ChemistryStepByStep

Combined Gas Law Calculator

Enter five of pressure, volume and temperature (before and after) to solve for the sixth using P1V1/T1 = P2V2/T2.

V2 -- final volume
5.0000

P1V1/T1 = P2V2/T2 (temperatures used in Kelvin), solved for V2 = 5.0000.

P1V1/T1 = P2V2/T2

The combined gas law tracks a fixed amount of gas through a change in pressure, volume, or temperature -- since for a set amount of gas, PV/T stays constant. Knowing the "before" state (P1, V1, T1) and any two of the three "after" values lets you solve for the third.

Temperature must be absolute (Kelvin) for the ratio to hold -- this calculator accepts Celsius and converts internally, but the underlying math always runs in Kelvin.

Worked example

A gas starts at 1 atm, 2 L, 300 K. It's compressed to 3 atm at 350 K. Find the new volume.

V2 = P1V1T2 / (T1P2) = (1 × 2 × 350) / (300 × 3) = 0.7778 L.

Common mistake

Plugging in Celsius directly. 300°C and 300 K are wildly different temperatures (300°C ≈ 573 K) -- always convert to Kelvin (K = °C + 273.15) before solving.

Keep going

  • The combined gas law compares one gas sample under two different conditions; the ideal gas law instead relates pressure, volume, moles and temperature for a single condition at once. Ideal Gas Law Calculator

Frequently Asked Questions

What is the combined gas law?

It combines Boyle's, Charles's, and Gay-Lussac's laws into one relationship: P1V1/T1 = P2V2/T2. It describes how pressure, volume, and temperature relate for a fixed amount of gas as any of those three conditions change.

Why does temperature need to be in Kelvin?

The gas laws come from the ideal gas relationship PV = nRT, which only holds with an absolute temperature scale, where zero really means zero volume/pressure. Celsius has an arbitrary zero point (water's freezing point), so plugging Celsius directly into P1V1/T1 = P2V2/T2 gives a wrong answer -- always convert to Kelvin first.

Does this calculator handle Celsius input?

Yes -- select Celsius for the temperature unit and it converts to Kelvin internally before solving, then converts a temperature result back to Celsius for display.

What if pressure and temperature both stay constant, and volume changes?

That's just this same equation with one variable held fixed -- it's still the combined gas law, it just simplifies to whichever single law applies (Boyle's law if T is constant, Charles's law if P is constant, Gay-Lussac's law if V is constant).

How is this different from the ideal gas law?

The combined gas law compares the SAME fixed amount of gas under two different conditions and doesn't need to know how many moles are present. The ideal gas law (PV = nRT) works for a single condition but requires knowing the moles of gas.

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