The water displacement method is a technique for measuring the volume of any object by submerging it in water and observing how much the water level rises. The rise in water level equals the volume of the object. It’s one of the simplest and most reliable ways to measure irregularly shaped items that can’t be measured with a ruler, and it works in settings ranging from science classrooms to kitchens.
How It Works
The core idea is straightforward: when you place an object in water, it pushes aside (displaces) a volume of water exactly equal to its own volume. To capture that measurement, you partially fill a graduated cylinder or measuring cup with water and record the starting level. Then you lower the object in, let it sink, and record the new level. The difference between the two readings is the object’s volume.
Volume of object = final water level − initial water level
If the water starts at 12.4 mL and rises to 20.2 mL after you drop in a small battery, the battery’s volume is 7.8 mL. That’s all there is to it. No geometry, no formulas for surface area. The water does the measuring for you.
The Science Behind It
The principle dates back to the ancient Greek mathematician Archimedes, who reportedly discovered it while taking a bath. He noticed that sinking into the tub caused the water to rise, and he realized that the amount of water pushed aside was directly related to the volume of his body. The broader physical law named after him, Archimedes’ principle, states that any object submerged in a fluid experiences an upward (buoyant) force equal to the weight of the fluid it displaces.
Gravity is fundamental to how this works. Water pressure increases with depth because of the weight of the water above. That increasing pressure pushes upward on a submerged object, creating buoyancy. For volume measurement purposes, the practical takeaway is simpler: the water has to go somewhere when an object takes up space, so it rises by a predictable amount.
Step-by-Step Procedure
You’ll need a container with volume markings, such as a graduated cylinder for lab work or a liquid measuring cup in the kitchen. Here’s the basic process:
- Fill the container partway. Use enough water that the object will be fully submerged, but not so much that the water overflows when you add the object.
- Record the starting water level. Read the water line at eye level for accuracy.
- Lower the object gently into the water. Dropping it in can cause splashing and throw off your reading.
- Record the new water level. Again, read at eye level.
- Subtract. The difference is your object’s volume.
For objects that float, the standard method doesn’t work on its own because the object won’t fully submerge. The usual workaround is to use a thin rod or pin to push the object just below the surface, then read the water level. You’d need to account for the volume of whatever tool you used to push it down, or use a sinker method where a heavy object is attached to the floating item.
Using It in the Kitchen
Water displacement isn’t just for science class. It’s an old-fashioned cooking trick for measuring solid fats like shortening, coconut butter, or a block of butter that hasn’t been pre-scored into tablespoon markings.
Say you need half a cup of shortening. Fill a 2-cup liquid measuring cup with cold water to the 1.5-cup line. Then add spoonfuls or small slices of the shortening until the water reaches the 2-cup mark. Pour off the water, and what remains is exactly half a cup of fat. A few tips make this more reliable: use cold water and cold fat to prevent melting, make sure the fat is fully submerged, and always read the water level at eye level. It also helps to use a container much larger than the amount of fat you’re measuring. Trying to measure 1 cup of fat in a 2-cup container will likely be messy and inaccurate.
Measuring Body Composition
A more advanced version of the same principle is used to estimate body fat percentage. Called hydrostatic weighing, the method involves sitting on an underwater swing inside a steel tank, exhaling all the air from your lungs, and being fully submerged for 3 to 5 seconds while a scale records your underwater weight. The test is typically repeated up to five times to get a consistent reading.
Your body volume is calculated from your underwater weight, the density of the water, and a correction for the small amount of air left in your lungs after exhaling. From there, a standard equation converts body density into a body fat percentage using a two-compartment model that separates your weight into fat mass and fat-free mass. For decades, this was considered one of the most accurate lab methods for body composition analysis.
Common Sources of Error
The method is simple, but a few things can throw off your results. Air bubbles clinging to the surface of a submerged object inflate the apparent volume, because the bubble displaces water just like the object does. Tapping the container gently or tilting the object as you lower it in helps release trapped air.
Reading the water level inaccurately is the most common mistake. Water curves slightly upward where it meets the walls of a container, forming a concave surface called a meniscus. You should always read from the bottom of that curve, not the edges. In a standard graduated cylinder, readings are only precise to about ±0.5 mL, so both your initial and final readings carry some uncertainty. That means the calculated volume has roughly ±1 mL of potential error, which matters more for small objects than large ones.
Water temperature also plays a minor role. Water is densest at about 4°C (39°F), where it hits almost exactly 1 gram per milliliter. At room temperature, around 21°C (70°F), the density drops slightly to 0.998 g/mL. At near-boiling temperatures, it falls to about 0.959 g/mL. For most everyday measurements, this difference is negligible. But in precise lab work, the temperature of the water needs to be recorded and factored in.
When to Use It
Water displacement is ideal when you need the volume of something with an irregular shape: a rock, a piece of fruit, a small machine part, a chunk of metal. If the object is a perfect cube or cylinder, you can calculate the volume with a ruler and a formula. But most real-world objects aren’t perfect geometric shapes, and that’s where displacement shines.
The method does have limits. The object needs to be waterproof, or at least not absorb water in the time it takes to get a reading. Porous materials like sponges or unglazed ceramics will soak up water and give you a falsely low volume. Soluble materials, like a sugar cube, will dissolve. And very large objects simply need impractically large containers. For those situations, other techniques like 3D scanning or gas displacement are more practical. But for the vast majority of small-to-medium solid objects, a container of water and some basic arithmetic are all you need.