Boat Draft Calculator

Archimedes, Applied to a Hull

A floating boat displaces exactly its own weight in water — the physical principle Archimedes described more than two thousand years ago. This calculator uses that relationship directly: it converts the boat's displacement mass into the volume of water it must push aside, then spreads that volume across the hull's waterline length, beam, and shape (block coefficient) to find how deep the hull sits.

The Formula

Block coefficient describes how "boxy" versus "fine" the underwater hull shape is, from near 1.0 (a barge-like rectangular cross-section) down toward 0.3–0.4 for a sharp, fine-entry hull; 0.5 is a common general-purpose default.

Worked Example

The same boat sits very slightly deeper in fresh water than in salt water, because fresh water is less dense and provides marginally less buoyant support for the same displaced volume — a real effect boaters notice when moving between the ocean and a freshwater lake or river.

Where This Matters

• Shallow-water and anchorage planning — comparing draft against a chart's depth soundings is a basic grounding-avoidance check, especially at low tide.
• Loading and trim — adding weight (fuel, water, gear, crew) increases displacement and therefore draft; this formula shows how much a given load will sink the hull.
• Moving between salt and fresh water — boats transiting from the ocean into a river or lake system sit marginally deeper, worth accounting for in tight channels.

How to Use This Calculator

1. Enter the Displacement (kg).
2. Enter the Waterline Length (m) and Beam (m).
3. Optionally adjust the Block Coefficient (0–1, defaults to 0.5).
4. Select Salt Water or Fresh Water.
5. Select Calculate to see the volume displaced and resulting draft.

Related Calculations

Check anchoring depth requirements with the Anchor Chain Length Calculator, or estimate hull speed with the Displacement Hull Speed Calculator.