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The ideal gas law ties together the four state variables of a gas in one equation, PV = nRT: pressure times volume equals the number of moles times the gas constant times the absolute temperature. The constant R = 8.314462618 J/(mol·K) in SI units. Rearranged, you can solve for any variable — P = nRT/V, V = nRT/P, n = PV/RT or T = PV/nR — and this calculator does exactly that, converting pressure (Pa, kPa, bar, atm, mmHg, psi), volume (m³, L, mL, cm³) and temperature (K, °C, °F) internally so you can work in whatever units you have.

The classic worked example is one mole of gas at standard temperature and pressure: at 0 °C (273.15 K) and 1 atm, V = nRT/P = 1 × 8.314 × 273.15 / 101,325 ≈ 0.0224 m³, or the familiar 22.4 litres per mole. Heat that gas to 300 K at constant pressure and the volume rises in proportion, since V ∝ T. A crucial detail: temperature must be absolute — always convert °C or °F to kelvin (add 273.15 to °C) before the math, which the calculator handles for you.

The law is called "ideal" because it assumes gas molecules have no volume and do not attract one another. Real gases follow it closely at ordinary pressures and temperatures well above their boiling point, and it is the everyday tool for scuba and diving gas calculations, chemistry stoichiometry of gases, HVAC and pneumatics, and estimating how a balloon or tyre responds to heating and cooling. It becomes less accurate at very high pressure or near condensation, where equations like van der Waals do better.