Redundant use of contraception occurs when periods of contraceptive use overlap with periods of reduced fecundity, and will downwardly bias estimates of contraceptive failure rates. This paper investigates this bias using calendar data from the Demographic and Health Surveys. The paper presents unadjusted and adjusted 12-month failure rates for each of nine countries. The impact of redundant use on failure rates is generally modest. It tends to be greater in Indonesia, however, where both the incidence and the duration of overlap are relatively large.