Fresh water allowance or FWA. This is the change in draught of a vessel when it moves from salt water (density 1.025 tonnes/cu.m) to fresh water (1 tonne/cu.m). The FWA for a vessel for a draught at or near the summer loadline can be calculated by the formula:
Displacement / 4 x TPC
where, TPC = the tonnes per centimetre immersion.