Boyle's law formula
At constant temperature and fixed amount of gas, pressure times volume is constant: P1·V1 = P2·V2. Rearranged for the new volume, V2 = P1·V1 ÷ P2. Pressure and volume are inversely proportional — squeeze a gas into half the volume and its pressure doubles. To vary temperature too, use the combined gas law calculator.
How to use this calculator
Enter the initial pressure P1, the initial volume V1 and the final pressure P2. Keep both pressures in the same unit (atm here) and the volume in litres; the result V2 comes out in the same volume unit. The calculator guards against a zero final pressure, which would be physically impossible.
Because the law depends only on the ratio P1 ÷ P2, the pressure unit cancels. You can enter both pressures in kPa, bar, psi or mmHg and still get a correct answer, as long as you do not mix units. Gauge pressure is the usual trap: convert it to absolute pressure first, or the ratio will be wrong. Holding pressure constant instead and letting temperature vary gives Charles’s law.
Worked example
A 4 L gas sample at 2 atm is compressed to 1 atm at constant temperature. V2 = (2 × 4) ÷ 1 = 8 L. Halving the pressure doubles the volume, exactly as Boyle's law predicts.
Read the result as a check on the direction of the change: dropping the pressure should always give a larger volume, and raising it should give a smaller one. If your answer moves the other way, P1 and P2 have probably been swapped. When more than one variable moves at once, step up to the ideal gas law calculator.
Frequently asked questions
- What is Boyle's law?
- Boyle's law states that the pressure of a fixed amount of gas is inversely proportional to its volume when temperature is held constant.
- What units should I use?
- Any consistent units work. Keep P1 and P2 in the same pressure unit and V1 in your chosen volume unit; V2 returns in that same volume unit.
- Does temperature have to stay constant?
- Yes. Boyle's law only holds at constant temperature. If temperature changes, use the combined gas law instead.
- Why must P2 not be zero?
- Dividing by a zero final pressure is undefined, and a gas cannot exist at zero pressure, so the calculator requires P2 greater than zero.
- When does Boyle's law stop being accurate?
- It breaks down at very high pressures and very low temperatures, where a gas no longer behaves ideally. Under those conditions the molecules occupy a meaningful share of the container and attract each other, so real volumes deviate from P1V1 = P2V2 and a real-gas equation is needed.