Ideal Gas Law Calculator
Pick the unknown, enter the other three in whatever units your problem uses.
Using PV = nRT
Convert temperature to kelvin, pick an R that matches your pressure and volume units (or let the calculator convert), rearrange for the unknown, and substitute. Example: 2.00 mol of gas at 300 K in a 10.0 L flask: P = 2.00 × 0.08206 × 300 ÷ 10.0 = 4.92 atm.
The same law gives gas density, ρ = PM ÷ RT, and molar mass from a measured density, M = ρRT ÷ P. For mass-based problems, the molar mass calculator gives M from a formula.
Questions students ask
What is the ideal gas law?
PV = nRT. P is pressure, V volume, n moles of gas, T absolute temperature in kelvin, and R the gas constant: 8.314 J/(mol·K), or 0.08206 L·atm/(mol·K).
Which value of R should I use?
Match R to your units. With litres and atmospheres use 0.08206 L·atm/(mol·K); with kPa and litres use 8.314 L·kPa/(mol·K); with pascals and cubic metres use 8.314 J/(mol·K). The calculator converts everything to SI internally, so you can mix units freely.
Why must temperature be in kelvin?
The law is proportional to absolute temperature. At 0 °C a gas does not have zero volume; at 0 K (−273.15 °C) it would. Add 273.15 to a Celsius value.
What is molar volume at STP?
At IUPAC STP (0 °C, 100 kPa) one mole of ideal gas occupies 22.71 L. With the older definition (0 °C, 1 atm) it is 22.41 L. At 25 °C and 1 atm it is 24.47 L.
When does the ideal gas law fail?
At high pressures and low temperatures, when molecules are close together and attract each other. Real gases then deviate, and equations such as van der Waals are used.