How to Find the Volume of an Irregular Object
Two methods cover everything: for a solid object with no formula — a rock, a sculpture, a machine part — use water displacement: submerge it and measure the rise. For an irregular space — an oddly shaped room, pond or excavation — split it into simple shapes, calculate each, and add them up.
Method 1 — Water displacement
This is the method Archimedes is said to have discovered in his bath. A submerged object pushes aside exactly its own volume of water, so the rise in water level tells you the volume directly.
- Fill a measuring container — a measuring jug, graduated cylinder, or any straight-sided vessel with a scale — partway with water. Note the level, say 500 mL.
- Lower the object in until it is completely underwater, without letting any water spill.
- Read the new level, say 650 mL.
- Subtract. 650 − 500 = 150 mL, so the object's volume is 150 cm³ (since 1 mL = 1 cm³).
The overflow variant: fill a container to the very brim, place it in a tray, submerge the object, and collect the water that overflows. Measure that overflow — it equals the object's volume. This is handy for objects too big for a graduated cylinder.
Watch out for
- Floating objects. They must be fully submerged, so push them under with a thin rod or attach a sinker and subtract the sinker's volume.
- Absorbent objects. Wood, sponge and fabric soak up water and give a false reading; seal them first or use a different method.
- Trapped air. Tilt the object as it goes in so no air bubbles cling to it.
Method 2 — Split into simple shapes
When the object is too large to submerge, or you are measuring an empty space rather than a solid, break it into pieces you do have formulas for. An L-shaped room is two rectangular boxes. A silo is a cylinder plus a cone. A swimming pool with a shallow and deep end is a box plus a wedge.
Worked example — an L-shaped room. Split the floor plan into two rectangles: one 4 m × 3 m and one 2 m × 2 m, both 2.5 m high.
- Piece 1: 4 × 3 × 2.5 = 30 m³
- Piece 2: 2 × 2 × 2.5 = 10 m³
- Total: 40 m³
Each piece can be worked out with its own shape calculator, then simply added. The tank calculator uses exactly this approach internally for capsule and dished-end tanks (a cylinder plus two end caps).
Averaging depths for ponds and excavations
For a natural feature with an uneven bottom, like a pond, take several depth readings on a grid, average them, and multiply by the surface area:
The more readings you average, the better the estimate. The pond volume calculator builds in a correction factor for the sloping sides that most natural ponds have.
Frequently asked questions
How do you find the volume of an irregular object?
Submerge it in water and measure how much the water level rises. The rise, converted to volume, equals the object's volume exactly. This is the water displacement method, and it works for any solid that does not absorb water or float.
What is the water displacement method?
It uses the fact that a submerged object pushes aside a volume of water equal to its own volume. Measure the water level before and after submerging the object, and the difference is the volume. One millilitre of rise equals one cubic centimetre of object.
How do you measure the volume of an object that floats?
Push it fully under with a thin rod or weigh it down, since it must be completely submerged to displace its true volume. Alternatively, attach a known sinker and subtract the sinker's volume afterwards.
How do you find the volume of an irregular room or pond?
Divide the space into simple shapes — rectangles, cylinders, wedges — calculate the volume of each, and add them together. For a pond, averaging several depth readings across a measured surface area gives a good estimate.