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Combined Gas Law Calculator
Your gas values
Temperatures must be in Kelvin. To convert Celsius to Kelvin, add 273.15 — for example 0 °C = 273.15 K, and 25 °C = 298.15 K.
Example: P1 1.0 atm, V1 2.0 L, T1 273 K, P2 2.0 atm, T2 546 K → V2 = 2.0 L.
Behind the numbers
How the combined gas law is calculated
The combined gas law links the pressure, volume and temperature of a fixed amount of gas before and after a change: P1V1 ÷ T1 = P2V2 ÷ T2. Choose the one value you do not know, enter the other five, and the calculator rearranges the equation to solve for it. Pressure and volume can use any units as long as both pressures use the same unit and both volumes use the same unit — but temperature must always be in Kelvin, never Celsius or Fahrenheit.
Worked example — solve for V2. Start from P1V1 ÷ T1 = P2V2 ÷ T2, so V2 = P1 × V1 × T2 ÷ (P2 × T1). Put in the numbers: V2 = 1.0 × 2.0 × 546 ÷ (2.0 × 273) = 1092 ÷ 546 = 2.0 L. The pressure doubled and the temperature doubled, so the two changes cancel and the volume stays the same.
Examples
Examples
| Known values | Solve for | Answer |
|---|---|---|
| P1 1.0 atm, V1 2.0 L, T1 273 K, P2 2.0 atm, T2 546 K | V2 | 2.0 L |
| P1 1.0 atm, V1 1.0 L, T1 300 K, P2 1.0 atm, V2 2.0 L | T2 | 600 K |
| P1 2.0 atm, V1 3.0 L, T1 300 K, P2 1.0 atm, V2 3.0 L | T2 | 150 K |
| P1 1.0 atm, V1 4.0 L, T1 273 K, V2 2.0 L, T2 273 K | P2 | 2.0 atm |
| P1 1.0 atm, V1 2.0 L, P2 2.0 atm, V2 2.0 L, T2 546 K | T1 | 273 K |
Tips before you use it
Always convert temperatures to Kelvin first: K = °C + 273.15. Using Celsius directly gives a wrong answer.
Keep pressure units matching: if P1 is in atm, P2 must be in atm too (the same goes for kPa, mmHg or psi).
Keep volume units matching as well: liters with liters, milliliters with milliliters.
The law assumes the amount of gas stays the same — no gas is added or escapes during the change.
Kelvin can never be zero or negative in this calculator: 0 K is absolute zero, the lowest possible temperature.
Good to know
Frequently asked questions
What is the combined gas law?
It combines Boyle's law, Charles's law and Gay-Lussac's law into one equation, P1V1/T1 = P2V2/T2, for a fixed amount of gas. It tells you how pressure, volume and temperature change together.
Why must temperature be in Kelvin?
The gas laws use ratios of temperatures, so the scale must start at absolute zero. Celsius starts at the freezing point of water, not at zero energy, so a Celsius ratio does not work. Add 273.15 to a Celsius value to get Kelvin.
What if only pressure changes?
Then temperature is constant and the law reduces to Boyle's law, P1V1 = P2V2. This calculator still works — just enter the same temperature for T1 and T2.
Can I use this for a gas that escapes or is added?
No. The combined gas law only applies when the amount (moles) of gas is fixed. If gas is added or removed, use the ideal gas law, PV = nRT, instead.