Tank names round the gallons, and the glass, the gap at the top and the gravel all take room, so the water in a tank is less than its name. Measuring gives the real number you need for medicine, water conditioner, salt mix and the stand.
How to use this calculator
- Choose the shape: rectangle or cube, bow front, cylinder or hexagon.
- Measure inside the glass. For a bow front, take the width at the straight ends and the bow at the middle; for a hexagon, the width of one glass panel.
- Choose freshwater or saltwater: salt water weighs more.
- Under More options, set how far below the top you fill and how deep the gravel or sand is, to get the water you will really add.
Worked examples
A long rectangle tank
Size: 30 × 12 in · Height: 12 in
- Tank volume
- 18.7 US gallons
- Watts of heater
- 95
- Liters
- 70.79 L
- Water to fill
- 17.1 US gallons
- Weight of the water
- 143 lb
- Set-up tank, about
- 187 lb
A reef tank
Size: 36 × 18 in · Height: 24 in · Water: Saltwater (reef or marine)
- Tank volume
- 67.3 US gallons
- Watts of heater
- 200
- Liters
- 255 L
- Water to fill
- 64.5 US gallons
- Weight of the salt water
- 552 lb
- Set-up tank, about
- 673 lb
A bow front
Tank shape: Bow front · Size: 36 × 15 in · Bow: 3 in · Height: 20 in
- Tank volume
- 53.0 US gallons
- Watts of heater
- 265
- Liters
- 201 L
- Water to fill
- 50.4 US gallons
- Weight of the water
- 420 lb
- Set-up tank, about
- 530 lb
A cylinder tank
Tank shape: Cylinder · Diameter: 18 in · Height: 24 in
- Tank volume
- 26.4 US gallons
- Watts of heater
- 130
- Liters
- 100 L
- Water to fill
- 25.3 US gallons
- Weight of the water
- 211 lb
- Set-up tank, about
- 264 lb
A hexagon tank
Tank shape: Hexagon · Width of one glass panel: 12 in · Height: 24 in
- Tank volume
- 38.9 US gallons
- Watts of heater
- 195
- Liters
- 147 L
- Water to fill
- 37.3 US gallons
- Weight of the water
- 311 lb
- Set-up tank, about
- 389 lb
How the gallons are worked out
Multiply the inside length, width and height in inches and divide by 231, the cubic inches in a US gallon. A bow front is the rectangle plus the curved part in front of it, a slice of a circle; a cylinder is the circle times the height; a hexagon is six equal triangles.
That is the volume of the glass box. The water you pour in is less: you fill to a little below the rim, and the gravel, rock and decorations take up room. The calculator takes off the gap and the substrate you enter.
How much a full aquarium weighs
Fresh water weighs 8.34 lb per US gallon. Salt water is heavier by its specific gravity, about 1.026 for natural seawater; reef keepers hold their tanks between 1.02 to 1.026.
With the glass, gravel and decorations, Aqueon puts a set-up tank at about 10 lb per gallon per gallon. Put it on a stand made for that weight, check that the stand is level and does not rock, and shim it if needed. For a large tank on an upper floor, ask someone who knows framing.
What size heater the tank needs
Aqueon's rule of thumb is 5 watts per gallon for tanks up to 55 gallons and 3 watts per gallon for bigger ones, for a room at 68 to 72 °F. In a cooler room, or for tropical fish that like it warm, go a size up; in a big tank, two smaller heaters at each end spread the heat and back each other up.
Common questions
How many gallons is my fish tank?
Measure the inside length, width and height in inches, multiply them and divide by 231. The calculator does it for rectangle, bow front, cylinder and hexagon tanks, and shows the water to fill it too.
Why does my tank hold less than its name?
Tank names round the size, and the outside measurements include the glass and the frame. Filling below the rim and the gravel take away some more, so the water inside is usually a bit less than the name.
How much does a full fish tank weigh?
Set up with water, gravel and decorations, about 10 lb per gallon per gallon. The water alone weighs 8.34 lb per US gallon, a little more for salt water.
What size heater do I need for my aquarium?
About 5 watts per gallon for tanks up to 55 gallons, and 3 watts per gallon for bigger tanks, in a room at normal indoor temperature. The calculator works it out for your tank.
Does salt water weigh more than fresh water?
Yes, by its specific gravity: natural seawater is about 1.026 times as heavy as fresh water, so a saltwater tank weighs a little more.