Aquarium tool

Aquarium Tank Volume Calculator

This aquarium volume calculator turns inside tank measurements into gross and net water volume in litres and US gallons. Add substrate depth, rock or equipment displacement if you want the water volume your fish actually get.

Tank shape and units
Inside measurements
in

Inside length, left to right.

in

Inside width, front to back.

in

From the inside bottom to your normal water line.

Measure inside the glass, not the outside. Glass thickness, bracing and the rim all reduce the water the tank holds.

Displacement (optional)
in

Average depth across the tank footprint.

Measured or estimated water displacement.

Substrate is estimated at 60% of its bulk volume for typical gravel or sand. Rock, driftwood and equipment are not guessed for you.

Runs in your browser. Your measurements are not uploaded or saved.

How to measure your tank

  1. Measure inside the glass, not the outside. Glass thickness can add an inch or more to each dimension.
  2. Use your normal water line for height, not the rim. A tank filled to the brace holds less water than its outside height suggests.
  3. For a bow-front tank, measure the front-to-back depth at the flat sides and again at the centre of the bow. The difference is the curve the calculator adds.
  4. For a round tank, measure the inside diameter across the middle.

How the volume is calculated

Rectangular tanks use length × width × height. Cylindrical tanks use π × radius² × height. Bow-front tanks use the rectangular volume at the side depth plus a circular segment for the bow, which is why you enter both end depth and centre depth.

Results show litres and US gallons. One US gallon is 3.785 litres, and one litre of fresh water weighs about one kilogram.

Worked example

A rectangular tank measuring 36 × 18 × 18 inches inside, filled to 18 inches, holds about 191.1 litres or 50.5 US gallons gross. With 2 inches of substrate and 5 US gallons of rock and equipment, net water volume drops to about 159.5 litres or 42.1 US gallons.

What the displacement fields do

Substrate displacement is calculated from your tank footprint and average substrate depth. The calculator assumes gravel or sand displaces about 60% of its bulk volume, because water fills the gaps between grains. Fine sand, coarse gravel and planted substrates vary, so treat the result as an estimate.

Rock, driftwood and equipment displacement is not estimated for you. Enter a number if you know it, or leave it at zero if you want gross volume only.

What this calculator does not include

  • Sump, filter, plumbing or canister volume.
  • Glass thickness, bracing, rim or overflow cut-outs.
  • Water displacement from fish, plants and open rock pores.
  • Custom curves that are not close to a circular arc.

For stocking decisions, use net water volume and then check a species guide's minimum tank size rather than assuming a nominal tank size is enough.

Frequently asked questions

How do I calculate aquarium volume in gallons?

Measure the inside length, width and height in inches, multiply them together, then divide by 231. That gives gross US gallons. This calculator does the same conversion and subtracts substrate and decor displacement for net water volume.

Should I measure inside or outside the tank?

Measure inside the glass and up to your normal water line. Outside dimensions include glass thickness and the rim, so they overstate the water the tank can actually hold.

How much water does gravel or sand displace?

Substrate displaces roughly 60% of its bulk volume for typical gravel or sand, because water fills the gaps between grains. The calculator uses that estimate. Fine sand, coarse gravel and planted substrates can differ.

Does the volume include my sump or filter?

No. The result covers the display tank only. Add sump, filter and plumbing volume separately if you need the whole system's water volume.

How accurate is the bow-front calculation?

It treats the bow as a circular segment using your end depth and centre depth. That is close for standard bow-front tanks, but custom curves or unusually shallow bows are estimates only.

Why is net volume lower than gross volume?

Substrate, rock and equipment take up space that water cannot fill. Net volume is the water your fish actually swim in, which is the better number for stocking and dosing.

Next step

Choosing fish for the tank you just measured? Our freshwater fish guide for beginners compares species groups by tank size, temperature and temperament.