For a rectangular tank with straight vertical sides, enter its inside length and width in centimetres, the actual fill depth in centimetres, and user-measured displacement in litres. Measure fill depth from the inside bottom to the water surface, including the height occupied by substrate. Do not use external dimensions, advertised tank capacity or the full tank height unless the water reaches that height.
Displacement is the submerged volume that excludes water, such as solid substrate material, rocks and equipment. Do not subtract the entire bulk volume of gravel if water fills its gaps. Measure displacement separately, for example by measuring the increase in water volume when suitable objects are submerged in a graduated container. Count only the submerged portions and avoid counting the same displacement twice.
This estimates water in the tank itself. Water in external filters, plumbing and sumps is excluded.
Inside length and width must each be 0.1–1,000 cm. Fill depth must be 0–1,000 cm and must not exceed the actual internal tank height. Displacement must be nonnegative and must not exceed the filled rectangular volume shown by the calculator. The absolute displacement input limit is 1,000,000 litres; the smaller, tank-specific limit still applies. These are calculator input bounds, not aquarium design recommendations.
If fill depth is zero, displacement must also be zero. Displacement equal to the filled rectangular volume gives zero remaining water. A negative result indicates inconsistent measurements and is invalid; it is not a physical water volume. This version cannot automatically enforce the relationship between displacement and dimensions, so check it before using the result. Unavailable results are not zero.
Enter only the units printed in the labels. The calculation uses numerical measures in those fixed units; the three conversion-factor inputs have equal minimum and maximum values and must remain at their defaults. Results are rounded to two decimal places for display, without implying that the measurements are that precise.
A rectangular solid has volume equal to length × width × height; see OpenStax’s rectangular-solid volume explanation. Using centimetres gives cubic centimetres. Divide by 1,000 to obtain litres, then subtract measured displacement.
One US liquid gallon is 3.785411784 litres; see NIST Handbook 44, Appendix C. One imperial gallon is 4.54609 litres; see NIST’s conversion-factor table. The two gallon results describe the same amount of water using different units.
| Inside length × width × actual fill depth | Displacement | Calculation in litres | Working litres | US liquid gallons | Imperial gallons |
|---|---|---|---|---|---|
| 80 × 35 × 35 cm | 12 L | (80 × 35 × 35) / 1000 − 12 | 86.00 | 22.72 | 18.92 |
| 60 × 30 × 35 cm | 9 L | (60 × 30 × 35) / 1000 − 9 | 54.00 | 14.27 | 11.88 |
| 60 × 30 × 0 cm | 0 L | (60 × 30 × 0) / 1000 − 0 | 0.00 | 0.00 | 0.00 |
| 60 × 30 × 35 cm | 64 L | 63 − 64 = −1: displacement exceeds available space | Invalid | Invalid | Invalid |
Can I use the dimensions printed on the tank packaging?
Only if they are confirmed inside dimensions and you separately measure the actual fill depth. External dimensions include the tank walls, and nominal capacity need not represent the water present.
Should I measure depth above the gravel?
For this formula, measure from the inside bottom to the water surface, then subtract measured displacement. Measuring only above the gravel excludes water between the grains and can double-count a deduction if you also subtract substrate displacement.
Does this work for bow-front, cylindrical or irregular tanks?
No. The rectangular formula assumes a constant rectangular cross-section. For other shapes, use a suitable geometric method or directly measure the water added.
Can the calculator estimate displacement from a percentage?
This calculator requires a user-measured displacement volume. It does not assume a percentage for rocks, substrate or equipment. Entering zero means no displacement is being subtracted.