Combined Gas Law Calculator

It merges Boyle's law (pressure and volume), Charles's law (volume and temperature) and Gay-Lussac's law (pressure and temperature) into one relation, P1V1/T1 = P2V2/T2, for a fixed amount of an ideal gas. The quantity PV/T stays constant as the gas moves between two states.

The law only holds on an absolute scale where zero really means zero thermal energy. Celsius is offset, so the calculator adds 273.15 to convert your Celsius values to Kelvin before solving, then subtracts it to hand the answer back in Celsius. Using raw Celsius directly would give a wrong ratio.

Any consistent units, because only the ratios enter the equation. Use the same unit for both pressures (atm, kPa, mmHg or bar) and the same unit for both volumes (litres, m^3 or mL). The result comes back in whatever units you supplied.

Exactly five of the six. From any five of P1, V1, T1, P2, V2 and T2 the sixth is fully determined. The 'Solve for' menu blanks out one field, so you naturally fill the other five.

Yes. Hold temperature equal (T1 = T2) and it becomes Boyle's law; hold pressure equal for Charles's law; hold volume equal for Gay-Lussac's law. Just enter the same value in both boxes for the quantity you want to keep constant.

Temperatures must convert to a value above 0 K. In Celsius that means above -273.15 C. A colder entry, or one that solves to zero or below, is rejected as non-physical. Pressures and volumes must also be greater than zero.

The combined gas law compares two states of the same sealed sample, so the amount of gas cancels out. The ideal gas law PV = nRT is the fuller statement that also fixes the amount in moles, n, through the gas constant R. Use the combined law for before-and-after problems where the moles do not change.

They are exact for an ideal gas and are rounded to six decimal places for display. Real gases drift from the ideal at high pressure or near condensation, so treat the output as a very good approximation rather than a laboratory-grade measurement.