Glass, water and gravel
Type inches like 24 or 20 1/4. Measure the tank from the outside — or pick The inside under "I measured". Everything is saved on this device.
About the aquarium volume calculator
This free aquarium volume calculator tells you how much water your fish tank really holds. Pick the shape — rectangle, cube, cylinder, bow front, corner or hexagon — and type the size. You get gallons and litres.
A "20 gallon" tank never holds 20 gallons of water. The glass takes room, the water stops below the top, and the sand or gravel fills the bottom. The tool takes all three off, and shows the outside size too. It also tells you the weight of the full tank, so you can check your stand or shelf.
Features
- Real water volume — after the glass, the gap at the top and the sand or gravel layer
- 6 tank shapes — rectangle, cube, cylinder, bow front, corner (quarter circle) and hexagon
- Outside or inside sizes — measure the outside and type the glass thickness, or measure the inside
- Weight when full — water, sand and glass together, for fresh or salt water
- Sand or gravel — the weight to buy for the layer you want
- Several tanks — add them up, with each tank's own number
- Inches or cm — US gallons and litres in both
- Just volume — no stocking rules; this is a measuring tool
How the volume is worked out
- Inside size = outside size − 2 × glass thickness for each width, and − 1 × glass thickness for the height (the bottom pane).
- Water height = inside height − the gap at the top − the sand or gravel layer.
- Water = the inside footprint × the water height.
- One US gallon is 231 cubic inches (3.785 litres). One litre is 1,000 cm³.
| Shape | Footprint |
|---|---|
| Rectangle | length × width |
| Cube | side × side |
| Cylinder | 0.785 × diameter × diameter |
| Bow front | length × width at the sides + the curved part (a segment of a circle) |
| Corner (quarter circle) | 0.785 × side × side |
| Hexagon | 2.598 × side × side |
Common tank sizes
Outside sizes, rounded, with 1/4 in glass, the water 1 in below the top and 2 in of gravel:
| Tank | Outside (L × W × H) | Tank size | Real water | Full weight (about) |
|---|---|---|---|---|
| 10 gallon | 20 × 10 × 12 in | 10.4 gal | 7.0 gal | 101 lb |
| 20 gallon high | 24 × 12 × 16 in | 19.9 gal | 14.9 gal | 188 lb |
| 20 gallon long | 30 × 12 × 12 in | 18.7 gal | 12.9 gal | 177 lb |
| 29 gallon | 30 × 12 × 18 in | 28.1 gal | 21.7 gal | 262 lb |
| 40 gallon breeder | 36 × 18 × 16 in | 44.9 gal | 34.3 gal | 412 lb |
| 55 gallon | 48 × 13 × 21 in | 56.7 gal | 45.6 gal | 521 lb |
| 75 gallon | 48 × 18 × 21 in | 78.5 gal | 63.9 gal | 711 lb |
| 125 gallon | 72 × 18 × 21 in | 117.8 gal | 96.1 gal | 1,062 lb |
In metric, with 6 mm glass, the water 2.5 cm below the top and 5 cm of gravel:
| Tank (L × W × H) | Tank size | Real water | Full weight (about) |
|---|---|---|---|
| 60 × 30 × 30 cm | 54 L | 37.1 L | 61 kg |
| 80 × 35 × 40 cm | 112 L | 85.0 L | 124 kg |
| 100 × 40 × 50 cm | 200 L | 160.6 L | 218 kg |
| 120 × 50 × 50 cm | 300 L | 242.9 L | 324 kg |
Small tanks often have thinner glass (3/16 in or 4–5 mm). Type yours for a closer number.
Other shapes, worked out
With the same 1/4 in glass, 1 in gap and 2 in of gravel:
- Bow front, 36 in long, 12 in deep at the sides and 16 in in the middle, 21 in high: 48.1 gal outside, 38.7 gal of water
- Cylinder, 18 in across and 24 in high: 26.4 gal outside, 21.6 gal of water
- Corner, 24 in along each wall and 24 in high: 47.0 gal outside, 38.7 gal of water
- Hexagon, 12 in sides and 24 in high: 38.9 gal outside, 32.0 gal of water
- Cube, 16 in: 17.7 gal outside, 13.3 gal of water
How much does a full tank weigh?
Fresh water weighs about 8.3 lb per gallon (1 kg per litre). Salt water is about 2.5% heavier. Add the glass and the sand or gravel, and a full tank weighs about 9 to 10 lb per gallon of tank size. A full 55 gallon tank is about 520 lb. Use a stand made for the tank, on a level, solid floor.
Frequently asked questions
How do I work out how many gallons my fish tank holds?
Multiply length × width × height in inches and divide by 231. A 24 × 12 × 16 in tank is 4,608 cubic inches = 19.9 gallons from the outside. After the glass, the gap and 2 in of gravel, the real water is about 14.9 gallons.
Why is the real water less than the tank size?
The size on the box is the outside size. The glass is inside that size, the water stops below the rim, and the gravel and decorations take room. Together they take about 20–35% off a small tank.
How much does a full 55 gallon tank weigh?
About 520 lb with 2 in of gravel and 1/4 in glass. The water alone is about 380 lb.
How much gravel do I need?
For a 2 in layer, a 55 gallon tank (48 × 13 in) needs about 69 lb of sand or gravel. The tool shows the weight for your layer. Bags list their weight, so buy by the pound or kilogram.
How do I measure a bow-front tank?
Type the length, the width at the ends (front to back) and the width in the middle of the bow. The tool adds the curved part as a slice of a circle.
Can I get litres?
Yes. Litres are always shown. Switch to Metres & cm to type the size in cm and get litres first.
Related tools
- Pool Volume Calculator — gallons in a swimming pool
- Gallons to Liters — convert any amount of water
- Liters to Gallons — from litres back to gallons
- Unit Converter — inches, cm, pounds and kilograms
How to use it
- Choose Feet & inches or Metres & cm
- Pick the shape of your tank and type its size
- Say whether you measured the outside or the inside, and check the glass thickness
- Type how far the water stays below the top, and how deep the sand or gravel is
- Use Add another tank to add up several tanks
- Read the gallons, litres and weight, and press Copy the list
