The ideal gas law, PV = nRT, ties together everything about a gas in one equation: its pressure, volume, amount in moles and absolute temperature, linked by the universal gas constant R. Squeeze a gas and the pressure rises; warm it and it expands; add more gas and it pushes harder. Because the four quantities are locked together, knowing any three fixes the fourth — which is why this single relation handles balloons, scuba tanks, engine cylinders and lab gas measurements alike.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the ideal gas law PV = nRT.
The ideal-gas equations
The catch is units: this calculator uses atmospheres, litres, moles and kelvin so that R = 0.08206 L·atm/(mol·K). Temperature must always be absolute — convert Celsius by adding 273.15 — because the law assumes that doubling the kelvin temperature doubles the gas's tendency to expand or push. Choose the R whose units match your pressure and volume, and the rest is straightforward algebra.
Worked example — molar volume at STP
Scenario: What volume does 1 mole of an ideal gas occupy at standard temperature and pressure (0 °C = 273.15 K, 1 atm)?
One mole of any ideal gas fills about 22.4 litres at STP — the famous molar volume, the same whether it is hydrogen, oxygen or carbon dioxide. Warm it to room temperature (25 °C) and it expands to ~24.5 L, because volume rises in proportion to absolute temperature at fixed pressure. Double the gas to 2 moles at STP and you need 44.8 L; halve the pressure to 0.5 atm and the same mole expands to ~44.8 L as well.
Frequently Asked Questions
PV = nRT, solve for the unknown. 1 mol at 273.15 K, 1 atm → V ≈ 22.4 L. T in kelvin, R = 0.0821 L·atm/(mol·K).
8.314 J/(mol·K) in SI, or 0.08206 L·atm/(mol·K) with atm and litres. Same constant, different units.
The law uses absolute temperature (zero = no thermal energy). K = °C + 273.15. Celsius gives wrong answers.
At 0 °C and 1 atm, 1 mole of any ideal gas ≈ 22.4 L. Handy for mole↔volume conversions.
At high pressure / low temperature (near liquefaction). Use van der Waals there; PV=nRT is great otherwise.