An empirical formula for semiconductive metal oxides is proposed relating nonstoichiometric value x to a temperature or an oxygen partial pressure such that experimental data can be represented more accurately by the formula than by the well-known Arrhenius-type equation. The proposed empirical formula is log x = A + $B{cdot}1000/T;+;C{cdot}$exp$(-D{cdot}1000/T)$ for a temperature dependence and $log;{ imes};=a;+b{cdot}log;Po_2;+;c{cdot}$exp$(-d{cdot}log;Po_2)$ for an oxygen partial pressure dependence. The A, B, C, D and a, b, c, d are parameters which are evaluated by means of a best-fitting method to experimental data. Subsequently, this empirical formula has been applied to the n-type metal oxides of $Zn_{1+x}O,; Cd_{1+x}O,;and;PrO_{1.8003-x}$, and the p-type metal oxides of $CoO_{1+x},; FeO_{1+x},;and;Cu_2O_{1+x}$. It gives a very good agreement with the experimental data through the best-fitted parameters within 6% of relative error. It is also possible to explain approximately qualitative characters of the parameters A, B, C, D and a, b, c, d from theoretical bases.