Block Coefficient Calculator
Calculate a hull's block coefficient (Cb) from displacement, waterline length, beam and draft — and see what it says about hull fullness.
What is block coefficient?
Block coefficient (Cb) describes how "full" a hull is — the ratio of the underwater volume to the box that would just enclose it (length × beam × draft at the waterline). A fine, wedge-shaped racing hull has a low Cb; a boxy cargo barge has a high one. It's a fundamental figure in naval architecture for comparing hull form, resistance and carrying capacity.
The block coefficient formula
Cb = ∇ ÷ (LWL × B × T)
where ∇ (nabla) is the displaced volume of water, and LWL, B and T are the waterline length, beam and draft. This calculator converts your displacement in tonnes to a volume using water density (1.025 t/m³ salt, 1.0 t/m³ fresh) before applying the formula.
Typical block coefficients
| Vessel type | Cb |
|---|---|
| Sailing yacht / planing boat | 0.35–0.45 |
| Motor cruiser / fast ferry | 0.45–0.55 |
| Trawler / workboat | 0.55–0.70 |
| Bulk carrier / barge | 0.70–0.85 |
Why it matters
A low Cb hull slips through the water with less resistance and suits higher speeds; a high Cb hull carries more load for its length but pushes more water. Pair this with the displacement-length ratio, hull volume and hull speed calculators to build a full picture of a hull's behaviour.
Note: use waterline dimensions, not overall length or maximum beam, for an accurate Cb.
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See what's insideFrequently Asked Questions
What is a block coefficient?
Block coefficient (Cb) is the ratio of a hull's underwater volume to the rectangular block that would just enclose it (waterline length × beam × draft). It measures how full or fine the hull is — low for sleek fast hulls, high for boxy load-carriers.
How do you calculate block coefficient?
Cb = displaced volume ÷ (LWL × beam × draft), all measured at the waterline. If you know displacement in tonnes, divide by water density first (1.025 t/m³ for salt water, 1.0 for fresh) to get the volume in cubic metres, then apply the formula.
What is a typical block coefficient for a boat?
Sailing yachts and planing boats run about 0.35–0.45, motor cruisers and fast ferries 0.45–0.55, trawlers and workboats 0.55–0.70, and bulk carriers or barges 0.70–0.85. A lower Cb generally means less resistance and higher speed potential for the length.