Because the height is often 8 to 12 inches, a 2-inch substrate bed and a 1-inch fill gap can remove a third of the nominal volume. Use both adjustment toggles to get a realistic number for dosing and water changes.
How to measure a shallow tank
- Measure the interior height precisely; a quarter inch matters on a 9-inch tank.
- Enter the water line you actually run, which on paludariums may be far below the rim.
- Deep aquasoil slopes are common in shallow scapes; enter the average depth, not the peak at the back.
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.
Shallow volume formula
V = L × W × H
Shallow tanks are rectangular prisms. Because the height is small, every inch of substrate or air gap is a large share of the total, so the adjustment options matter more here than on any other shape.
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: Length 24 inWidth 14 inHeight 9 in
A 24 × 14 × 9 inch shallow tank holds 3,024 cubic inches, or 13.1 US gallons (49.6 litres) to the rim. With a 1-inch gap and 2 inches of soil it drops to about 9 gallons.
Why fill height matters most on shallow tanks
Volume adjustments are proportional to height. One inch out of 24 is 4%, but one inch out of 8 is 12.5%. Shallow tanks therefore show the largest gap between the volume printed on a label and the water that actually circulates, which is why medication and fertiliser dosing on these tanks should always use the adjusted figure.
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.