The equation and its constant
Ideal gas law
PV = nRT
P is pressure, V volume, n moles, T temperature in kelvin, and R the gas constant. R's numeric value depends on your units — 0.0821 L·atm/(mol·K) for atmospheres, 8.314 for kPa. This calculator converts everything to atm/L/K internally, so you never have to pick.
Worked example
What volume does 2.0 mol of gas occupy at 27 °C and 1.5 atm?
- Convert: 27 °C = 300.15 K.
- V = nRT ÷ P = (2.0 × 0.0821 × 300.15) ÷ 1.5
Answer: ≈ 32.9 L
💡 Tip: Sanity-check answers with the STP landmark: 1 mole of ideal gas ≈ 22.4 L at 0 °C and 1 atm. Two moles near room temperature should be somewhere in the 40–50 L range at 1 atm.
When the ideal gas law breaks down
The law treats molecules as size-less points with no attraction to each other. That's excellent near room conditions (within ~1%) but fails at high pressure (molecules crowd together) and low temperature (attractions dominate — gases liquefy). For schoolwork at ordinary conditions, PV = nRT is the right tool.
⚠ Common mistake: Temperature must be kelvin. Using 25 instead of 298 doesn't just shift the answer — it wrecks it by more than a factor of ten. This calculator converts °C and °F automatically, but on paper the conversion is on you.