A new method was proposed for calculating reaction rates using the Weibull distribution, which represents the reliability of items in reliability engineering. First, the Weibull distribution was linearized, and parameters were determined by the least squares method. Next, to obtain a reaction rate, the equation obtained by multiplying the Weibull distribution by the initial concentration of the reactant was differentiated with respect to the reaction time. This method was applied to the hydrolysis of ethyl acetate and the hydrolysis of sucrose using a typical pseudo-primary reaction with an acid catalyst. It was found that the coefficient of determination of the regression equation between the logarithm of the reaction rate and the logarithm of the concentration of the reactant calculated by the method was close to 1, indicating high linearity. Furthermore, the order of the reaction determined from the regression analysis was between 0.973 and 1.05, which was close to 1. The reaction rate calculation method using the Weibull distribution is therefore effective for pseudo-primary reactions.