As of 2006, the most precise measurement of R had been obtained by measuring the speed of sound c(P, T) in argon at the temperature T of the triple point of water  at different pressures P, and extrapolating to the zero-pressure limit c(0, T). The value of R is then obtained from the relation:
where:
*γ is the heat capacity ratio ( for monatomic gases such as argon);
*T is the temperature, T = 273.16 K by the definition of the kelvin at that time;
*A(Ar) is the relative atomic mass of argon and M =  as defined at the time.
However, following the 2019 redefinition of the SI base units, R now has an exact value defined in terms of other exactly defined physical constants.