What is displacement tonnage?

Displacement tonnage is the mass of water displaced by a floating vessel, which equals the mass of the vessel itself (by Archimedes' principle). It is measured in metric tons (1 metric ton = 1,000 kg) or long tons (1 long ton ≈ 1,016 kg). Full load displacement includes the hull, machinery, fuel, crew, cargo, and stores.

What is the block coefficient (Cb)?

The block coefficient is the ratio of the underwater hull volume to the bounding rectangular box (L × B × T). A Cb of 1.0 would be a perfect rectangular barge. Typical values range from 0.50 for fast naval vessels to 0.85 for slow bulk carriers. The formula is: Volume = LBP × B × T × Cb.

What is the difference between light ship and full load displacement?

Light ship displacement (also called lightship weight or LWT) is the mass of the vessel with all its permanent equipment but without cargo, fuel, stores, or crew. Full load displacement adds the deadweight tonnage (DWT) — the total carrying capacity — on top of the light ship weight.

Why is salt water density 1.025 t/m³?

Standard ocean salt water has a salinity of about 3.5%, giving a density of approximately 1.025 metric tons per cubic meter (1,025 kg/m³). Fresh water is 1.000 t/m³. A vessel will sit slightly deeper in fresh water because it must displace a larger volume to achieve the same upward buoyant force.

What is deadweight tonnage (DWT)?

Deadweight tonnage is the maximum load a vessel can carry, including cargo, fuel, ballast water, fresh water, provisions, and crew. It equals the difference between full load displacement and light ship displacement. DWT is the primary commercial capacity measure for cargo ships.

Can this calculator look up the deadweight of a specific ship?

No. It holds no vessel database. A ship's deadweight and lightship come from its particulars, class register or stability book. Once you know the summer DWT and displacement for the same draft, enter them here to see the lightship share.

How do I convert tanker deadweight into cubic meters of cargo?

Deadweight is a mass, so divide the cargo part of it by the cargo density. For example, 30,000 t of cargo at 0.85 t/m³ fills about 35,294 m³. Remember that DWT also covers fuel, fresh water, stores and crew, so not all of it is cargo. This calculator does not do that step.

How do you calculate TPC (tonnes per centimetre immersion)?

TPC = waterplane area × water density ÷ 100. The waterplane area is Cwp × L × B; for a 180 m × 30 m ship with Cwp 0.827 it is 4,464 m², so TPC in seawater is 4,464 × 1.025 ÷ 100 = 45.76 t/cm. Loading 457.6 t would sink that ship about 10 cm. In imperial units, TPI (long tons per inch) = waterplane area in ft² ÷ 420 for seawater. TPC changes with draft, so use the hydrostatic table value for real loading.

What is the fresh water allowance (FWA) and how is it calculated?

FWA is how much deeper a ship floats when it moves from seawater (1.025 t/m³) to fresh water (1.000 t/m³) at the same displacement. FWA (mm) = summer displacement (t) ÷ (4 × TPC in seawater). For 45,054.9 t and TPC 45.76 that is about 246 mm. In brackish dock water the dock water allowance is FWA × (1.025 − dock density) ÷ 0.025.

How do I calculate displacement in long tons from feet?

Multiply length × beam × draft × Cb in feet to get the underwater volume in ft³, then divide by 35 for seawater (35 ft³ of seawater weigh one long ton) or by about 35.9 for fresh water. A 500 × 70 ft hull at 25 ft draft with Cb 0.60 has 525,000 ft³, about 15,000 long tons. Choose Imperial units here to enter feet and read long and short tons directly.

How accurate is this calculator?

The formula is Displacement (t) = LBP × B × T × Cb × ρ. Its reliability depends entirely on using a block coefficient that matches the same draft and hull condition. Real hull geometry, appendages and trim are simplified, so use approved hydrostatic data for operational or regulatory work.

Ship Displacement Tonnage Calculator

Enter LBP, breadth, mean draft and block coefficient in metres or feet to get displacement in metric, long and short tons, plus TPC/TPI and the fresh water allowance. Displacement is weight, not Gross Tonnage (GT).

Displacement (t) = LBP × breadth × mean draft × block coefficient × water density, using 1.025 t/m³ for seawater.

Hull Principal Dimensions

Water density in t/m³ in both unit systems.

Units

Length Between Perpendiculars — LBP (m)

Moulded Breadth — B (m)

Mean Draft at Condition — T (m)

Block Coefficient — C_b (0.40 – 0.95)

Typical: 0.50 fast vessel · 0.65 container · 0.74 tanker · 0.82 bulk carrier

Waterplane Coefficient — C_wp (optional, for TPC)

Water Density — ρ (t/m³)

Known Deadweight Tonnage — DWT (t) (optional)

Must correspond to the same entered draft. Estimated lightweight = displacement at condition − DWT.

