The Fresh Water Allowance is the number of millimetres by which the mean draft changes when a ship passes from salt water to fresh water, or vice versa, whilst floating at the loaded draft.
FWA(in mm) = Displacement (in tonnes)/(4*TPC)
When the ship is loading in dock water which is of a density between these two limits ‘S’ may be submerged such a distance that she will automatically rise to ‘S’ when the open sea and salt water is reached. The distance by which ‘S’ can be submerged, called as Dock Water Allowance.
DWA =[ FWA(1025- ρdw)]/25