Freeboard is the distance from the waterline to the deck edge at the lowest point of the hull, measured amidships. Greater freeboard means more reserve buoyancy and improved seakeeping.

What is the difference between Type A and Type B?

Type A applies to vessels carrying liquid cargo in bulk (tankers) with inherently low permeability. Type B applies to all other vessels. Type A ships are allowed lower freeboard because flooding risk is reduced by enclosed cargo tanks.

What is block coefficient (Cb)?

Cb is the ratio of the displaced volume to the product of LBP × B × T. A full-form tanker might have Cb ≈ 0.85; a fine-form sailing yacht might be 0.35. Values outside 0.60 are corrected by a multiplier per ILLC rules.

How accurate is this calculator?

It provides a close approximation based on the simplified ILLC tabular method. It does not include all corrections (deck line, bow height, sheer) and should not replace a formal load line survey or classification society calculation.

What units does the calculator use?

Input lengths are in meters. The output minimum freeboard is in millimeters, which is the conventional unit used in load line certificates.

Freeboard Calculator

Name: Freeboard Calculator
Availability: InStock
Author: Nham Vu

Calculate minimum freeboard for a vessel based on its length, depth, and load line rules.

Vessel Dimensions

Length Between Perpendiculars — L (m)

Measured from stem to rudder post. Min 24 m for ILLC tables.

Moulded Breadth — B (m)

Moulded Depth — D (m)

Vertical distance keel to upper deck.

Block Coefficient — Cb

Typical range: 0.60 (fine) – 0.85 (full tanker).

Superstructure Effective Length (% of L)

0 = no superstructure credit; 100 = full-length superstructure.

Freeboard Type

Enter vessel dimensions and click Calculate.

Summary

Calculate minimum freeboard for a vessel based on its length, depth, and load line rules.

How it works

Enter the vessel's length between perpendiculars (LBP), moulded breadth (B), and moulded depth (D).
Enter the block coefficient (Cb) — the ratio of displaced volume to the bounding box — typically 0.60–0.85 for most hulls.
Enter the superstructure effective length as a percentage of LBP (0 if none).
Select the freeboard type: Type A (tankers / low permeability), Type B (general cargo / others).
Click Calculate. The tool applies the ILLC tabular baseline, Cb correction, D/L correction, and superstructure deduction to produce minimum freeboard in millimeters.

Use cases

Preliminary design check for new vessel construction.
Estimating load line position for stability studies.
Educational reference for naval architecture students.
Comparing Type A vs Type B freeboard requirements for a given hull.
Verifying that a proposed loaded draft meets minimum safety margins.
Planning cargo loading limits before a full classification society review.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu