What is the ideal gas law?

The ideal gas law is PV = nRT, where P is pressure, V is volume, n is the number of moles of gas, R is the universal gas constant (8.314 J/mol·K), and T is absolute temperature in Kelvin. It describes how an ideal gas behaves under varying conditions.

What is the value of the gas constant R?

The universal gas constant R = 8.314 J/(mol·K) = 8.314 Pa·m³/(mol·K). In other common units: R = 0.08206 L·atm/(mol·K) = 62.36 L·mmHg/(mol·K) = 8.314 L·kPa/(mol·K) × 0.001.

What are standard temperature and pressure (STP)?

STP is defined as 0°C (273.15 K) and 1 atm (101.325 kPa). At STP, one mole of an ideal gas occupies 22.414 liters. Some sources use SATP (25°C, 100 kPa) where one mole occupies 24.789 liters.

When does the ideal gas law fail?

The ideal gas law assumes gas molecules have no volume and no intermolecular forces. It is less accurate at very high pressures (where molecular volume matters) or very low temperatures (where intermolecular attractions are significant). Real gas behavior is better described by the van der Waals equation.

Why must temperature be in Kelvin?

The ideal gas law requires absolute temperature (Kelvin) because Celsius or Fahrenheit can be negative, which would give nonsensical negative pressures or volumes. Kelvin always starts at absolute zero (0 K = −273.15°C), so all values are non-negative.

How do I convert between pressure units?

1 atm = 101.325 kPa = 101,325 Pa = 760 mmHg = 760 torr = 14.696 psi. This calculator accepts atm, kPa, Pa, mmHg, and psi and converts automatically.

Ideal Gas Law Calculator

Name: Ideal Gas Law Calculator
Availability: InStock
Author: Nham Vu

Select the unknown variable, enter the other three, and instantly solve PV = nRT for pressure, volume, moles, or temperature.

PV = nRT Solver

Solve for:

Pressure (P)

Volume (V)

Amount (n)

Temperature (T)

Select what to solve for, enter the other three values, and click Calculate.

Pressure

—

atm

Equivalent Values

Common Gases at STP (0°C, 1 atm)

Gas	Molar Mass (g/mol)	Density at STP (g/L)

Copied!

Summary

Select the unknown variable, enter the other three, and instantly solve PV = nRT for pressure, volume, moles, or temperature.

How it works

Select which variable you want to solve for: Pressure, Volume, Moles, or Temperature.
Enter values for the other three variables and choose their units.
Click Calculate to get the result instantly.
View the result converted into multiple common units.
Use the gas reference table to look up standard conditions for common gases.

Use cases

Calculate the pressure inside a closed container after heating a gas.
Determine the volume a gas will occupy at a given temperature and pressure.
Find the number of moles in a gas sample from its pressure, volume, and temperature.
Solve chemistry and physics homework problems involving ideal gases.
Convert between pressure and volume units for engineering calculations.
Estimate temperature changes in gas-filled systems.
Check gas behavior against STP (standard temperature and pressure) conditions.
Teach or learn ideal gas law concepts interactively.

Frequently Asked Questions

Last updated: 2026-06-13 · Reviewed by Nham Vu