Required Displacement (t) (optional, solves draft)

Vessel Type Presets

Enter hull dimensions to calculate displacement tonnage.

Displacement at Entered Draft

—

(— long tons)

LBP × B × T × C_b × ρ

Hull Volume

—

m³

Displacement

—

metric tons

Displacement

—

long tons

Displacement

—

short tons

Immersion & Fresh Water Allowance

Waterplane area

—

TPC (t/cm)

—

TPI (long tons/in)

—

FWA

—

Formula Used

—

Displacement (t) = LBP × B × T × C_b × ρ
Long tons = metric tons ÷ 1.01605 · short tons = metric tons ÷ 0.90718
TPC (t/cm) = C_wp × L × B × ρ ÷ 100 · FWA (mm) = Δ ÷ (4 × TPC)

Displacement is not Gross Tonnage. Displacement and DWT are mass measures. GT and NT are dimensionless regulatory measures derived from enclosed-space volumes under the IMO tonnage convention. Use approved hydrostatic data and survey records for vessel decisions. IMO tonnage reference

Copied!

Quick answer

Displacement (t) = LBP × breadth × mean draft × block coefficient × water density, using 1.025 t/m³ for seawater. Displacement also equals lightship weight plus deadweight, so lightship = displacement − DWT at the same draft.

Example: A 180 m × 30 m hull at 11 m draft with Cb 0.74 displaces 43,956 m³ × 1.025 = 45,054.9 t (44,343.3 long tons). With an estimated Cwp of 0.827 its TPC is 45.76 t/cm, so the fresh water allowance is 45,054.9 ÷ (4 × 45.76) ≈ 246 mm; with 33,000 t deadweight the lightship is 12,054.9 t.

Enter hull dimensions in metres or feet for displacement, TPC, FWA and lightship, then copy the result.

Summary

The calculator estimates underwater volume as LBP × breadth × mean draft × block coefficient, then multiplies by water density to estimate displacement at the entered condition, in metric tons, long tons (1,016.05 kg) and short tons (907.18 kg). From the waterplane area (Cwp × L × B) it gives tonnes per centimetre immersion, TPC = Awp × ρ ÷ 100, long tons per inch, and the fresh water allowance FWA = Δ ÷ (4 × TPC). It can also solve the draft for a required displacement and, if deadweight for that same condition is known, estimate lightweight by subtraction. It does not calculate regulatory Gross Tonnage or replace a vessel hydrostatic table, stability book, survey, or inclining test.

How it works

Choose metric (metres, tonnes) or imperial (feet, long tons) units.
Enter the Length Between Perpendiculars (LBP), moulded breadth (B) and mean draft (T).
Enter the block coefficient (Cb) — typically 0.50 to 0.85 for commercial vessels — and, if known, the waterplane coefficient (Cwp) for TPC.
Optionally adjust the water density (1.025 t/m³ for salt water, 1.000 for fresh water).
Enter deadweight only if it belongs to the same draft/loading condition, or leave it blank.
Optionally enter a required displacement to solve the mean draft that gives it.
Click Calculate to see underwater volume, displacement, TPC/TPI and the fresh water allowance at the entered draft.

Use cases

Preliminary hull weight estimation during concept design.
Checking preliminary dimensions against a known displacement before using hydrostatic tables.
Teaching naval architecture students the volumetric displacement formula.
Converting between metric tons and long tons for international cargo documentation.
Exploring how draft, density, and block coefficient affect a preliminary displacement estimate.
Evaluating block coefficient impact on cargo capacity for bulk carrier design.
Quick sanity-check of displacement figures in ship sale and purchase negotiations.
Comparing displacement at different draft values for ballast condition planning.

Frequently Asked Questions

Last updated: 2026-10-09 · Reviewed by Nham Vu

Ship Displacement Tonnage Calculator

Hull Principal Dimensions

Immersion & Fresh Water Allowance

Draft for Required Displacement

Weight Breakdown

Formula Used

Quick answer

Summary

How it works

Use cases

Frequently Asked Questions

Ship Displacement Tonnage Calculator

Use the Ship Displacement Tonnage Calculator

Hull Principal Dimensions

Immersion & Fresh Water Allowance

Draft for Required Displacement

Weight Breakdown

Formula Used

Quick answer

Summary

How it works

Use cases

Frequently Asked Questions