CALCULATOR · VOLUME
Aquarium volume calculator
The short answer
A tank’s volume is length × width × height in inches ÷ 231. A 48 × 18 × 20-inch tank holds about 75 gallons (283 L) of glass-box water — but rock, sand and the fill line leave roughly 64 gallons of actual water, which is what you size heaters, flow and dosing against.
The ReefRig Volume Method
Gallons = length × width × height (in) ÷ 231. Then net out ~15% for glass, rock and sand to get the water volume you actually dose and heat.
The rule: gallons = L × W × H (in) ÷ 231, then plan around ~85% of that as real water.
Common reef tanks: box volume vs. rated
Manufacturer “gallons” is the rated display volume; the glass box itself computes larger. The gap is rear chambers, the fill line and displacement — proof you should plan around water volume, not the number on the box.
| Tank | Type | L×W×H (in) | Box (gal) | Rated (gal) |
|---|---|---|---|---|
| Fluval SEA EVO XII 13.5 | AIO | 22×11.5×12.5 | 13.7 | 13.5 |
| Red Sea MAX NANO G2 | AIO | 18×18×20 | 28.1 | 16.5 |
| Innovative Marine NUVO Fusion Pro 2 20 | AIO | 23.62×15×13 | 19.9 | 20 |
| Waterbox AIO 20 Cube | AIO | 17.7×17.7×15.7 | 21.3 | 20 |
| Waterbox AIO 35.2 | AIO | 23.6×19.7×17.7 | 35.6 | 33 |
| Red Sea MAX NANO G2 XL | AIO | 20×24×20 | 41.6 | 33 |
| Waterbox Marine X 60.2 | Sump | 23.6×19.7×19.7 | 39.6 | 36 |
| Innovative Marine NUVO Fusion Pro 2 40L (Long) | AIO | 35.4×19.7×13 | 39.2 | 40 |
| Waterbox AIO 50.3 | AIO | 35.4×19.7×17.7 | 53.4 | 49.1 |
| Red Sea REEFER 250 G3 | Sump | 35.4×19.7×20.9 | 63.1 | 54 |
| Waterbox AIO 65.4 | AIO | 47.2×19.7×17.7 | 71.2 | 65.8 |
| Red Sea REEFER 350 G3 | Sump | 47.2×19.7×20.9 | 84.1 | 72 |
| Waterbox Marine X 110.4 | Sump | 47.2×19.7×21.7 | 87.3 | 79.6 |
| Waterbox Infinia Reef 150.4 | Sump | 48.2×25.3×21.7 | 114.6 | 105.5 |
| Red Sea REEFER 525 G3 | Sump | 59.1×23.6×21.7 | 131 | 112 |
| Red Sea REEFER 625 G3 | Sump | 59.1×25.6×23.6 | 154.6 | 131 |
Box (gal) is this calculator’s output for each tank’s outside glass dimensions. Rated (gal) is the manufacturer’s published display volume. Tank dimensions verified 2026-07-03.
Unit conversions
| From | Equals |
|---|---|
| 1 US gallon | 3.785 L · 231 in³ |
| 1 liter | 0.264 US gal |
| 1 imperial gallon | 1.201 US gal |
| 1,000 cm³ | 1 L · 0.264 US gal |
LIVE CALCULATOR
Measure your tank
Enter inside dimensions in inches or centimeters. Effective water volume nets out ~15% for rock, sand and the fill line — the same basis our engine uses to size gear.
Effective water volume nets out ~15% for rock, sand and the fill line (via the ReefRig engine’s displacement factor). Size heaters, flow and dosing against this number, not the glass-box figure.