Buoyancy calculator
Buoyancy is the upward force water exerts on anything in it. This calculator gives the buoyant force, whether the object floats or sinks, and the weight it appears to have in water. Use Any object for gear, weights and tanks, or Me in the water to see how much you weigh in water with lungs full or empty.
How to calculate buoyancy
Apparent weight = mass − (fluid density × volume)
This is Archimedes’ principle. A 10 L object fully under seawater (1025 kg/m³) is pushed up with the force of 10.25 kg. If it has a mass of 12 kg it sinks with an apparent weight of 1.75 kg. A buoyancy calculator like this one does that arithmetic for you.
How much do you weigh in water?
With a full breath most people float in seawater by a few kilograms; exhaled, many sink slowly. That is exactly what a buoyancy check relies on: float at eye level with an empty BCD and a normal breath, then exhale and sink slowly. Fat floats and muscle sinks, which is why body build changes the result.
How many pounds of buoyancy do I need?
Your BCD must be able to float you at the surface at the start of the dive, when the tank is full: it has to lift the lead you carry plus any gear that is heavier than water. Check the lift rating for your BCD size and leave a margin. Ask your instructor if you are unsure.
Common questions
How do you calculate buoyancy?
Multiply the density of the fluid by the volume displaced and by gravity. Compare that force with the weight of the object to see if it floats.
Why do divers float in seawater but not always in lakes?
Seawater is about 2.5 percent denser than freshwater, so it pushes up harder on the same volume.
Is this a calculator for BCD lift?
No. It computes buoyant force from mass and volume. Use your BCD manufacturer lift rating for BCD capacity.
