Rectangular Aquarium Volume Calculator

1. Length: Measure the longest horizontal inside edge, from glass to glass.
2. Width: Measure the inside width from the front glass to the back glass.
3. Height: Measure from the inside bottom of the tank up to the water line.

Measure inside dimensions, glass to glass, inside any frame or trim, and measure each dimension twice.

This formula calculates the interior volume in cubic units matching your input measurements. If you measure in inches, the result is cubic inches; divide by 231 to convert to US gallons, or multiply gallons by 3.785 for liters. If you measure in centimeters, the result is cubic centimeters; divide by 1,000 for liters.

Consider a rectangular aquarium with the following inside dimensions: 24 inches long, 12 inches wide, 16 inches tall.

1. Multiply length by width: 24 × 12 = 288 square inches
2. Multiply by height: 288 × 16 = 4,608 cubic inches
3. Convert to gallons: 4,608 ÷ 231 = 19.95 US gallons
4. Convert to liters: 19.95 × 3.785 = 75.51 liters

Result: This rectangular aquarium holds approximately 19.95 US gallons (75.51 liters) of water when filled to the measured height.

Fill Height

The calculator above assumes you fill the tank to the full measured height. In practice, most aquarists keep the water level 1–2 inches below the rim to prevent overflow during water changes, when adding hands and equipment, or if fish splash. Use the fill height adjustment toggle in the calculator to enter your actual water line height for a more realistic volume estimate.

For tanks with open-top (rimless) designs, the gap between water surface and glass top is especially important to track. A 2-inch gap on a 20-inch tall tank reduces effective volume by 10%, which is significant for medication dosing.

Substrate Displacement

Gravel, sand, and planted substrates occupy space that would otherwise hold water. A 2-inch layer of gravel in a standard rectangular tank displaces roughly 5–8% of the total tank volume. Use the substrate displacement toggle in the calculator to subtract this volume from your water capacity estimate.

Rock and Decoration Displacement

Large rocks, driftwood, and dense decorations also displace water, which this calculator does not subtract. Use the aquarium displacement calculator to estimate how much.

For how glass thickness reduces interior volume on your specific tank size, see the glass thickness and volume loss guide.

The calculator displays estimated water weight for both freshwater (8.34 lbs per gallon) and saltwater (8.55 lbs per gallon at 1.025 specific gravity). These estimates help you determine whether your floor and stand can safely support the filled tank, substrate, and equipment combined.

Water weight estimates are approximate and based on standard conditions. Freshwater weight assumes pure water at 62 °F (16.7 °C) with a density of 8.34 lbs/gal (1.0 kg/L). Saltwater weight assumes a specific gravity of 1.025, typical of reef aquariums. Actual weight may vary depending on water temperature, salinity, dissolved minerals, and substrate displacement. Always verify that your aquarium stand and floor can support the total weight of the filled tank, substrate, equipment, and decorations before filling.

• Measuring outside dimensions: Glass or acrylic thickness ranges from 3 mm to 19 mm per panel. Using outside dimensions overstates volume, especially on smaller tanks where glass thickness is a larger proportion of total width.
• Mixing measurement units: Entering some dimensions in inches and others in centimeters produces nonsensical results. Use one unit system consistently and let the calculator handle conversions.
• Confusing length and width:On a rectangular tank, length is the longest horizontal side and width is measured front to back. Swapping them doesn't change the total volume, but it does change which measurement you're comparing against a manufacturer spec sheet.
• Rounding a near-square footprint to a cube:If your tank's length and width are close but not identical, use this rectangular calculator rather than the cube calculator — even a small difference between length and width changes the volume more than it looks like it should.

Most aquarium volume questions arrive as raw dimensions — if you have a tank with specific measurements, enter them directly into the calculator above. The worked examples below cover the most commonly searched dimension combinations.

All values use interior dimensions — if measuring from outside the glass, subtract glass thickness on each side before calculating. See the glass thickness volume loss guide for precise adjustments.

The last column uses the standard sizes listed on the aquarium volume by size page: an exact match shows that size's name, the same length and width shows the matching footprint, and a dash means no standard size matches.