In short: a floating object displaces a volume of water whose weight equals its own weight — a 1 kg object floating in fresh water at room temperature displaces about 1.002 liters. A fully submerged object always displaces exactly its own volume, no matter how heavy or light it is. Use the calculator above with your own numbers, or see our buoyancy calculator if you need to know whether an object will float or sink in the first place.
These figures are planning aids based on published density data and assume no trapped air. For structural, regulatory or safety-critical work, confirm with a licensed engineer.
How to calculate water displacement
Archimedes' principle gives two different rules depending on whether the object floats or is fully underwater. A floating object sinks in just far enough that the water it pushes aside weighs exactly as much as the object itself:
A fully submerged object, on the other hand, displaces a volume of water equal to its own volume — full stop, regardless of weight:
Worked example (floating). A 1 kg rubber duck floats in fresh water at 20 °C, where water's density is 998.2 kg/m³. Volume displaced = 1 ÷ 998.2 = 0.0010018 m³, which is about 1.002 L (1,001.8 mL).
Worked example (submerged). A 50 g steel bolt (density 7,850 kg/m³) is fully underwater. Its volume is 0.05 ÷ 7,850 = 6.369×10⁻⁶ m³, or about 6.369 mL — and that's exactly how much water it displaces, weighing roughly 6.359 g (0.0140 lb). That's far less than the bolt's own 50 g weight, which is exactly why it sinks instead of floating.
Why fluid density and temperature change the answer
Water is denser when it's cold and lighter when it's hot, reaching its maximum density at about 4 °C. This calculator uses the Kell equation — the standard model for pure water's density from 0 °C to 150 °C — to look up the right figure for your temperature. You can read the full method on our methodology page, and check the underlying numbers on our water density calculator.
The fluid's density only changes the floating calculation. Seawater (about 1,025 kg/m³, roughly 2.6% denser than fresh water) supports the same weight with slightly less displaced volume — which is why ships float a little higher in the ocean than in a freshwater lake. For a fully submerged object, the fluid's density doesn't affect the displaced volume at all, since that's fixed by the object's own size — it only changes the weight of the water being pushed aside.
How much water floating loads displace
Volume of fresh water displaced at 20 °C, by the weight of the floating object. Because floating only depends on weight and fluid density, this table applies no matter what the object is made of.
| Floating weight | Volume displaced |
|---|---|
| 1 kg (2.2 lb) | 1.00 L (1,001.8 mL) |
| 5 kg (11.0 lb) | 5.01 L |
| 10 kg (22.0 lb) | 10.02 L |
| 50 kg (110.2 lb) | 50.09 L |
| 70 kg (154.3 lb) (average adult) | 70.13 L |
| 100 kg (220.5 lb) | 100.18 L |
Submerged displacement by material
For 1 kg of each material, fully underwater in fresh water at 20 °C. Volume displaced depends only on the material's density; the weight of displaced water follows from that volume.
| Material | Density (kg/m³) | Volume displaced | Weight of water displaced |
|---|---|---|---|
| Pine wood * | 500 | 2,000.0 mL | 1,996.4 g |
| Oak wood * | 750 | 1,333.3 mL | 1,330.9 g |
| Ice * | 917 | 1,090.5 mL | 1,088.6 g |
| Aluminum | 2,700 | 370.4 mL | 369.7 g |
| Glass | 2,500 | 400.0 mL | 399.3 g |
| Concrete | 2,400 | 416.7 mL | 415.9 g |
| Cast iron | 7,200 | 138.9 mL | 138.6 g |
| Steel | 7,850 | 127.4 mL | 127.2 g |
| Lead | 11,340 | 88.2 mL | 88.0 g |
| Gold | 19,300 | 51.8 mL | 51.7 g |
* Less dense than water — this material would float on its own; the figures assume it's held fully underwater. For materials denser than water, notice the water displaced weighs less than the material's own 1 kg, which is exactly why they sink. See our specific gravity calculator to compare any material's density directly to water's.
Worked examples
A floating rubber duck
1.002 L
A 1 kg bath toy floating in room-temperature fresh water displaces just over 1 liter — almost exactly its own weight in milliliters, since 1 L of water weighs about 1 kg.
A rock dropped in a fish tank
500 mL
A 500 mL rock, fully underwater, displaces exactly 500 mL. In an 800 cm² tank, that raises the water level about 6.25 mm (0.246 in), and the displaced water weighs about 1.100 lb (0.4991 kg).
A steel bolt sinking in a bucket
6.369 mL
A 50 g steel bolt displaces only 6.369 mL of water, weighing about 6.359 g (0.0140 lb) — far less than the bolt itself weighs, which is exactly why it sinks.
What else affects the result
Trapped air. An object with air pockets — a hollow toy, a life jacket, a boat's hull — displaces more water than its solid material volume alone would suggest, because the trapped air counts too. This calculator assumes no trapped air unless you account for it in the volume or mass you enter.
Partial submersion. This tool covers two clean cases: fully floating at rest, or fully submerged. An object that's partly out of the water — like an iceberg or a person wading — displaces only the volume of the part that's underwater, which this calculator doesn't estimate directly.
Container shape. The optional water-level-rise figure assumes a container with a constant, flat cross-section (like a rectangular tank). In a container that tapers or curves, the level will rise unevenly and this estimate will be approximate.
Salinity and temperature. Both change the fluid's density, which changes how much volume a floating object needs to displace to support its weight — see the section above for how much that matters in practice.
Frequently asked questions
How much water does a floating object displace?
A floating object always displaces a volume of water whose weight equals the object's own weight. In fresh water at room temperature, a 1 kg object displaces about 1.002 liters. This is Archimedes' principle: the buoyant force from the displaced water exactly balances the object's weight.
How much water does a fully submerged object displace?
A fully submerged object displaces a volume of water exactly equal to its own volume, regardless of how heavy or light it is. A 50 g steel bolt and a 50 g block of foam of the same volume displace the same amount of water when both are fully underwater.
Does a heavier object always displace more water?
No. For a submerged object, displacement depends on volume, not weight — a small, dense object like a steel bolt displaces very little water even though it is heavy. For a floating object, displacement depends on weight, not volume. To work out whether an object will actually float or sink, use our buoyancy calculator, which compares the object's weight to the buoyant force.
Does salt water change how much water is displaced?
For a floating object, yes: seawater is denser than fresh water, so the object needs to displace slightly less volume to support the same weight — which is why ships float a little higher in the ocean than in a freshwater lake. For a fully submerged object, the displaced volume stays the same either way, because it equals the object's own volume; only the weight of that displaced water changes.
How accurate is this calculator?
Water density comes from the Kell equation, the reference standard between 0 °C and 150 °C, and unit conversions use exact internationally defined factors. The calculator assumes the object has no trapped air and, for the submerged-mass mode, a single uniform material density. Full detail is on our methodology page.
Sources & references
- Archimedes (c. 250 BCE). On Floating Bodies.
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105.
- National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluid Systems.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)