Ideal Gas Law Units
Chemistry Tools | May 20, 2026 | 8 min read
How to use PV = nRT without unit mistakes in pressure, volume, temperature, and gas constant.
| Variable | Meaning | Common unit |
| P | Pressure | atm |
| V | Volume | L |
| n | Moles | mol |
| T | Temperature | K |
| R | Gas constant | 0.08206 L atm mol^-1 K^-1 |
The Equation
The ideal gas law connects pressure, volume, moles, and temperature. It works best for gases at relatively low pressure and high temperature where particle volume and intermolecular forces are small.
The equation is simple, but unit consistency is everything. Choose R to match pressure and volume units.
Temperature Must Be Kelvin
Always convert Celsius to kelvin before using PV = nRT. Kelvin equals Celsius plus 273.15. Using Celsius directly is one of the most common wrong-answer sources.
Temperature on the kelvin scale is proportional to average kinetic energy, which is why the gas law needs kelvin.
Choosing R
If pressure is in atm and volume is in liters, use R = 0.08206 L atm mol^-1 K^-1. If pressure is in kPa, use a matching R value or convert pressure first.
A quick unit check can save the problem: after rearranging, units should cancel to the variable you are solving for.
Useful Site Tools
Use the Ideal Gas Law Calculator for pressure, volume, mole, and temperature problems with unit conversion support.