An empirical rule, the 10°C rule, states that chemical reaction rates are doubled for every 10°C temperature increase. This is often used in thermally accelerated medical device polymer aging studies. Here, theoretical evidence and limitations for the rule are analyzed. Thus, a new more accurate rule based on averaging Arrhenius chemical reaction rate ratios over typical activation energies 0.1 eV-0.9 eV in the normal medical device accelerated test temperature interval 25°C-70°C is proposed. Comparison with the 10°C rule shows that the 10°C rule provides similar estimates, but only at the reference temperature 25°C. Fitting the reaction rate ratio based on the Arrhenius equation using the reference temperature 25°C to the 10°C rule data reveals that best agreement is achieved with a thermal aging activation energy of 0.67 eV.