Abstract In everyday life, we often estimate rather than know. It was the goal of this study to assess the effect of depressed mood on cognitive estimation in old age. Cognitive estimation was performed in 44 subjects with major depressive disorder (MDD; DSM-IV) and 48 age-matched healthy subjects (HS). Severity of depressive symptoms was rated with the Montgomery–Åsberg Depression Rating Scale (MADRS, mean = 18.6 ± S.D. 4.85). Estimation tasks comprised the dimensions length (coin diameter), weight (pile of paper), quantity (number of marbles in a glass jar), and time (estimation of time it takes for a marble to roll down a marble track both before and after having observed it). Other than the procedure followed in previous tests on cognitive estimation, the tasks were performed by observing objects rather than pictures thereof. MDD patients overestimated time (before and after observation) and underestimated quantity. Cognitive estimation was not correlated to measures of frontal functioning or semantic knowledge. We conclude that MDD patients in old age are impaired to some extent in cognitive estimation and in the ability to correct themselves, deficits that are likely to affect the performance of everyday activities.