About Ideal Gas Law Calculator
PV = nRT ties together the four quantities that describe a gas's state: pressure, volume, the amount present (in moles) and temperature. It's an approximation — real gas molecules have volume and attract each other slightly, neither of which this model accounts for — but it's an extremely good one for most everyday conditions, and the standard starting point for any gas calculation.
The gas constant R depends entirely on which units the other four quantities are expressed in, and mixing units without converting first is the single most common ideal gas law mistake — using atmospheres for one calculation and kPa for the next, without swapping which R value goes with it, silently produces a wrong answer that still looks plausible. This fixes the units throughout — atmospheres, litres, moles, kelvin — specifically so that mismatch can't happen.
Temperature has to be in kelvin, not Celsius or Fahrenheit, because the relationship is proportional (double the kelvin temperature, double the pressure at fixed volume and amount) and that only holds on a scale where zero actually means zero — Celsius's zero is arbitrary, kelvin's is absolute zero. Convert with +273.15 before entering a Celsius temperature here.
How to use the ideal gas law calculator
Enter three of the four values
Pressure (atm), volume (L), amount (mol), temperature (K).
Convert temperature to kelvin first
°C + 273.15 = K.
Read the fourth value
Calculated directly from the other three.