About the Buoyancy Calculator
An object in water feels an upward push equal to the weight of the water it pushes aside. That is Archimedes' principle. If the push is larger than the object's weight, the object floats. This calculator takes the mass and volume of the object and the fluid it is in, and tells you whether it floats or sinks, the buoyant force, and how much of a floating object sits below the surface. Type the mass, the volume and the fluid and the buoyant force appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Buoyancy Calculator
- 1Enter the mass, the volume and the fluid.
- 2Pick the units or options from the menus if the buoyant force tool offers them.
- 3Read the buoyant force in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the buoyant force
Boat builders, divers and students use buoyancy to size floats, decide how much ballast a diver needs, and understand why steel ships float while a steel block sinks: the hull encloses a lot of air so the average density is below water.
Sea water is denser than fresh water, so objects float a little higher in the sea. The calculator includes several fluids, including mercury, in which even iron floats, and air, which explains how balloons and airships rise.
How the buoyant force is worked out
Buoyant force = fluid density × gravity × volume of fluid displaced. An object floats if its average density (mass ÷ volume) is lower than the density of the fluid. A 10 kg object with a volume of 2 litres has a density of 5,000 kg/m³ and sinks in water: the buoyant force is 19.6 N against a weight of 98.1 N, so a net 78.5 N pulls it down. A 0.5 kg object of 1 litre floats with 50.2 percent of it submerged.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Six fluids
- Fraction submerged
- Net force shown
Mistakes to avoid with buoyant force
The volume to use is the whole volume of the object, including any hollow parts that stay dry. A boat's displacement depends on the hull below the waterline, not on the material. For an object that floats, the buoyant force simply equals its weight.
On phone and computer
On a phone, type into the boxes one after another and read the buoyant force underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the buoyant force move. The buoyancy calculator works the same on both. Buoyancy depends on density and volume, so check them with Density Calculator and Volume Calculator. Pressure Converter helps with depth and Weight on Other Planets Calculator shows how gravity changes things.
Features
- Use the whole volume
- Sea water is denser
- Floating means force equals weight
- Clear messages when the buoyant force inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate buoyant force?
Buoyant force = fluid density × gravity × volume of fluid displaced. An object floats if its average density (mass ÷ volume) is lower than the density of the fluid. A 10 kg object with a volume of 2 litres has a density of 5,000 kg/m³ and sinks in water: the buoyant force is 19.6 N against a weight of 98.1 N, so a net 78.5 N pulls it down. A 0.5 kg object of 1 litre floats with 50.2 percent of it submerged.
What is the formula for buoyant force?
Buoyant force = fluid density × gravity × volume of fluid displaced. An object floats if its average density (mass ÷ volume) is lower than the density of the fluid. The buoyancy calculator applies it exactly and rounds only the displayed answer to six significant figures.
Why does a ship float?
Its hull holds so much air that its average density is lower than water's. It sinks until the displaced water weighs as much as the ship.
Does a heavy object always sink?
No. Weight alone does not decide it; the density does. A large wooden raft is heavy and floats.
How do I find the volume of an irregular object?
Lower it into a full container and measure the water that spills over, or use the displaced volume of a measuring jug.
Is this buoyancy calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.