The most frequently introduced forms are:where:
*  is the absolute pressure of the gas,
*  is the volume of the gas,
*  is the amount of substance of gas (also known as number of moles),
*  is the ideal, or universal, gas constant, equal to the product of the Boltzmann constant and the Avogadro constant,
*  is the Boltzmann constant,
*  is the Avogadro constant,
*  is the absolute temperature of the gas,
*  is the number of particles (usually atoms or molecules) of the gas.
In SI units, p is measured in pascals, V is measured in cubic metres, n is measured in moles, and T in kelvins (the Kelvin scale is a shifted Celsius scale, where 0.00 K = −273.15 °C, the lowest possible temperature). R has for value 8.314 J/(mol·K) = 1.989 ≈ 2 cal/(mol·K), or 0.0821 L⋅atm/(mol⋅K).