The variable that determines which "R" value to use in a calculation is pressure (P). The ideal gas law equation, PV = nRT, includes the universal gas constant (R), which has different values depending on the units used for pressure. If pressure is measured in atmospheres (atm), the value of R is 0.08206 L·atm/mol·K. If pressure is measured in pascals (Pa), the value of R is 8.314 J/mol·K. Similarly, if pressure is measured in other units such as torr, bar, or psi, a different value of R should be used. Therefore, the choice of "R" value to use in a calculation depends on the units of pressure being used in the problem.