The only trap is double counting. Decide where to cut the L, then make sure the second section's dimensions describe only the part that is not already inside the first section.
How to measure a l-shaped tank
- Choose a cut line. The simplest is to extend one inner edge straight across, giving one long rectangle and one shorter one.
- Measure section 1 as a full rectangle, then measure section 2 only up to the cut line.
- Both sections must share the same height; if they do not, calculate each with the rectangular calculator and add the results.
Always measure inside the glass. Exterior sizes include the panel thickness and any trim, which on larger tanks can remove 5 to 10 percent of the apparent volume. See the measuring guide for the full method.
L-Shaped volume formula
V = (L1 × W1 + L2 × W2) × H
Split the L into two rectangles that do not overlap, add their floor areas and multiply by the common height.
With inches, divide the cubic result by 231 to get US gallons and multiply gallons by 3.785 for litres. With centimetres, divide by 1,000 for litres directly.
Worked example
Inputs: Section 1 Length 60 inSection 1 Width 24 inSection 2 Length 36 inSection 2 Width 24 inHeight 24 in
Section 1: 60 × 24 = 1,440 in². Section 2: 36 × 24 = 864 in². Floor 2,304 in² × 24 = 55,296 in³ = 239.4 US gallons (906.2 litres).
Avoiding double-counted volume
If both legs of an L are measured along their full outside length, the corner square where they meet gets counted twice. Picture the L as a long bar with a shorter bar attached to one end at right angles: the long bar runs the full length and the short bar starts where the long bar's side ends. Measured that way, the two rectangles tile the L exactly.
Getting to real water volume
The formula gives the geometric volume of the tank to the rim. Three things make the real water volume smaller, and the calculator above lets you account for two of them directly:
- Fill height. Most tanks run an inch or two below the rim. Enter the actual water line and the result scales to match.
- Substrate. Gravel and sand sit in the water column. The substrate option removes the solid fraction of the bed (62 percent of its depth for typical gravel) from the total.
- Rock, wood and equipment. Use the displacement calculator to subtract hardscape by weight and equipment by type.
Water weight
Fresh water weighs about 8.34 pounds per US gallon (1 kg per litre). Reef water at a specific gravity of 1.025 is about 8.55 pounds per gallon. The calculator shows both so you can check what your stand and floor will carry before adding glass, substrate and rock on top.