Gallons and liters your fish tank really holds after glass, the gap below the rim and substrate, plus water weight, the load on the stand and a water change table.
Aquarium Volume Calculator - Gallons, Liters and Weight
Gallons and liters your fish tank really holds after glass, the gap below the rim and substrate, plus water weight, the load on the stand and a water change table.
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How many gallons is my fish tank?
Measure the tank, pick its shape, and the aquarium volume calculator returns the gallons and liters of water it actually holds, what that water weighs, how many gallons each inch of water level is worth, and a water change table. Glass thickness, the gap you leave below the rim and the depth of the substrate are optional, and each one takes water away from the number on the box. Rectangular, bow front, cylinder and hexagon tanks, in inches or centimeters.
Measuring the tank, box by box
- Unit and shape - inches or centimeters, and one of four shapes. A tank with a curved front panel is a bow front; a six-sided column is a hexagon.
- Outside dimensions - length, front-to-back width and height for a rectangle; for a bow front the depth at the side panels and at the middle of the bow; the diameter of a cylinder; one side of a hexagon. Measure the glass, not the trim or the lid.
- Glass thickness - optional, in the same unit. A quarter inch is 0.25, 3/8 inch is 0.375.
- Gap below the rim and substrate depth - optional. Measure from the water line to the top edge, and from the bottom glass to the top of the gravel or sand.
- Specific gravity and dry weight - optional. Enter the hydrometer or refractometer reading for saltwater, and the weight of the empty tank, substrate and rock (lb for inches, kg for centimeters) for the load on the stand.
The area formula behind each shape
Volume is the floor area times the height of water. The calculator works in cubic inches or cubic centimeters and converts at 231 cubic inches per US gallon, the exact NIST definition, and 1,000 cubic centimeters per liter.
| Shape | Floor area | What the glass takes away |
|---|---|---|
| Rectangular | length x width | two thicknesses from length and width, one from height |
| Bow front | length x side depth + 2/3 x length x bow | two from length and from both depths, so the bow stays the same |
| Cylinder | 3.1416 x radius² | one thickness from the radius |
| Hexagon (regular) | 2.598 x side² | one thickness from the distance between the center and each wall |
The bow is treated as a parabolic arc: the 2/3 is the area of a parabolic segment relative to the rectangle around it. A circular arc of the same depth differs very little; on a 36 inch front with a 4 inch bow it adds 0.94 square inches of floor, about 0.2% of the volume. Water weight uses the USGS density of freshwater at 80 °F, 0.99669 g/cm³, which comes to about 8.3 lb per gallon; saltwater multiplies it by the specific gravity.
Ten tanks run through the calculator
Every figure below came out of the calculator with the inputs listed. Where glass and a gap are given, the gallons are water, not the outside volume.
20 x 10 x 12 in, bare measurement
10.4 gal, 39.3 L, about 86 lb of water. One inch of level is 0.87 gal.
The same tank, set up
3/16 in glass, 1 in gap, 2 in sand: 7.2 gal of water. Close to a third less than the bare figure.
30 x 12 x 12 in, long and low
18.7 gal, about 156 lb, 1.56 gal per inch of level.
30 x 12 x 18 in, same footprint, taller
28.1 gal and 233 lb. Still 1.56 gal per inch, because the floor did not change.
36 x 18 x 16 in, wide and shallow
44.9 gal, 373 lb, 2.81 gal per inch.
48 x 13 x 21 in, 1/4 in glass, 1 in gap
Outside 56.7 gal, water 50.8 gal weighing 422 lb. A 25% change is a 5 inch drop, 12.85 gal.
48 x 18 x 21 in with 140 lb dry weight
1/4 in glass, 1 in gap, 2 in substrate: 63.9 gal, 531 lb of water, 671 lb on the stand.
36 x 24 x 24 in reef at 1.025
3/8 in glass, 1 in gap, 1.5 in sand: 74.9 gal of saltwater weighing 639 lb, against an outside volume of 89.8 gal.
Bow front 36 x 12 in, 16 in at the bow, 22 in tall
1/4 in glass, 1 in gap: 45.2 gal. A flat front with the same 12 inch depth holds 36.7, so the bow adds 8.5 gal.
Hexagon, 12 in sides, 24 in tall
1/4 in glass, 1 in gap: 35.1 gal, 292 lb, 1.54 gal per inch.
Where the gallons go between the box and the water
The second and the eighth tank above show it best. Taking the reef tank from its outside volume to the water you dose, step by step:
| 36 x 24 x 24 in tank | Gallons | Lost at this step |
|---|---|---|
| Outside dimensions | 89.8 | - |
| Inside 3/8 in glass, filled to the rim | 83.8 | 6.0 |
| Water 1 in below the rim, over 1.5 in of sand | 74.9 | 8.9 |
The water held between sand grains is not counted, so the real figure sits slightly above the last row, and rock and decorations pull it back down. For dosing salt, medication or conditioner, the last row is the honest starting point; the outside volume overstates it by about a fifth here.
Fish tank volume questions
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