Air Density Calculator

Air density is the mass of air per unit volume, usually expressed in kilograms per cubic meter (kg/m³). Under standard sea-level conditions (15°C, 101325 Pa), dry air has a density of about 1.225 kg/m³. Density falls as temperature rises, as altitude rises, or as humidity increases, and it rises when air gets colder or pressure increases.

This calculator uses the CIPM-2007 formula from the International Committee for Weights and Measures: ρ = p·Ma / (Z·R·T) · (1 − xv·(1 − Mv/Ma)). You enter temperature, pressure, and relative humidity; the formula adds a compressibility factor Z and a water-vapor mole fraction xv (from the Sonntag saturation-vapor-pressure equation and the enhancement factor), so moist air is treated correctly instead of being approximated as dry.

At the standard sea-level reference (15°C, 101325 Pa, dry air) this calculator gives 1.2255 kg/m³, which is the widely quoted textbook value of 1.225 kg/m³ rounded to three decimals. Real air at sea level is rarely exactly at the standard point: at 20°C and 50% humidity the density is about 1.199 kg/m³, roughly 2% lower.

Humidity decreases air density. Water molecules (molar mass 18 g/mol) are lighter than the nitrogen and oxygen molecules they displace (average ~29 g/mol for dry air), so at the same temperature and pressure, moist air is less dense. At 15°C and sea-level pressure, raising relative humidity from 0% to 50% lowers density from 1.2255 to 1.2216 kg/m³ — the classic reason a baseball is said to carry farther on humid days.

Density drops with altitude because both pressure and temperature fall. Under the International Standard Atmosphere, density at 1000 m is about 1.112 kg/m³, at 3000 m about 0.910 kg/m³, and at 11000 m (typical cruising level) about 0.364 kg/m³. The Altitude lookup tab on this page shows ISA temperature, pressure, and density for any altitude up to 20000 m.

Denver sits at about 1609 m (5280 ft). Under standard-atmosphere conditions its temperature is 4.5°C and pressure about 83431 Pa, giving an air density of roughly 1.047 kg/m³ — about 14.5% lower than at sea level. That thinner air is why engines make less power and aircraft need longer takeoff runs in Denver; our density altitude lab covers the flight-performance side in depth.

Air density is a physical quantity: how much mass is in each cubic meter of air (kg/m³). Density altitude is the altitude in the standard atmosphere that would have that same density — a single number pilots and engineers use to compare conditions. This calculator shows density altitude as a derived readout; for flight performance (takeoff distance, climb, true airspeed) see our dedicated density altitude lab, which goes far deeper into the aviation side.

At standard temperature and pressure (0°C, 101325 Pa), dry air has a density of about 1.293 kg/m³, roughly 5.5% denser than the 15°C reference of 1.225 kg/m³. Note that "STP" is defined differently by different organizations (some use 20°C), so always check which reference a table or dataset is using before comparing numbers.

Multiply by 0.06243: 1 kg/m³ = 0.06243 lb/ft³. So standard sea-level air (1.2255 kg/m³) equals about 0.0765 lb/ft³. This calculator displays both units automatically, along with grams per cubic meter.

It implements the full CIPM-2007 equation of state for moist air (Picard, Davis, Gläser & Stock, Metrologia 2008), including the compressibility factor Z, the Sonntag 1990 saturation vapor pressure, and the humidity enhancement factor. For meteorological ranges (0–1000 hPa, −50…+60°C) it agrees with published reference values to about 1 part in 10⁴ — far better than the simple ideal-gas approximation ρ = p/(R·T), which ignores both compressibility and moisture.

Three common reasons. First, the textbook 1.225 kg/m³ is a rounded ideal-gas value; the more complete CIPM-2007 equation gives 1.2255 kg/m³ at the same conditions. Second, real air contains humidity, which lowers density. Third, your actual pressure may differ from 101325 Pa — weather systems routinely swing sea-level pressure by ±20 hPa, which changes density by about ±2%.

Lift, drag, engine power, and propeller thrust all scale directly with air density, so hot, high, or humid conditions ("high density altitude") degrade aircraft and engine performance. Ballistic trajectories also depend on density through air resistance. This calculator gives you the density number; for takeoff-distance and climb estimates, our density altitude lab applies the Koch chart and compressible-flow corrections that the aviation use case needs.

Pressure altitude is the altitude in the standard atmosphere corresponding to your measured pressure — it tells you what the atmosphere "thinks" your altitude is from pressure alone, independent of temperature. Comparing pressure altitude with the true density altitude shows how much temperature and humidity are shifting conditions away from the standard model. It is shown here as a derived readout for context.

It is a good sanity check for back-of-envelope estimates — for example, checking that 20°C indoor air at 50% humidity is about 1.199 kg/m³ before sizing a fan. It is not a substitute for psychrometric software or engineering standards (such as ASHRAE tables) when real design decisions, safety margins, or compliance are involved, because this tool computes a single-point density and does not model the full moist-air state the way dedicated HVAC tools do.

Four honest limits. (1) It computes density from the conditions you enter — it is not live weather data, so enter your own station readings for real situations. (2) The ISA altitude lookup is the standard model, not the actual atmosphere; real pressure at altitude varies with weather. (3) Inputs are clamped to −50…+60°C and 100…1100 hPa, the validated range of the Sonntag vapor-pressure equation. (4) Above 11000 m the density-altitude readout uses the stratosphere extension (isothermal layer), which is correct for ISA but approximate for real high-altitude conditions.

No. Every calculation runs entirely in your browser with JavaScript — temperature, pressure, and humidity never leave your device. There are no accounts, no analytics beacons from this tool, and no external requests of any kind.