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P₁V₁/T₁ = P₂V₂/T₂

Combined gas law calculator

Fill the first state, leave exactly one field of the second state empty, and the missing pressure, volume or temperature is solved for.

First state (known)

Second state (leave exactly one empty)

Short answer: 22.4 L at 273.15 K heated to 546.3 K at constant pressure becomes44.8 L — the volume doubles because the absolute temperature doubled. Doubling 273.15 in Celsius instead would give an answer that is simply wrong.

The three ratios in one equation

Hold temperature fixed and it is Boyle’s law; hold pressure fixed and it is Charles’s law; hold volume fixed and it is Gay-Lussac’s law. The page prints both sides of the equation after solving, so the equality can be checked rather than assumed.

Worked example

A tyre at 2.2 bar and 300 K is driven until it reaches 320 K at the same volume: P₂ = 2.2 × 320 ÷ 300 = 2.347 bar, a rise of about 7%. That is the real reason a hot tyre reads high, and it is also why 2.2 is specified cold.

Combined gas law questions

What does the combined gas law say?

P₁V₁ ÷ T₁ = P₂V₂ ÷ T₂ for a fixed amount of gas. It puts Boyle’s law (pressure and volume), Charles’s law (volume and temperature) and Gay-Lussac’s law (pressure and temperature) into a single equation.

Why must temperature be in kelvin?

Because the law is a ratio and 0 °C is not the bottom of the scale — entering 25 where 298 is meant gives an answer about twelve times too small. Every temperature field here says kelvin, and no Celsius conversion is applied silently.

Can I leave more than one value empty?

No. With two of the second state’s values missing the problem is underdetermined and any answer would be a guess, so the page asks for exactly one unknown and says so instead of inventing a number.

Does it hold for any gas?

It is exact only for an ideal gas, and a good approximation at ordinary temperatures and pressures. Near condensation or at high pressure real gases deviate, which is when an equation of state with a compressibility factor is needed instead.

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