Combined Gas Law Calculator

Enter your initial and final gas state values — P1 (initial pressure), V1 (initial volume), T1 (initial temperature), plus any two of P2, V2, T2 — and the Combined Gas Law Calculator solves for the missing variable using the formula P1·V1/T1 = P2·V2/T2. Choose the thermodynamic process type to lock the appropriate variable. Results show the unknown state value along with a visual breakdown of how pressure, volume, and temperature relate across both states.

Select the process type to pre-lock the constant variable.

Choose which final state variable to calculate.

kPa

Absolute pressure of the initial gas state.

L

Total volume occupied by the gas in the initial state.

Temperature of the gas in the initial state.

kPa

Absolute pressure of the final gas state.

L

Total volume occupied by the gas in the final state.

Temperature of the gas in the final state.

Results

Solved Variable (Result)

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P1 (kPa)

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V1 (L)

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T1 (K)

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P2 (kPa)

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V2 (L)

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T2 (K)

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Gas Constant k (P·V/T)

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State 1 vs State 2 — Pressure, Volume & Temperature

Results Table

Frequently Asked Questions

What is the Combined Gas Law formula?

The Combined Gas Law merges Boyle's Law, Charles's Law, and Gay-Lussac's Law into a single equation: P1·V1/T1 = P2·V2/T2. It states that the ratio of the product of pressure and volume to temperature remains constant for an ideal gas undergoing a state change. All temperature values must be in Kelvin for the formula to work correctly.

How do I solve for T2 in the Combined Gas Law?

Rearrange the formula to isolate T2: T2 = (P2·V2·T1) / (P1·V1). Simply plug in your known values for P1, V1, T1, P2, and V2, and calculate. Make sure T1 is in Kelvin — convert from Celsius by adding 273.15, or from Fahrenheit using K = (°F + 459.67) × 5/9.

Are pressure and temperature directly proportional?

Yes — but only when volume is held constant (an isochoric process). In that case, the law simplifies to P1/T1 = P2/T2, known as Gay-Lussac's Law. This means if you double the absolute temperature of a gas in a sealed rigid container, the pressure also doubles.

What are the four main thermodynamic processes?

The four common thermodynamic processes for ideal gases are: Isobaric (constant pressure), Isochoric (constant volume), Isothermal (constant temperature), and Adiabatic (no heat exchange with surroundings). Each process holds one variable constant, simplifying the combined gas law into a two-variable relationship.

What is an isobaric process?

An isobaric process is one where pressure remains constant throughout the gas transformation. Under constant pressure, the combined gas law simplifies to V1/T1 = V2/T2, also known as Charles's Law. A common everyday example is a gas expanding in a balloon placed in a warmer environment.

What is the T2 of an isochoric process where T1 = 300 K, P1 = 100 kPa, and P2 = 125 kPa?

In an isochoric (constant volume) process, V1 = V2, so the formula simplifies to T2 = T1 × P2/P1. Substituting the values: T2 = 300 × (125/100) = 375 K. This means the temperature increased from 300 K to 375 K as the pressure rose from 100 kPa to 125 kPa.

Do I need to convert temperature to Kelvin?

Yes. The Combined Gas Law requires absolute temperature values, which means Kelvin must be used in the formula. This calculator handles the conversion automatically — you can enter temperatures in Celsius or Fahrenheit, and the results will also be displayed in your chosen output unit. Never use Celsius or Fahrenheit directly in the P·V/T formula.

Can this calculator handle unit conversions for pressure and volume?

Yes. This calculator supports multiple units for pressure (kPa, Pa, bar, atm, psi, mmHg) and volume (litres, millilitres, m³, cm³, ft³, in³). All inputs are internally converted to SI base units (kPa and litres) before applying the gas law, ensuring consistent and accurate results regardless of the unit combination you choose.

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