Ideal Gas Law Calculator (PV = nRT)

The ideal gas law, PV = nRT, links the pressure, volume, temperature and amount of a gas. It works well for most gases at everyday conditions and underlies gas flow and tank calculations.

How to use the ideal gas law calculator

Enter any three of pressure, volume, moles and temperature to find the fourth. Pressure must be absolute (not gauge); temperature can be in °C, °F or K.

Ideal gas law formula

P V = n R T (R = 8.314 J/mol·K)

Worked example

1 mol of gas at 0 °C (273.15 K) and 101.325 kPa:

V = nRT ÷ P = 1 × 8.314 × 273.15 ÷ 101,325 = 22.41 L

At 25 °C it expands to 24.47 L.

Combined gas law

For a fixed amount of gas, P₁V₁/T₁ = P₂V₂/T₂. A tyre at 220 kPa absolute and 20 °C warms to 50 °C: P₂ = 220 × 323.15 ÷ 293.15 ≈ 243 kPa.

Molar volume at different conditions

The volume of 1 mole of an ideal gas:

Conditions	Temperature	Pressure	Volume
STP (IUPAC)	0 °C	100 kPa	22.71 L
Older STP	0 °C	101.325 kPa	22.41 L
Room conditions	25 °C	101.325 kPa	24.47 L

Always check which standard a textbook or datasheet uses.

The gas constant R

Units	Value of R
J/(mol·K) or kPa·L/(mol·K)	8.314
L·atm/(mol·K)	0.08206
L·bar/(mol·K)	0.08314

Using mass instead of moles

n = mass ÷ molar mass. For 64 g of oxygen (O₂, 32 g/mol), n = 2 mol. Common molar masses: N₂ 28, O₂ 32, CO₂ 44, air about 28.97 g/mol.

Density of a gas

Rearranging gives gas density: ρ = P × M ÷ (R × T). Air at 20 °C and 101.325 kPa has a density of about 1.204 kg/m³.

Always use absolute values

Temperature must be in kelvin (or entered in °C so the calculator converts it) and pressure must be absolute, not gauge. A tyre at 2.2 bar gauge is about 3.2 bar absolute.

When it is less accurate

Real gases deviate at very high pressures and low temperatures, close to condensing. For air and most gases at everyday conditions, the error is well under 1%.

Frequently asked questions

What is the ideal gas law?

PV = nRT, linking pressure, volume, amount of gas and absolute temperature. R = 8.314 J/(mol·K).

What volume does one mole of gas occupy?

About 22.41 L at 0 °C and 1 atm, and about 24.47 L at 25 °C and 1 atm.

Why must pressure and temperature be absolute?

The law is based on absolute zero and zero pressure. The calculator converts °C and °F to kelvin automatically; enter absolute pressure (gauge + atmospheric).