Flat Back Hexagon Aquarium Volume Calculator

1. Back Length: Measure the full inside width of the straight rear panel, the glass that sits against the wall, from one straight side panel to the other.
2. Overall Depth: Measure the greatest inside distance from the rear panel to the front panel, at a right angle to the rear panel and not along a side panel or an angled panel.
3. Front Panel Width: Measure the inside width of the flat front panel only, between the two angled front-corner panels. It is never wider than the back length.
4. Straight Side Panel Length: Measure the inside length of one straight side panel, from its rear corner forward to where the angled front-corner panel begins. It is never longer than the overall depth. If the angled front-corner panels are at 45 degrees, it equals the overall depth minus half of the difference between the back length and the front panel width.
5. 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.

• If the angled front-corner panels start at the back corners, so the tank has no straight side panels, use the Trapezoid calculator instead.
• If the tank has six equal panels and no flat wall side, use the Hexagonal calculator instead.

The calculator models a rectangle, back length by overall depth, with a right triangle cut off each front corner by an angled front-corner panel. The straight side panels run forward from the back corners and the flat front panel sits between the two angled panels. The cut-away area is (L - F) × (W - P) ÷ 2, so the base is L × W minus that, and no panel angle is needed: any angle works as long as the panels are straight and flat. A rounded or bent corner is only approximated. A front panel wider than the back, or straight side panels longer than the overall depth, describe no flat back outline, and the calculator then reports that the dimensions are outside the supported calculation range.

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 flat back hexagon aquarium with the following inside dimensions: 36-inch back length, 12-inch overall depth, 24-inch front panel width, 6-inch straight side panel length, 16 inches tall.

1. Rectangle area: 36 × 12 = 432 square inches
2. Cut-away corners: ½ × (36 - 24) × (12 - 6) = ½ × 12 × 6 = 36 square inches
3. Base area: 432 - 36 = 396 square inches
4. Multiply by height: 396 × 16 = 6,336 cubic inches
5. Convert to gallons: 6,336 ÷ 231 = 27.43 US gallons

Result: This flat back hexagon aquarium holds approximately 27.43 US gallons (103.82 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.
• Entering the whole front as the front panel width:The front panel width is only the flat panel between the two angled front-corner panels. Entering the whole front, angled panels included, makes it as wide as the back and removes the cut corners from the result. In the worked example that raises the result from 27.43 to 29.92 US gallons, about 9% too high.
• Entering an angled panel's length as the straight side panel length:The straight side panel runs straight forward from the rear corner. The angled front-corner panel is longer and is not an input. In the worked example, entering the angled panel's 8.49-inch length instead of the 6-inch straight side panel raises the result by about 3.8%.
• Swapping the back length and front panel width, or the depth and side panel length:The front panel cannot be wider than the back, and the straight side panels cannot be longer than the overall depth. If two fields are swapped, the numbers describe no flat back outline and the calculator reports that the dimensions are outside the supported calculation range.