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Fish Tank Volume Calculator

Pick your tank shape and enter its measurements in inches or centimetres. You get the volume in US gallons, UK gallons and litres, plus the weight of the water. For a rectangular tank, the rule is length × width × height ÷ 231 (in inches) to get US gallons.

Tank volume calculator

Drawn to scale from your numbers.

Tank shape
Measurements
Units
Common tanks

10.4US gallons

Actual water once set up: about 8.8–9.4 US gal estimate

Litres
39.3
UK (imperial) gallons
8.7
Water weight
87 lb
US gallons
–
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Next for your 10-gallon tank

  • Heater size: wattage for your volume and room temperature
  • Substrate: how much gravel or sand for your footprint
  • Stocking: how many fish, with a tank-mate check
  • Tank weight: full weight including glass and substrate

How the calculator works

Every shape works the same way: find the area of the tank's footprint (seen from above), multiply by the water height, then convert cubic inches or cubic centimetres into gallons and litres.

Volume = footprint area × height US gallons = cubic inches ÷ 231 Litres = cubic centimetres ÷ 1,000 (1 US gal = 3.785 L, 1 UK gal = 4.546 L) Water weight ≈ 8.34 lb per US gallon ≈ 1 kg per litre (fresh water)

Rectangle

Area = length × width

Worked example: a standard 10-gallon tank measures 20 × 10 × 12 in. 20 × 10 × 12 = 2,400 in³. 2,400 ÷ 231 = 10.4 US gal (39.3 L). See the full 10 gallon tank dimensions.

Cylinder

Area = π × (diameter ÷ 2)²

Worked example: 12 in across, 18 in tall. Radius 6 in, so area = 3.1416 × 36 = 113.1 in². × 18 = 2,035.8 in³ ÷ 231 = 8.8 US gal (33.4 L).

Bow-front

A bow-front is a rectangle (using the depth at the sides) plus the curved "bow" in front. We treat the bow as a slice of a circle whose chord is the tank length and whose bulge is the centre depth minus the side depth.

bulge s = centre depth − side depth circle radius R = (length²/4 + s²) ÷ (2 × s) bow area = R² × arccos((R − s)/R) − (R − s) × √(2Rs − s²) Area = length × side depth + bow area

Worked example: 36 in long, 12 in deep at the sides, 16 in deep at the centre, 20 in tall. s = 4; R = (324 + 16) ÷ 8 = 42.5 in; bow area = 96.9 in². Area = 432 + 96.9 = 528.9 in². × 20 = 10,578.8 in³ ÷ 231 = 45.8 US gal (173.4 L).

Hexagon

For a regular hexagon (all six glass panels the same width):

Area = 2.598 × side² (exactly 3√3 ÷ 2 × side²)

Worked example: panels 10 in wide, 24 in tall. 2.598 × 100 = 259.8 in². × 24 = 6,235.4 in³ ÷ 231 = 27.0 US gal (102.2 L).

Corner (quarter-circle)

For a corner tank whose front is a quarter circle; the radius is the length along each back wall.

Area = π × radius² ÷ 4

Worked example: 24 in along each back wall, 20 in tall. 3.1416 × 576 ÷ 4 = 452.4 in². × 20 = 9,047.8 in³ ÷ 231 = 39.2 US gal (148.3 L).

How to measure your tank

  • Measure the inside of the glass if you can. Outside measurements include the glass, so they slightly overstate the volume.
  • For height, measure to the water line you actually keep, not the rim.
  • Metric users: enter centimetres; the calculator converts everything.

FAQ

Why does my tank hold less water than its label says?

Tank sizes are usually rounded and based on outside dimensions. Glass thickness, substrate, rocks, decor and the gap below the rim all take up space, so the water you actually have is typically around 10–15% less than the calculated volume (an estimate; it depends on how much hardscape you use). Our 75 gallon page shows a worked example.

Should I use inside or outside measurements?

Inside measurements are more accurate. If you only have the maker's outside dimensions, the result will be a little high, so treat it as an upper limit.

How many litres are in a gallon?

One US gallon is 3.785 litres. One UK (imperial) gallon is 4.546 litres. Most US tank labels use US gallons.

How much does the water in my aquarium weigh?

Fresh water weighs about 8.34 lb per US gallon, or 1 kg per litre. Salt water is slightly heavier. The glass, stand, substrate and rock add more on top, so check the full weight before choosing a stand or floor spot.

How do I measure a hexagon tank?

Enter the width of one glass panel. If you can only measure straight across from one corner to the opposite corner, divide that by 2: for a regular hexagon, that equals the panel width.

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