Volume is the dominant metaphor underlying the European Credit Transfer and Accumulation System (ECTS). Credits are used to 'express the volume of learning based on the defined learning outcomes and their associated workload', with the latter based on volume of student effort. But the convenience of volume can leave the first part overlooked, i.e. 'the defined learning outcomes' and in particular their relative importance or relative weight for achieving the overall programme outcomes. Paying attention to issues of relative weight and volume at module design stage is necessary to ensure overall programme balance and coherence. The second part of the article uses metaphor analysis to draw attention to this. Density, based on volume and weight, provides a more satisfactory metaphor for credit allocation, drawing attention to programme substance, which is what ultimately matters.