Gas Constant (R) Calculator
The universal gas constant, R = 8.314462618 J/(mol·K), changes numeric value depending on your unit system. Pick your pressure, volume, and amount units below and see the matching value of R instantly.
R Unit Converter
Common Values of R
The gas constant is one of the most-quoted numbers in chemistry and physics, and because it can be expressed in so many different unit combinations, students often memorize several versions of it without realizing they are all the same underlying constant. The table beside this text lists the values you will most commonly need.
In SI units, R = 8.314462618 J/(mol·K). This is the value to use whenever pressure is in pascals and volume is in cubic meters, and it is the value this site's ideal gas law calculator uses internally regardless of which units you actually type in, since it always converts to SI before solving.
In chemistry classrooms, R = 0.082057 L·atm/(mol·K) is by far the most commonly used form, since gas volumes are usually measured in liters and pressures in atmospheres.
| Value of R | Units |
|---|---|
| 8.314462618 | J/(mol·K) |
| 8.314462618 | Pa·m³/(mol·K) |
| 0.082057 | L·atm/(mol·K) |
| 62.363 | L·mmHg/(mol·K) |
| 83.14462618 | L·mbar/(mol·K) |
| 1.987204 | cal/(mol·K) |
| 8.314462618 × 10⁻² | L·bar/(mol·K) |
Worked Example: Using R to Find Pressure
Problem: 0.75 mol of nitrogen gas is held in a 2.00 L cylinder at 310 K. What is the pressure inside the cylinder?
Notice that choosing R = 0.082057 L·atm/(mol·K) instead of 8.314462618 J/(mol·K) avoided an extra unit conversion step entirely, since volume was already in liters and the answer came out directly in atmospheres. Picking the right form of R for the units you already have is the whole point of this calculator, try the same problem with the ideal gas law calculator to see it solved with automatic unit conversion instead.
Common Mistakes When Using the Gas Constant
The single most common error with R is mismatched units, using R = 8.314462618 J/(mol·K) while pressure is in atm and volume is in liters, or using R = 0.082057 L·atm/(mol·K) while volume is actually in cubic meters. Because R carries its own implicit units, plugging the wrong version into PV = nRT produces an answer that is wrong by whatever conversion factor was missed, often by a large, easy-to-miss margin like a factor of 100 or 1,000.
A second mistake is forgetting that R's numeric value changes with the amount unit too, not just pressure and volume. R per kilomole is 1,000 times larger than R per mole, since a kilomole represents 1,000 times more gas particles for the same measured pressure and volume. If a problem gives you moles but you accidentally use a kmol-based R (or vice versa), the resulting pressure or volume will be off by exactly that factor of 1,000.
A third pitfall is treating R as if it varies between different gases. It does not. R is identical for helium, nitrogen, carbon dioxide, or any other (approximately ideal) gas. What changes between gases is molar mass, which is a completely separate quantity used to convert between moles and grams, not a substitute for or variant of R itself.
Finally, remember that R only appears in the ideal gas law and its direct derivatives (like the Arrhenius or Nernst equations). It does not appear in the standalone forms of Boyle's, Charles', Gay-Lussac's, or Avogadro's Laws, or in the standard combined gas law, since R algebraically cancels out of all of those comparisons between two states of the same fixed amount of gas.