Free Surface Effect Calculator

Calculate free surface moment and GM reduction for rectangular tanks.

Tank Length (m)

Tank Breadth (m)

Liquid Density (t/m³)

Ship Displacement (tonnes)

Free Surface Moment —

GM Reduction (GG₁) —

Formula (rectangular tank)

i = (L × B³) / 12

Free Surface Moment = i × density

GG₁ (GM reduction) = Free Surface Moment / Displacement

Useful for stability officers calculating the effect of slack tanks on GM.