Aquarium volume and water weight calculator for five tank shapes
Short answer
Inside length × depth × water height, in centimeters, divided by 1000 gives liters; a 60 × 30 × 36 cm tank holds 64.8 L to the rim. The net water volume is lower once you subtract the glass, the gap below the rim and the substrate, and that net figure is the one to use for dosing. Water at 25 °C weighs 0.997 kg per liter [3].
What to measure
Measure the inside of the tank if you can. If you can only measure the outside, enter the glass thickness: for rectangular and cylindrical tanks the calculator subtracts it twice from length and depth (or once per side from the diameter) and once from the height, for the bottom pane. Then enter how far the water surface sits below the top edge, and the average depth of the substrate.
The calculator returns three volumes: to the top edge, the water at your fill line, and that water minus the substrate layer. The last one is the figure to type into the dosing calculators.
Shapes and formulas
All lengths in centimeters; 1 liter is 1000 cm³ [2].
| Shape | Footprint area | Inputs |
|---|---|---|
| Rectangle or cube | length × depth | inside length, depth, height |
| Cylinder | π × diameter² ÷ 4 | inside diameter, height |
| Bow-front | rectangle + circular segment | width, depth at the ends, depth at the center of the bow |
| Quarter-circle corner | π × side² ÷ 4 | length of one straight side |
| Regular hexagon | 3√3 ÷ 2 × side² | length of one side |
Volume is the footprint area times the water column height. For a bow-front tank, the curved front is treated as a circular arc: the straight back edge is the chord, and the extra depth at the center is the arc’s height (its sagitta). From those two numbers the radius follows, r = chord² ÷ (8 × sagitta) + sagitta ÷ 2, and the segment area is r² ÷ 2 × (θ − sin θ), with θ the angle the arc subtends. Real bow-fronts are not always true circular arcs, so treat the result as close rather than exact.
Worked example
A rimless tank measures 60 × 30 × 36 cm on the outside with 6 mm glass. The water stands 2 cm below the top and the substrate averages 5 cm.
- Inside: 58.8 × 28.8 cm footprint, 35.4 cm high → 59.9 L to the rim.
- Water column: 35.4 − 2 − 5 = 28.4 cm → 48.1 L of water above the substrate.
The 60 L label overstates the water by about a fifth. Dosing 1 g of potassium nitrate into 48 L instead of 60 L raises nitrate by 12.8 mg/L instead of 10.2 mg/L.
Weight
Water weight uses the density of pure water at the temperature you enter, from the formula of Tanaka and colleagues adopted for metrology [3]: 998.2 kg/m³ at 20 °C and 997.05 kg/m³ at 25 °C. The difference is small, so for practical purposes a liter of tank water weighs about 1 kg (2.2 lb). Pounds and gallons match the conversion factors in NIST’s guide to the SI [1].
The weight is only the water. Glass, substrate, rock and wood add to the load on the stand, and the calculator cannot know them; check the cabinet maker’s rated load before filling a large tank.
Limits
- Pore water inside the substrate is not counted, so the real water volume is somewhat larger than the net figure when you enter a substrate depth.
- Rock, wood and filter volume are not subtracted.
- Corner and hexagonal tanks are assumed to be regular shapes; measure the inside where the manufacturer’s dimensions describe the outside.
Questions
Should I dose for the tank's nominal size or the real water volume?
The real water volume. A tank sold as 60 L often holds noticeably less once glass, substrate, rock and the gap below the rim are taken out, and every ppm figure in the dosing calculators scales with the liters you enter.
How many liters are in a US gallon?
3.785412 L, and an imperial (UK) gallon is 4.54609 L [1]. The calculator reports both.
Other calculators
- Dry fertilizer dosingConvert grams of KNO3, KH2PO4, K2SO4, Epsom salt or a trace mix into mg/L for your tank, or work back from a target ppm. Every molar mass is shown.
- EI and PPS-Pro plannerTurn the Estimative Index and PPS-Pro, as their authors published them, into grams or milliliters for your tank, with the build-up each schedule produces.
- Substrate volumeHow many liters of aqua soil, sand or gravel a tank needs for a flat bed or a front-to-back slope, and how many bags that is.
Background reading
- FertilizersEstimative Index (EI) dosing: what the method prescribesEI as Tom Barr wrote it up: dose a surplus three times a week, change 50 % of the water weekly, and why that caps build-up at twice the weekly dose.
- FertilizersHow ppm from dry fertilizer salts is calculatedThe molar-mass arithmetic behind every dosing chart: grams of salt to mg/L of nitrate, phosphate or potassium, with worked examples and the common traps.
Sources
- NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically. National Institute of Standards and Technology (SP 811). US gallon 3.785 412 L; imperial gallon 4.546 09 L; inch 2.54 cm; pound 0.453 592 4 kg. Accessed October 12, 2026.
- SI units: volume. National Institute of Standards and Technology, Office of Weights and Measures. The liter is one cubic decimeter (1000 cm³). Accessed October 12, 2026.
- Recommended table for the density of water between 0 °C and 40 °C based on recent experimental reports. Metrologia 38: 301–309. Tanaka, M., Girard, G., Davis, R., Peuto, A. and Bignell, N. (2001). Density formula for pure water used for the weight result (997.05 kg/m³ at 25 °C). Accessed October 12, 2026.