This invention relates to a method and a circuit for estimating the bit error rate in a data transmission system. Symbols are detected (u) by a maximum likelihood detector (1), which provides path metrics of the decoded path and the best competitor at predetermined symbol positions. Absolute path metric differences (2) are calculated between the decoded path and the best competitor at said predetermined symbol positions. Events (5) are counted when an absolute path metric difference (2) is equal to one of a set of difference values. The bit error rate is estimated based on the number of counted (4) events (5). The invention further comprises a method and a circuit in which a function is applied onto said absolute path metric difference. The function maps quantized logarithms of probabilities to probabilities.