Fish tank volume calculator

Use inside measurements for a regular tank shape, or add up the water you actually poured in. Enter gravel, decor and any sump water separately.

For the example: 18.70 US gal of water. Liters and buckets ↓

Worked example: a 30 × 12 × 12 in tank with nothing displacing water. Replace the numbers with yours.

Measurement method

Tank shape or measured fillLength unitsMeasuredInside the glass, or outside: give the glass and we take it off

LengthInside left to rightWidthInside front to backWater heightBase to the waterline

Top view, not to scale. Measure the inside length and width, then the depth to the waterline.

Gravel, decor and sump water

Water displaced by gravel and decor, US galMeasured or estimatedSump or canister water, US galOnly if you have one

Gravel, rock and decor take up water: enter how much if you know it. We don’t guess it for you.

Your water change in gallons

Percentage to changeContainer capacity, US gal

25% is only an example; enter the amount you usually change.

Measure the water, not the label.

Outside arrows include glass and trim. Measure between the inside glass faces for water volume.

The water stops below the rim, and gravel and decor take up some of the space, so your tank holds less than its size suggests.

A tank sold as 20 gallons doesn’t always hold that much at its real waterline. Outside dimensions include glass and trim. Use the inside length and width, and measure to the actual water surface. If you can only measure the outside, for a rectangular tank set “Measured” to “Outside” and enter the glass thickness and the gap from the water to the top edge: the calculator takes the glass off each side and the bottom, and the gap off the height.

Substrate, rocks and equipment take up room that would otherwise be water. A sump or canister holds water of its own. You enter both yourself; the calculator doesn’t take off a hidden percentage.

For a cylinder, measure the internal diameter and water height. For a regular hexagon or octagon, enter one inside edge length; all sides and angles must be equal. Ellipse inputs are the two full inside diameters. For a quarter cylinder, diameter is twice the straight side (radius). Clipped-corner and L-shaped tanks use the full bounding rectangle plus the cut-out dimensions. The cut-out lengths run along the rectangle edges, not along a diagonal. Irregular bowfront tanks can use measured fill: total marked container amounts with all contents installed. Do not use their rectangular bounding box as water capacity. Count water held in porous rock and in a sump or canister once, not twice.

Know your water volume? Work out the filled weight →

Browse tank equipment plans · Choose practical water-change tools

How much substrate to buy

For a rectangular bed with an even front-to-back slope, use inside length × width × average depth. This estimates bulk substrate volume, including spaces between grains. It is not water displacement or weight.

Divide by the bag’s labeled volume and round up for whole bags. If it is sold by weight, you need that substrate’s own density, since sand, gravel and soil weigh very different amounts: the substrate calculator has CaribSea’s and counts the bags. This doesn’t choose a depth for your plants or fish. Follow the chosen material’s preparation instructions.