reason hot-air balloons rise, why aircraft need longer runways on hot days at high-altitude airports, and why naturally aspirated engines make less power in summer heat.
Humidity nudges density down, which surprises people. A water molecule (18 g/mol) is lighter than the average dry-air molecule (about 29 g/mol), so replacing some dry air with water vapor makes the mixture less dense at the same pressure. This tool splits the pressure into a dry part and a vapor part and applies each gas constant separately, so the humid-air result is physically correct rather than a rough fudge.
How it’s calculated
Dry air: ρ = P/(Rd·T), Rd = 287.05 J/kg·K, T in kelvin. Humid air: ρ = pd/(Rd·T) + pv/(Rv·T), Rv = 461.495 J/kg·K, where pv = (RH/100)·es, es from the Magnus form 611.2·exp(17.625·Tc/(243.04+Tc)) Pa, and pd = P − pv. Pressure converts to Pa (hPa ×100, kPa ×1000, inHg ×3386.389, psi ×6894.757); lb/ft³ = kg/m³ × 0.0624280.
Treats air as an ideal gas, valid to well under 1% at ordinary conditions. The 1.225 kg/m³ reference is the ISA sea-level value at 15 °C and 1013.25 hPa.
Dry air density by temperature at sea level
| Temperature | Density (kg/m³) | Density (lb/ft³) |
|---|---|---|
| 0 °C (32 °F) | 1.292 | 0.0807 |
| 15 °C (59 °F) | 1.225 | 0.0765 |
| 20 °C (68 °F) | 1.204 | 0.0752 |
| 25 °C (77 °F) | 1.184 | 0.0739 |
| 35 °C (95 °F) | 1.146 | 0.0716 |
Computed with ρ = P/(287.05·T) at 1013.25 hPa, dry air; rounded.
Common mistakes
- Using temperature in Celsius directly in the gas law — it must be absolute temperature in kelvin.
- Assuming humid air is denser than dry air; water vapor actually lowers density slightly.
- Entering station pressure as sea-level pressure at a high-altitude site, which overstates density.
- Mixing pressure units — 1013 hPa and 1013 inHg differ by a factor of about 34.
Frequently asked questions
What is the air density formula?
For dry air, ρ = P/(R·T) with R = 287.05 J/kg·K and T in kelvin. Humid air adds a vapor term: ρ = pd/(287.05·T) + pv/(461.5·T).
What is the density of air at sea level?
About 1.225 kg/m³ (0.0765 lb/ft³) at the standard 15 °C and 1013.25 hPa. It rises in cold, high-pressure air and falls in hot, low-pressure, or humid air.
Does humidity make air denser or lighter?
Lighter. Water vapor molecules are lighter than the nitrogen and oxygen they displace, so more humidity means slightly lower density at the same pressure and temperature.
Why does air density matter for engines and aircraft?
Both breathe air by volume, so thinner air means less oxygen and lift per stroke or wingspan. That is why performance drops on hot days and at high-altitude airports.