What formula does this calculator use?

It uses the ideal gas law rearranged for density: rho = (P x M) / (R x T), where P is absolute pressure in Pa, M is molar mass in kg/mol, R = 8.31446 J per mol per K (universal gas constant, CODATA 2018), and T is absolute temperature in K. The same equation rearranges to P = (rho x R x T) / M and T = (P x M) / (rho x R) for the other two solve modes.

What is the density of air at room temperature?

Dry air (M = 28.97 g/mol) at 25 degrees C and 1 atm (101325 Pa) has a density of approximately 1.184 kg/m3. At 20 degrees C (NTP) it is about 1.204 kg/m3, and at 0 degrees C (old STP) about 1.293 kg/m3. This calculator reproduces all three values in its reference table.

Does the formula work for all gases?

It works well for gases well above their boiling point and at pressures well below the critical pressure (roughly below 5-10 MPa for most industrial gases). For high-pressure pipelines, near-critical conditions, or liquefied gases, a real-gas equation of state such as van der Waals or Peng-Robinson gives more accurate results. For engineering-grade accuracy at elevated pressure, apply a compressibility factor Z: rho = PM / (ZRT).

Why do I need to enter molar mass?

The ideal gas law itself (PV = nRT) treats every gas the same. Molar mass M connects the amount of substance (n, in mol) to the actual mass (m = nM). Without M the formula cannot distinguish between a heavy molecule like CO2 (44 g/mol, density 1.96 kg/m3 at STP) and a light one like hydrogen H2 (2.016 g/mol, density 0.090 kg/m3 at STP). Selecting a preset gas fills in the correct IUPAC molar mass automatically.

What is STP and how does it differ from NTP?

STP (Standard Temperature and Pressure) is defined by IUPAC since 2014 as 0 degrees C (273.15 K) and 100 kPa. The older pre-1982 STP used 1 atm (101.325 kPa) instead of 100 kPa, giving a slightly higher density. NTP (Normal Temperature and Pressure) is a US convention using 20 degrees C and 1 atm and is common in HVAC and compressed-gas specifications. The reference table in the calculator shows all three alongside room temperature (25 degrees C / 1 atm).

Can I use this for gas mixture density?

Yes, as long as you know the mixture molar mass. For a mixture, the effective molar mass is the mole-fraction-weighted average of the component molar masses: M_mix = sum(y_i x M_i) where y_i is the mole fraction of component i. Enter this value using the Custom molar mass option. For example, a 60/40 (by moles) methane/propane mix has M_mix = 0.60 x 16.04 + 0.40 x 44.10 = 27.26 g/mol.

Gas Density Calculator

Gas density tells you how much mass a given volume of gas contains. It is a fundamental quantity in aerodynamics, combustion engineering, HVAC design, pipeline sizing, altitude physiology, and atmospheric science. This calculator derives density from first principles using the ideal gas law, one of the most precisely verified relationships in physics.

How it works

The ideal gas law states:

PV = nRT

where P is absolute pressure (Pa), V is volume (m³), n is amount (mol), R is the universal gas constant (8.31446 J·mol⁻¹·K⁻¹, CODATA 2018), and T is absolute temperature (K). Because density ρ = mass/volume = nM/V (where M is the molar mass in kg/mol), the law rearranges directly to:

ρ = (P × M) / (R × T)

All three quantities can be the unknown. The calculator supports solving for:

ρ — the most common use: what is the density of gas X at this P and T?
P — what pressure produces a target density at a given temperature?
T — what temperature corresponds to a measured density at a known pressure?

Every input accepts multiple units (Pa / kPa / bar / atm / psi / mmHg for pressure; K / °C / °F for temperature; kg/m³ / g/L / g/cm³ / lb/ft³ for density). Conversions happen automatically before the formula runs.

Worked example — CO₂ at pipeline conditions

Suppose you need the density of carbon dioxide at 5 MPa and 40 °C — typical conditions near the inlet of a CO₂ compression train.

M = 44.01 g/mol = 0.04401 kg/mol
P = 5 × 10⁶ Pa
T = 40 + 273.15 = 313.15 K
ρ = (5 000 000 × 0.04401) / (8.31446 × 313.15)
ρ = 220 050 / 2603.8 ≈ 84.5 kg/m³

That is roughly 70 times denser than CO₂ at atmospheric pressure (1.96 kg/m³). Note that at 5 MPa the ideal gas approximation begins to diverge from measured values; real CO₂ near its critical point (73.8 atm, 31 °C) requires a compressibility correction. For conditions below ~1 MPa the formula is accurate to better than 1%.

Gas	Molar mass (g/mol)	Density at 25 °C / 1 atm (kg/m³)
Hydrogen H₂	2.016	0.082
Helium He	4.003	0.164
Methane CH₄	16.04	0.655
Dry air	28.97	1.184
Oxygen O₂	32.00	1.308
Argon Ar	39.95	1.633
CO₂	44.01	1.799
Propane C₃H₈	44.10	1.802

Formula note

The derivation is exact within the ideal-gas approximation. Deviations arise from intermolecular forces (van der Waals interactions) and finite molecular volume, both captured by the compressibility factor Z. At atmospheric pressure Z ≈ 1.000 ± 0.001 for most gases, so the ideal formula is engineering-grade accurate for most everyday calculations — weather balloons, lab-bench gas flow, ventilation rates, and molar mass determination from measured density. For high-pressure (above ~5 MPa) or low-temperature (within ~50 K of the boiling point) work, use ρ = PM / (ZRT) with a tabulated or equation-of-state Z value.