The aim of this paper is to calculate the contribution tax rate that equals expected present values of contributions and old-age benefits in a social security system. A theoretical exercise is developed to generate an analytical formula for the contribution rate, complemented for an empirical analysis, using Brazilian data. The tax rate is calculated in diverse situations, including differences of gender, educational level and type of retirement. The main conclusion is that contribution rates around 31%, as the observed in Brazil, are too high for retirement by contribution time, but insufficient for the retirement by age, especially for women.
