What is Gay-Lussac's Law?

Gay-Lussac's Law states that at constant volume, the pressure of a fixed amount of ideal gas is directly proportional to its absolute temperature. Mathematically: P1 / T1 = P2 / T2, where P is pressure, T is absolute temperature in Kelvin, and subscripts 1 and 2 refer to the initial and final states.

Why must temperature be in Kelvin?

Gay-Lussac's Law requires absolute temperature (Kelvin) because the relationship is proportional — doubling Kelvin doubles the pressure. Celsius and Fahrenheit have arbitrary zero points, so using them directly gives incorrect results. This calculator automatically converts °C and °F to Kelvin before solving.

Which units can I use for pressure?

This calculator accepts pressure in atm, kPa, mmHg (Torr), and psi. All values are converted to Pascals internally before solving, so you can mix units freely between P1 and P2.

What happens if I leave more than one field blank?

The calculator needs exactly three known values to find one unknown. If you leave two or more fields empty it will prompt you to fill in the missing values.

What conditions must hold for this law to apply?

Gay-Lussac's Law applies to ideal gases under two conditions: the volume must remain constant (isochoric process) and the amount of gas must not change. Real gases approximate ideal behavior at low pressures and high temperatures.

Can I use this for liquids or solids?

No. Gay-Lussac's Law applies to gases only. The pressure-temperature relationship for liquids and solids is governed by very different physics and cannot be described by this simple proportionality.

What is an everyday example of this law?

A common example is a car tire: on a hot summer day the tire pressure rises compared to a cold morning, even though no air was added or removed and the volume of the tire is approximately constant. Another example is a pressure cooker, where sealing the lid keeps volume fixed and heating increases internal pressure.

Gay-Lussac's Law Calculator

Name: Gay-Lussac's Law Calculator
Availability: InStock
Author: Nham Vu

Enter any three of P1, T1, P2, T2 and instantly solve for the unknown using Gay-Lussac's Law at constant volume.

Enter Three Known Values

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

P₁ / T₁ = P₂ / T₂

at constant volume & fixed amount of gas

Initial State

Initial Pressure (P₁)

Initial Temperature (T₁)

Final State

Final Pressure (P₂)

Final Temperature (T₂)

Fill in three values and click Calculate

Solved Variable

All Values Summary

Variable	Description	Value

Step-by-Step Solution

Summary

Enter any three of P1, T1, P2, T2 and instantly solve for the unknown using Gay-Lussac's Law at constant volume.

How it works

Enter values for three of the four variables: Initial Pressure (P1), Initial Temperature (T1), Final Pressure (P2), or Final Temperature (T2).
Leave exactly one field blank — that is the unknown the calculator will solve.
Select the appropriate unit for each pressure field (atm, kPa, mmHg, or psi) and each temperature field (K, °C, or °F).
Click Calculate to apply the formula P1 / T1 = P2 / T2.
The result is displayed with the solved value highlighted, plus a step-by-step solution breakdown.
Click Reset to clear all fields and start a new calculation.

Use cases

Solve chemistry homework problems involving gas pressure and temperature changes at constant volume.
Predict the pressure inside a sealed container when heated or cooled.
Calculate the temperature needed to reach a target pressure in a fixed vessel.
Verify lab results from constant-volume gas experiments.
Understand the direct relationship between pressure and absolute temperature for ideal gases.
Prepare for AP Chemistry, IB Chemistry, or university-level gas law exams.
Model autoclave or pressure cooker behavior where volume is constant.
Convert between pressure and temperature units while solving gas law problems.

Frequently Asked Questions

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Last updated: 2026-05-28 · Reviewed by Nham Vu