Aquarium volume calculator

Find how many gallons and liters your tank holds, from its size on the label down to the actual water once the glass, substrate and gap at the top are taken out.

Inputs

Tank shape

Most glass tanks use 3/16–1/2 in. Enter 0 if you measured the inside.

Sand or gravel bed. 0 for a bare bottom.

From the top rim down to the water line.

Tank volume

57gal

≈ 215 L. About 46 gal (173 L) of actual water after glass, substrate and headroom.

Nominal
57 gal · 215 L
Actual water
46 gal · 173 L
Water weight
380 lb
Substrate
5.1 gal · 19.5 L
Your tank, filled to the water line

A 48 in × 13 in × 21 in rectangular tank has a nominal volume of 57 gal (215 L). With 0.25 in glass, 2 in of substrate and the water 1 in below the rim, it holds about 46 gal (173 L) of water, 80% of nominal, which weighs about 380 lb.

How it works

Every tank is a shape seen from above (its footprint) times its height. The label size uses the outside dimensions, and the real water sits inside the glass, above the substrate and below the water line:

nominal volume = footprint area × height
actual water   = inner footprint × (height − bottom glass − substrate − gap at top)
gallons        = cubic inches ÷ 231        liters = cubic cm ÷ 1,000
footprint area by shape:
rectangle  L × W                     cube      side²
cylinder   π × diameter² ÷ 4         hexagon   2.598 × panel²
corner     π × wall² ÷ 4             (quarter circle)
bow-front  L × end depth + circular segment (chord L, bow = center − end depth)
  • Nominal volume is what the dimensions you entered enclose, which is how manufacturers label tanks. Checked against Aqueon’s chart of 20 standard tanks, the outside dimensions always come out at or above the label.
  • Inside the glass the walls are inset by the glass thickness. Aqueon’s labels line up closely with this interior volume: with 1/4 in glass, 16 of the 20 tanks land within 6% of their label.
  • Actual water also takes out the substrate bed and the gap between the rim and the water line.
  • Water weight is the actual water at 8.32 lb per gallon (fresh water at 77 °F), 1% more for brackish and 2.5% more for reef salt water.
  • Bow-fronts are a rectangle plus a circular segment: the bent front glass is an arc across the tank’s length, bulging by the difference between the center and end depths.

Worked example

Aqueon’s standard 55 gallon tank measures 48¼ × 12¾ × 21 in on the outside:

  • Nominal: 48.25 × 12.75 × 21 = 12,919 in³ ÷ 231 = 55.9 gal (212 L)
  • Inside 1/4 in glass: 47.75 × 12.25 × 20.75 in = 52.5 gal
  • With 2 in of sand and the water 1 in below the rim, the water is 17.75 in deep: 47.75 × 12.25 × 17.75 = 45 gal (170 L) of actual water
  • That water weighs about 374 lb (170 kg). Carry the tank size over to the weight calculator to add the glass, sand and rock.

So a “55” really holds about 45 gallons once it’s set up, which is the number to use for medication, fertilizer and salt dosing.

Tips

  • Measure outside, then set the glass. The outside is easiest to measure accurately. Enter the glass thickness (look at the top edge) and the calculator works out the inside.
  • Dose by actual water, size equipment by nominal. Filter and heater ratings are quoted for the tank’s label size. Medication and salt should be dosed for the water that’s really there.
  • Remember the sump. If you run a sump, add its water to the total for dosing, but not to the display tank’s size.
  • Rock and décor displace water too. A reef with 50 lb of live rock loses about 3 more gallons. Enter a bit more substrate depth if you want to account for it.

FAQ

How many gallons is a 48 × 13 × 21 inch tank?

48 × 13 × 21 = 13,104 cubic inches, and 13,104 ÷ 231 = 56.7 gallons (215 L). That's the nominal size, the standard "55 gallon" tank. Set up with 1/4 in glass, 2 in of sand and the water an inch below the rim, it holds about 46 gallons (173 L) of water.

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

Labels are based on the outside dimensions, rounded to a size class. The glass takes up some room, the substrate takes more, and most people keep the water an inch or so below the rim. Together that's usually 15–25% less water than the label, which matters when you dose medication or salt.

How do I measure a bow-front tank?

Measure the length across the back, the depth front to back at one end (along a side panel), the depth through the middle of the bow, and the height. The calculator treats the bow as a circular arc, which is how bow-front glass is bent.

How do I measure a hexagon or corner tank?

For a hexagon, measure one flat glass panel edge to edge, plus the height. For a corner tank, measure one of the two straight back walls from the corner out. We model the front as a quarter circle, like Aqueon's 54 gallon corner tank. A corner tank with a flat front holds about a third less.

Does the water in the sand count?

A little water does sit between sand and gravel grains, roughly a third of the bed's volume. We leave it out, so the actual-water figure is slightly low. For dosing, that's the safer direction.

Sources

  1. Aquarium catalog, "Critical dimensions for all aquariums" (p. 66), Aqueon. Outer dimensions of 22 rectangular tanks, the 54 gal corner and three bow-fronts, with their labeled gallons.
  2. NIST Handbook 44, Appendix C: General Tables of Units of Measurement, National Institute of Standards and Technology. 1 US gallon = 231 cubic inches exactly; 1 liter = 1,000 cubic centimeters.
  3. Hexagon; Circular Segment, Wolfram MathWorld. Area of a regular hexagon (3√3/2 × side²) and of a circular segment, used for hexagon and bow-front tanks.

Formula and sources last checked September 30, 2026. How we test formulas.