Estimate displacement from LWL, beam, draft, and block coefficient Cb. Volume = L×B×T×Cb; mass = volume × water density (64 lb/ft³ salt or 62.4 fresh). Preliminary only.
Reach for the displacement from dimensions calculator when a planning number depends on measurements you already have on the boat or trailer. Enter lwl (ft), beam (ft), draft (ft), and block coefficient cb.
Estimate displacement from LWL, beam, draft, and block coefficient Cb.
∇ (ft³) = LWL × Beam × Draft × Cb. Δ_lb = ∇ × density. Long tons ≈ Δ_lb ÷ 2240.
∇ = LWL × B × T × Cb
Δ_lb = ∇ × ρ (ρ = 64 salt or 62.4 fresh)
LT ≈ Δ_lb ÷ 2240
Cb must be yours or from hydrostatics — not invented.
Start with the calculator’s default entries: LWL (ft) = 30; Beam (ft) = 10; Draft (ft) = 4.5; Block coefficient Cb = 0.45; Water = Salt ≈ 64 lb/ft³.
With those entries the calculator returns:
38,880 lb
∇ 607.5 ft³ · ≈ 17.36 LT
∇ = 30 × 10 × 4.5 × 0.45 = 607.50 ft³
Δ = 607.50 × 64 = 38880 lb
LT ≈ 38880 ÷ 2240 = 17.357
Formula used: ∇ = LWL × B × T × Cb Δ_lb = ∇ × ρ (ρ = 64 salt or 62.4 fresh) LT ≈ Δ_lb ÷ 2240
One input at a time. Raise LWL (ft) from 30 to 45 and leave everything else alone. The result becomes:
58,320 lb
∇ 911.3 ft³ · ≈ 26.04 LT
∇ = 45 × 10 × 4.5 × 0.45 = 911.25 ft³
Δ = 911.25 × 64 = 58320 lb
LT ≈ 58320 ÷ 2240 = 26.036
Block coefficient = underwater volume ÷ (LWL × beam × draft). Fine racing hulls are lower; full displacement hulls higher.
This is a first-cut estimate. Real displacement needs offsets or a known hydrostatics package.
Use the draft that matches the load condition you care about. Light-ship vs full load changes Δ.