Starting from PV=nRT and using molar mass M, ideal-gas density is:

Pressure must be absolute and temperature must be in kelvins. The formula works best when the gas is sufficiently dilute and far from condensation.

A common engineering form includes the compressibility factor Z:

When Z differs from 1, an ideal-gas calculation can be materially wrong. High pressure, low temperature and proximity to the critical region are warning signs that a validated equation of state may be necessary.

Choose Ideal Gas or Real Gas with Z, select a gas preset or enter molar mass, then enter absolute pressure and temperature. The calculator reports density and compares the result visually with familiar gases.

Gas laws require absolute pressure. If a gauge reads zero at atmospheric conditions, that is not zero absolute pressure. Convert gauge pressure to absolute by adding local atmospheric pressure in compatible units.

At approximately fixed temperature, increasing pressure generally raises density. At approximately fixed pressure, increasing temperature generally lowers density. Real-gas departures become more important as molecules interact more strongly.

For an ideal mixture, calculate mixture molar mass from mole fractions, then use the same density relationship. Natural gas and flue gas can require detailed composition and a real-gas model for accurate work.

Relative gas density can be expressed as the ratio of gas density to air density at the same conditions. Helium and hydrogen are much lighter than air; carbon dioxide is heavier than air at comparable conditions.

Once state conditions and density are known, m=ρV and V=m/ρ can convert between gas mass and volume. Remember that the volume of a compressible gas changes if pressure or temperature changes.

Use specialized property models for high-pressure hydrogen, supercritical CO₂, saturated refrigerants or other conditions where phase behavior is important. The site provides dedicated tools for several of these cases.

At low pressure, common gases can be compared approximately by molar mass at the same temperature and pressure. Hydrogen and helium are light; methane is lighter than air; carbon dioxide is heavier. A reference table should always name the state conditions because gas density is not portable from one pressure-temperature pair to another.

For dry air approximated as an ideal gas at 20°C and 1.01325 bar absolute with molar mass 28.965 g/mol, the equation gives a density close to 1.20 kg/m³. Increase pressure while holding temperature constant and density rises; increase temperature at fixed pressure and density falls.

The most damaging mistakes are entering gauge pressure as absolute pressure, using °C directly instead of kelvins, using molar mass in g/mol where kg/mol is required internally, and applying an ideal-gas formula at a high-pressure state where Z differs materially from 1.