Buoyancy Calculator (Archimedes)
Weight ↔ displaced volume in salt or fresh water: the principle that floats everything, with the per-inch-of-immersion figure boat loaders need.
Archimedes, operationally: every cubic foot pushed underwater pushes back with 62–64 pounds. A boat loaded with 640 lb of gear sinks exactly 10 ft³ deeper into salt water — spread over the waterplane, that's the inches-of-freeboard you gave away.
Formula
⚠️ For planning and education only — verify with official charts, tide tables and your vessel's documentation. Not for navigation.
Weight ↔ displaced volume in salt or fresh water: the principle that floats everything, with the per-inch-of-immersion figure boat loaders need.
About Buoyancy Calculator (Archimedes)
One principle floats canoes, container ships and offshore platforms: immersed volume pushes back with the weight of the water it displaces — 64 pounds per cubic foot in the sea, 62.4 in the lake. This calculator runs Archimedes in both directions (force from volume, or volume demanded by a weight), in the units boaters and engineers actually mix, with the freeboard intuition attached: loading is literally buying immersion by the cubic foot.
How to use Buoyancy Calculator (Archimedes)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula buoyant force = displaced volume × water density (64 lb/ft³ salt, 62.4 fresh) substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Buoyancy Calculator (Archimedes)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula buoyant force = displaced volume × water density (64 lb/ft³ salt, 62.4 fresh) with sources cited on the page
- ✓Archimedes, operationally: every cubic foot pushed underwater pushes back with 62–64 pounds. A boat loaded with 640 lb of gear sinks exactly 10 ft³ deeper into salt water — spread over the waterplane, that's the inches-of-freeboard you gave away.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
How does pounds-per-inch of immersion work?+
Multiply your boat's waterplane area by water density per inch: a hull with 120 ft² of waterline plane sinks one inch for every 120 × (64/12) ≈ 640 lb loaded aboard. It's the single most useful loading number a skipper can know — and why the same gear visibly squats a narrow boat that a beamy one shrugs at. (Ships formalize it as TPI/TPC on their load documents.)
Why do boats float lower in fresh water?+
The 2.5% density gap: fresh water pushes back 62.4 lb/ft³ against salt's 64, so the same weight needs 2.5% more immersed volume. A loaded ship moving from sea to river visibly sinks — the Plimsoll mark's separate F (fresh) line above S (summer salt) legislates exactly this physics. Great Lakes freighters live their whole lives on the F line.
How can steel ships float when steel sinks?+
Shape is the cargo: the hull encloses mostly air, so the SHIP's average density (steel shell + air volume) is far below water's. She sinks until the displaced water's weight equals her total weight — Archimedes satisfied with most of the steel above the waterline. Breach the shell and let water replace the air, and the average density argument runs the other way; that's all sinking is.
Does buoyancy change with depth?+
Essentially no, for incompressible water: the force depends on displaced volume and density, both nearly constant with depth. (Submarines' real depth problem is hull compression reducing THEIR volume slightly, and seawater density layers.) For surface craft, the operative variables are exactly this tool's two: how much volume you immerse, and which water you're in.
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