Knowing the real volume of an aquarium matters for almost everything in fishkeeping: dosing medication, water conditioner and fertiliser, choosing a heater and filter, and deciding how many fish the tank can support. Manufacturers’ nominal sizes are rounded and measured to the rim, so a “55-gallon” tank is often a little more or less.
Choose the tank shape and enter its inside dimensions. For bow-front tanks give the length, the depth at the ends and the full depth at the middle of the bow; the curved front is treated as a circular arc, so the extra area is a circular segment. Corner bow fronts are a right-angled triangle against two walls plus a bowed front. Cylinders use the diameter, quarter cylinders the radius along each wall, and hexagons the length of one side.
Besides the gross volume, the calculator estimates the water you actually have after leaving a gap below the rim and allowing for gravel, rocks and decorations, the weight of that water (important for stands and floors), two common stocking rules of thumb and the filter flow for four tank turnovers per hour.
How to use the Aquarium Volume Calculator
- Tank shape
- Length
- Width (front to back) — For a bow front: the depth at the side panels.
- Full width at the centre of the bow
- Edge length
- Side length along each wall
- Depth from the back corner to the centre of the front
- Diameter
- Radius (side length along each wall)
- Side length of the hexagon
- Height
- Water level below the rim
- Substrate, rocks and decor
- Water
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Pick a shape, enter the dimensions in any unit, then set the water gap below the rim, the share of space taken by substrate and decor, and fresh or salt water. Footprint area × height gives the gross volume; the water volume uses the filled height and the displacement allowance.
Worked example
A rectangular tank of 48 × 13 × 21 inches has a volume of 13,104 in³ = 56.7 US gallons (214.7 L). Filling it to 1 inch below the rim and allowing 10% for substrate and decor leaves 48 × 13 × 20 × 0.9 ÷ 231 = 48.6 gallons, which weighs about 184 kg (405 lb) as freshwater. Its 624 in² footprint suggests about 52 inches of tropical fish by the surface rule, and the filter should move about 194 gal/h. A bow front 36 in long, 12 in deep at the ends and 16 in at the centre has a bow radius of 42.5 in and a segment of 96.9 in²; at 20 in high it holds (432 + 96.9) × 20 ÷ 231 = 45.8 gallons.
Common tank sizes
| Nominal size | Dimensions (L × W × H) | Gross volume |
|---|---|---|
| 10 gal | 20 × 10 × 12 in | 10.4 US gal / 39.3 L |
| 20 gal long | 30 × 12 × 12 in | 18.7 US gal / 70.8 L |
| 29 gal | 30 × 12 × 18 in | 28.1 US gal / 106.2 L |
| 55 gal | 48 × 13 × 21 in | 56.7 US gal / 214.7 L |
| 75 gal | 48 × 18 × 21 in | 78.5 US gal / 297.3 L |
| 125 gal | 72 × 18 × 22 in | 123.4 US gal / 467.2 L |
Inputs explained
| Field | What to enter |
|---|---|
| Tank shape | Choose one: Rectangular (standard), Cube, Bow front, Corner bow front … |
| Length | Units: in, cm, mm, ft, m |
| Width (front to back) | Units: in, cm, mm, ft, m. For a bow front: the depth at the side panels. |
| Full width at the centre of the bow | Units: in, cm, mm, ft, m |
| Edge length | Units: in, cm, mm, ft, m |
| Side length along each wall | Units: in, cm, mm, ft, m |
| Depth from the back corner to the centre of the front | Units: in, cm, mm, ft, m |
| Diameter | Units: in, cm, mm, ft, m |
| Radius (side length along each wall) | Units: in, cm, mm, ft, m |
| Side length of the hexagon | Units: in, cm, mm, ft, m |
| Height | Units: in, cm, mm, ft, m |
| Water level below the rim | Units: in, cm, mm, ft, m |
| Substrate, rocks and decor | In % of volume |
| Water | Choose one: Freshwater (998 kg/m³), Saltwater / reef (1,025 kg/m³) |
What each result means
- Tank volume = [Footprint area]×[Tank height]
- Actual water volume = [Footprint area]×max([Tank height] – [Water level below the rim], 0)×(1 – [Substrate, rocks and decor]÷100)
- Weight of the water = [Actual water volume]×[Water]
- Footprint (water surface) area = [Footprint area]
- Stocking – 1 inch of fish per gallon = [Actual water volume]÷0.003785411784
- Stocking – surface rule, tropical (12 in² per inch) = [Footprint area]÷0.00064516÷12
- Stocking – surface rule, coldwater (30 in² per inch) = [Footprint area]÷0.00064516÷30
- Filter flow for 4 turnovers per hour = [Actual water volume]×4÷3600
Formula
Rectangle: V = L × W × H · Cube: V = s³ · Cylinder: V = π·D²/4 × H · Half cylinder: π·D²/8 × H · Quarter: π·r²/4 × H
Bow front: V = (L·W + segment) × H, segment area = r²·acos((r − s)/r) − (r − s)·√(2rs − s²), r = (c²/4 + s²)/(2s)
Hexagon: V = (3√3/2)·a² × H · 1 US gal = 231 in³ = 3.785 L
Worked example
Inputs — Tank shape: Rectangular (standard); Length: 48 in; Width (front to back): 13 in; Height: 21 in; Water level below the rim: 1 in; Substrate, rocks and decor: 10 % of volume; Water: Freshwater (998 kg/m³).
Results — Tank volume = 56.73 US gal; Actual water volume = 48.62 US gal; Weight of the water = 405 lb; Footprint (water surface) area = 624 in²; Stocking – 1 inch of fish per gallon = 49 in of adult fish; Stocking – surface rule, tropical (12 in² per inch) = 52 in of adult fish; Stocking – surface rule, coldwater (30 in² per inch) = 21 in of adult fish; Filter flow for 4 turnovers per hour = 194 US gal/h.
Key terms
- Gross volume
- Volume of the inside of the tank up to the rim.
- Displacement
- Water volume taken up by gravel, rocks, wood and equipment.
- Turnover rate
- How many times per hour the filter circulates the full water volume.
- Circular segment
- The area between a chord and its arc – the extra area of a bowed front.
Frequently asked questions
How do I calculate aquarium volume in gallons?
Multiply length × width × height in inches and divide by 231. A 48 × 13 × 21 in tank holds 13,104 in³ ÷ 231 = 56.7 US gallons.
How do I calculate it in litres?
Multiply the dimensions in centimetres and divide by 1,000: 120 × 50 × 50 cm = 300 litres.
How much water is really in my tank?
Usually 10–20% less than the nominal volume: the water stops 2–3 cm below the rim and substrate, rock and decor displace water. The example 56.7-gallon tank holds about 48.6 gallons with a 1-inch gap and 10% decor.
How heavy is a full aquarium?
Water alone weighs about 8.3 lb per US gallon (1 kg per litre); add roughly 10–15% for glass, substrate and equipment. A 55-gallon setup weighs over 500 lb.
How do I measure a bow-front tank?
Measure the length, the front-to-back depth at the side panels and the depth at the middle of the bow; the calculator adds the circular segment in front.
What does the inch-per-gallon rule mean?
A rough guide of one inch of adult fish length per gallon for small, slim fish. It ignores body mass and filtration, so treat it as an upper limit for beginners.
How strong a filter do I need?
Most aquarists aim for the filter to pass the tank's volume 4–6 times per hour; for 48.6 gallons that is about 195–290 gal/h.