Octagon Aquarium Volume Calculator

1. Side Length: Measure the inside length of one panel, from seam to seam. If you only know the flat-to-flat width, the side length is that width multiplied by 0.4142.
2. 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 tank has one continuous curved wall instead of eight flat panels, use the Cylindrical calculator instead.

The calculator treats the base as a regular octagon: a square with its four corners cut at 45 degrees, so the base area is about 4.828 × s². All eight panels must be the same length; if they differ, the result is an approximation.

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 octagon aquarium with the following inside dimensions: 6-inch side length, 18 inches tall.

1. Calculate octagonal area: 2(1 + √2) × 6² = 4.828 × 36 = 173.82 square inches
2. Multiply by height: 173.82 × 18 = 3,128.76 cubic inches
3. Convert to gallons: 3,128.76 ÷ 231 = 13.55 US gallons
4. Convert to liters: 13.55 × 3.785 = 51.28 liters

Result: This octagon aquarium holds approximately 13.55 US gallons (51.28 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.
• Measuring across the tank instead of one panel:The formula needs the length of a single flat panel, not the distance across the tank between two opposite panels. Confusing the two — since the cross-tank distance is significantly larger — will badly overstate the calculated volume.
• Assuming alternating long and short panels are a true regular octagon:Some tanks marketed as octagons actually have alternating long and short sides rather than eight equal panels. If your tank has this pattern, this formula won't apply exactly — measure and calculate each distinct panel length separately.