# Air Density Calculator

Calculate air density from temperature, pressure, and humidity. Results in kg/m³ and lb/ft³ with density altitude estimate. Free physics calculator.

## What this calculates

Air density depends on temperature, atmospheric pressure, and humidity. Warmer air is less dense, higher pressure compresses air to higher density, and surprisingly, humid air is lighter than dry air because water vapor (molecular weight 18) replaces heavier nitrogen (28) and oxygen (32) molecules. This calculator uses the ideal gas law with humidity correction to give you accurate results.

## Inputs

- **Temperature** (°C) — Ambient air temperature.
- **Atmospheric Pressure** (hPa) — min 0 — Sea level standard: 1013.25 hPa (29.92 inHg).
- **Relative Humidity** (%) — min 0, max 100

## Outputs

- **Air Density** (kg/m³) — Density of air at the specified conditions
- **Air Density** (lb/ft³) — Air density in pounds per cubic foot
- **Approx. Density Altitude** (ft) — Approximate density altitude (ISA model)
- **Dry Air Density (0% RH)** (kg/m³) — Air density without humidity correction

## Details

The calculation combines Dalton's law of partial pressures with the ideal gas law:

rho = pd/(Rd x T) + pv/(Rv x T)

Where pd is the partial pressure of dry air, pv is the vapor pressure, Rd = 287.058 J/(kg·K) is the specific gas constant for dry air, Rv = 461.495 J/(kg·K) is the specific gas constant for water vapor, and T is absolute temperature in Kelvin.

Standard sea-level air density is 1.225 kg/m³ at 15°C, 1013.25 hPa, and 0% humidity (ISA standard atmosphere).

Air density matters in many fields:

  - Aviation: Lower air density reduces lift and engine power, increasing takeoff distance

  - Ballistics: Drag on a bullet depends directly on air density

  - Wind energy: Power output is proportional to air density (P = 0.5 x rho x A x v³)

  - HVAC: Fan performance and duct sizing depend on air density

  - Sports: A baseball travels farther in thin (warm, high-altitude) air

## Frequently Asked Questions

**Q: Why is humid air less dense than dry air?**

A: Water vapor has a molecular weight of 18 g/mol, while nitrogen is 28 g/mol and oxygen is 32 g/mol. When water vapor displaces these heavier molecules in a given volume, the overall mass decreases. At the same temperature and pressure, humid air is always lighter than dry air. This is why muggy summer days feel heavy but the air is actually less dense.

**Q: What is the standard air density at sea level?**

A: The International Standard Atmosphere (ISA) defines sea-level air density as 1.225 kg/m³ (0.0765 lb/ft³) at 15°C (59°F) and 1013.25 hPa (29.92 inHg) with 0% relative humidity. Real conditions almost always differ from this standard, which is why calculating actual air density matters.

**Q: How does altitude affect air density?**

A: Air density decreases roughly 12% per 1000 meters (3.6% per 1000 feet) of altitude gain. At 5000 feet elevation, air is about 15% thinner than at sea level. This is why aircraft performance degrades at high altitude and why athletes train at elevation to build endurance.

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Source: https://vastcalc.com/calculators/physics/air-density
Category: Physics
Last updated: 2026-04-08