Combined Gas Law Calculator
By Santaji GadepatilinUpdated October 27, 2026
The combined gas law brings Boyle's, Charles's and Gay-Lussac's laws together to show how pressure, volume and temperature of a fixed amount of gas change at the same time.
How to use the combined gas law calculator
Enter five of the six values (initial and final pressure, volume and temperature) to get the sixth.
Combined gas law formula
P₁V₁ ÷ T₁ = P₂V₂ ÷ T₂
Worked example
10 L of air at 101.325 kPa and 25 °C is compressed to 200 kPa and warms to 60 °C:
V₂ = 101.325 × 10 × 333.15 ÷ (298.15 × 200) = 5.66 L
Standard conditions
Gas volumes are often reported at standard conditions so they can be compared:
| Standard | Temperature | Pressure |
|---|---|---|
| STP (IUPAC, since 1982) | 0 °C (273.15 K) | 100 kPa |
| STP (older) | 0 °C | 101.325 kPa (1 atm) |
| NTP | 20 °C | 101.325 kPa |
| Normal (industrial Nm³) | 0 °C | 101.325 kPa |
Correcting gas volume to standard conditions
A gas meter reads 10 m³ at 2 bar(g) (3.013 bar absolute) and 30 °C. At normal conditions:
VN = 10 × (3.013 ÷ 1.01325) × (273.15 ÷ 303.15) = 26.8 Nm³
This is how gas flow is billed and compared in industry.
Adding the amount of gas
If the amount of gas changes too, use the full ideal gas law: PV = nRT. The combined gas law is the special case where n is constant.
Common errors
- Using gauge pressure instead of absolute.
- Using °C instead of kelvin.
- Mixing units — keep P₁ and P₂ in the same unit, and V₁ and V₂ in the same unit. The calculator handles this for you.
Special cases
- Constant temperature → Boyle’s law (P₁V₁ = P₂V₂)
- Constant pressure → Charles’s law (V₁/T₁ = V₂/T₂)
- Constant volume → Gay-Lussac’s law (P₁/T₁ = P₂/T₂)
Frequently asked questions
What is the combined gas law?
It combines Boyle's, Charles's and Gay-Lussac's laws — P₁V₁/T₁ = P₂V₂/T₂ for a fixed amount of gas.
When should I use the ideal gas law instead?
When you need the amount of gas (moles). The combined law only compares two states of the same gas sample.
Which units must I use?
Absolute pressure and absolute temperature. The calculator converts units automatically.
What is the combined gas law?
P₁V₁ ÷ T₁ = P₂V₂ ÷ T₂ for a fixed amount of gas, combining Boyle’s, Charles’s and Gay-Lussac’s laws.
Can I use it for real gases?
Yes for most gases near room conditions. At very high pressure or low temperature, real-gas corrections are needed.