OBJECTIVE To demonstrate a practical approach to describing the threshold of mental capacity. METHODS A retrospective analysis of 62 cases of assessments for capacity to manage property was carried out to record the type and frequency of errors. RESULTS For items on the Mini-Mental State Examination (MMSE), there was no clear demarcation of items associated with a judgement of capable. On the clinical criteria, several items almost completely distinguished capable from incapable subjects. CONCLUSIONS The permissible error approach makes the threshold of capacity explicit. It is well suited to the emphasis put on functional performance in the evaluation of mental capacity.