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Combined Gas Law Calculator

Solve for pressure, volume, or temperature at one of two gas states using the combined gas law, P1V1/T1 = P2V2/T2, given the other five quantities.

Initial pressure (P₁)
101.325 kPa
Initial volume (V₁)
2 L
Initial temperature (T₁)
300 K
Final pressure (P₂)
236.425 kPa
Final volume (V₂)
1 L
Final temperature (T₂)
350 K

How it works

The combined gas law tracks a fixed amount of a gas across two different states — a change in pressure, volume, and/or temperature — as P1×V1/T1 = P2×V2/T2. Unlike this site's existing Ideal Gas Law calculator, it never needs to know the amount of gas (moles) or the gas constant R, since both sides of the equation already cancel them out; it's the right tool when you know a gas's conditions before and after some change and want the third quantity at either state.

Pick which of the six quantities to solve for; the calculator cross-multiplies the same identity and rearranges it to isolate that one from the other five (e.g. P2 = P1×V1×T2 ÷ (V2×T1) when solving for final pressure).

Temperature must be entered in Kelvin, an absolute scale where 0 means no molecular kinetic energy at all — the same requirement the ideal gas law has, for the same reason. Holding temperature constant between the two states (T1 = T2) makes this reduce exactly to Boyle's law, P1V1 = P2V2; holding pressure or volume constant instead reduces it to Gay-Lussac's or Charles's law respectively.

FAQ

How is this different from the Ideal Gas Law calculator?

The ideal gas law, PV = nRT, needs the actual amount of gas in moles to relate pressure, volume, and temperature at a single state. The combined gas law instead compares the same fixed (but unspecified) amount of gas at two different states, so the amount cancels out of the equation entirely and never needs to be known.

Why kilopascals and liters instead of other pressure/volume units?

The combined gas law is a pure ratio, so it works in any consistent units — what matters is only that both pressures share one unit and both volumes share one unit, which kilopascals and liters here guarantee. Converting an existing measurement into kPa and L before entering it (or converting the result back afterward) won't change the ratio the law describes.

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