What is the combined gas law?

The combined gas law merges Boyle's Law (P1V1 = P2V2), Charles's Law (V1/T1 = V2/T2), and Gay-Lussac's Law (P1/T1 = P2/T2) into one equation: P1V1/T1 = P2V2/T2. It describes how pressure, volume, and temperature of a fixed amount of ideal gas are interrelated when all three can change.

Why must temperature be in Kelvin?

Gas law equations require an absolute temperature scale. Kelvin starts at absolute zero (−273.15 °C), so all values are positive and the ratios are physically meaningful. Using Celsius or Fahrenheit directly would produce incorrect results. This calculator automatically converts Celsius and Fahrenheit inputs to Kelvin before solving.

Which units are supported?

Pressure: atm, kPa, mmHg (Torr), psi. Volume: liters (L), milliliters (mL), cubic centimeters (cm³). Temperature: Kelvin (K), Celsius (°C), Fahrenheit (°F). All values are converted to consistent base units before solving, so you can freely mix units.

What happens if I leave more than one field blank?

The calculator requires exactly five known values to solve for one unknown. If two or more fields are empty, it will display an error asking you to fill in the missing values.

How does this differ from the ideal gas law (PV = nRT)?

The combined gas law assumes a fixed amount of gas (n is constant) and cancels out the gas constant R. It is useful when you know the initial and final states and do not need to know the exact amount of gas. Use the ideal gas law (PV = nRT) when the amount of gas (in moles) is known or must be calculated.

Can I hold one variable constant?

Yes. To simulate Boyle's Law, set T1 and T2 to the same value. For Charles's Law, set P1 and P2 equal. For Gay-Lussac's Law, set V1 and V2 equal. The combined gas law reduces to each special case automatically.

Combined Gas Law Calculator

Enter any five of the six variables (P1, V1, T1, P2, V2, T2) and instantly solve for the unknown using the combined gas law.

Enter Five Known Values

Leave exactly one field blank — that is the unknown to solve.

P₁V₁ / T₁ = P₂V₂ / T₂

Initial State

Initial Pressure (P₁)

Initial Volume (V₁)

Initial Temperature (T₁)

Final State

Final Pressure (P₂)

Final Volume (V₂)

Final Temperature (T₂)

Fill in five values and click Calculate

Solved Variable

All Values Summary

Variable	Description	Value

Step-by-Step Solution

Summary

The Combined Gas Law Calculator applies the relationship P1V1/T1 = P2V2/T2, which unifies Boyle's Law, Charles's Law, and Gay-Lussac's Law into a single equation for a fixed amount of ideal gas. Enter any five of the six variables and the tool instantly solves for the unknown. All calculations are performed client-side with no data sent to any server.

How it works

Enter values for five of the six variables: P1, V1, T1 (initial state) and P2, V2, T2 (final state).
Leave exactly one field blank — that is the unknown the calculator will solve.
Select the appropriate unit for each field (pressure, volume, or temperature).
Click Calculate to apply the formula P1 × V1 / T1 = P2 × V2 / T2.
The result appears instantly with the solved value and a full step-by-step breakdown.
Click Reset to clear all fields and start a new calculation.

Use cases

Solve chemistry homework problems involving simultaneous pressure, volume, and temperature changes.
Predict the final volume of a gas when both pressure and temperature change.
Calculate the new pressure when a gas is heated and compressed at the same time.
Verify lab results by cross-checking measured gas state variables.
Prepare for AP Chemistry or university-level gas law exams.
Model gas behavior in engineering scenarios such as pneumatic systems or HVAC.
Understand how all three gas law variables interact under a single equation.
Convert between different pressure, volume, and temperature units while solving problems.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu