What is the ideal gas law?

PV = nRT, linking pressure, volume, moles and temperature through the gas constant R of 8.314 J per mole per kelvin.

Do I have to use kelvin?

Yes. The equation needs absolute temperature. Using Celsius produces nonsense, including negative values. Add 273.15 to convert.

Why do crisp packets puff up on aeroplanes?

Cabin pressure is lower than at ground level, so the fixed amount of gas inside expands to a larger volume.

When does the ideal gas law fail?

At very high pressures or low temperatures near liquefaction, where molecular volume and intermolecular attraction stop being negligible.

Ideal Gas Law Calculator - PV = nRT | SimplyCalculators.co.uk

Ideal Gas Law Calculator

The ideal gas law links pressure, volume, temperature and quantity of gas in a single equation. Give this calculator any three and it returns the fourth.

Formula

PV = nRT · P₁V₁ ÷ T₁ = P₂V₂ ÷ T₂

R = 8.314 J/(mol·K)
T must be absolute: K = °C + 273.15
Using Celsius here is the single most common error

1 mol at 273.15 K and 101,325 Pa occupies 22.4 litres

The Ideal Gas Law

PV = nRT

P — pressure in pascals
V — volume in cubic metres
n — amount of gas in moles
R — the gas constant, 8.314 J/(mol·K)
T — temperature in kelvin

The single most common error is using Celsius. The equation requires absolute temperature: add 273.15 to convert.

The Combined Gas Law

When a fixed quantity of gas changes conditions, the moles and R cancel:

P₁V₁ ÷ T₁ = P₂V₂ ÷ T₂

This covers the familiar special cases: Boyle's law at constant temperature, Charles's law at constant pressure, and Gay-Lussac's at constant volume.

It explains why a sealed bag of crisps swells on a plane, and why tyre pressure reads higher after a motorway run.

Where "Ideal" Breaks Down

The law assumes gas molecules have no volume and do not attract one another. That is a good approximation at everyday temperatures and pressures, and a poor one at very high pressure or near the point of liquefaction, where real gases deviate noticeably.

One mole of any ideal gas occupies 22.4 litres at standard temperature and pressure — a useful figure to keep in mind for sanity-checking results.

Quick Reference

The named gas laws are each the combined law with one term held fixed.
Law	Held constant	Relationship
Boyle	Temperature	P₁V₁ = P₂V₂
Charles	Pressure	V₁/T₁ = V₂/T₂
Gay-Lussac	Volume	P₁/T₁ = P₂/T₂
Avogadro	P and T	V₁/n₁ = V₂/n₂
Combined	—	P₁V₁/T₁ = P₂V₂/T₂

Frequently Asked Questions

What is the ideal gas law?: PV = nRT, linking pressure, volume, moles and temperature through the gas constant R of 8.314 J per mole per kelvin.
Do I have to use kelvin?: Yes. The equation needs absolute temperature. Using Celsius produces nonsense, including negative values. Add 273.15 to convert.
Why do crisp packets puff up on aeroplanes?: Cabin pressure is lower than at ground level, so the fixed amount of gas inside expands to a larger volume.
When does the ideal gas law fail?: At very high pressures or low temperatures near liquefaction, where molecular volume and intermolecular attraction stop being negligible.