Objective. To investigate the association between menstrual flow weight measured from modern sanitary pads (converting liquid to non-evaporating gel) and clinically assessed normal, increased or decreased menstrual flow. Design. Objective method development study. Setting. Outpatient clinic, University Hospital, Reykjavik. Population. One hundred and thirteen volunteers included 26 normally menstruating adult women and 52 normally menstruating teenagers not using oral or intrauterine contraception, seven normally menstruating women using oral contraception, 17 women with clinically diagnosed menorrhagia, five women using oral contraception for clinical menorrhagia, and six teenage girls claiming heavy menstrual flow. Methods. Menstruation length, menstrual flow weight and history of iron deficiency were assessed. During the menstruation following recruitment, all women collected their used protective pads in a hygienic manner and returned them to the laboratory for accurate weighing. Main outcome measures. Menstrual flow total weight measured in grams. Results. Mean menstrual flow total weight in the 78 asymptomatic women was 51 g (median 44, range 5-144). The mean flow in 17 women clinically diagnosed with menorrhagia was 217 g (median 207, range 63 - 402) (p<0.0001 compared to healthy women). The seven healthy women using oral contraceptives discharged 13 g (13-19) (p=0.0004 compared with normals). Menstruation lasted<eight days in 77/78 healthy women and in 12 of 17 clinically diagnosed menorrhagic women. Conclusions. Measurement of menstrual flow total weight accurately reflects clinically assessed normal, increased and decreased flow. The method is an easy and accurate way of objectively estimating menstrual flow.