Flat Back Hexagon Aquarium Volume Calculator

Measure the front panel width (the widest flat glass at the front), the depth from the back flat panel to the front panel center, and the inside height. The angled wing panels are calculated from the front width and depth.

Always use a rigid tape measure or ruler for straight dimensions. For curved surfaces, a flexible sewing tape pressed against the glass gives the most accurate reading. Take each measurement at least twice and average the results to minimize error. If your tank has a plastic frame or trim, slide your measuring tool inside the frame so it contacts the glass directly. Accurate inside measurements give you the true water capacity of your fish tank — the number that matters for dosing, filtration, and stocking decisions.

For the most accurate volume calculation, measure to the actual water line rather than to the top rim of the tank. Most aquarists keep the water level 1–2 inches below the rim to prevent overflow during maintenance. This difference can reduce effective volume by 5–10% compared to a brim-full measurement.

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: 12-inch front panel, 10-inch depth, 18 inches tall.

1. Rectangle area: 12 × 10 = 120 square inches
2. Wing area: 2 × ½ × (12÷2) × 10 = 2 × ½ × 6 × 10 = 60 square inches
3. Total cross-section: 120 + 60 = 180 square inches
4. Multiply by height: 180 × 18 = 3,240 cubic inches
5. Convert to gallons: 3,240 ÷ 231 = 14.03 US gallons

Result: This flat back hexagon aquarium holds approximately 14.03 US gallons (53.10 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 stocking calculations and 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 displace additional water. In a heavily aquascaped tank, hardscape can reduce water volume by another 5–15%. The most accurate method to measure this is the water displacement approach: note the water level before and after adding hardscape, then calculate the volume of the water level change.

Once you have an accurate water volume — accounting for fill height, substrate, and hardscape — use it as the basis for all tank management decisions. Medication and supplement dosing should always be calculated against actual water volume, not the nominal gallon label. Filter turnover rate (typically 4–10× tank volume per hour) and heater wattage (approximately 3–5 watts per gallon) both depend on accurate volume. For a precise breakdown of 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.
• Including the frame or trim: Plastic frames add 0.5–1.5 inches per side. Always measure inside the frame, from glass surface to glass surface.
• Measuring to the rim instead of the water line: Unless you fill to the absolute top (which is impractical), measure to where you actually keep the water level for an accurate usable volume.
• 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 depth from the wrong reference point:Depth should be measured from the back flat panel to the center of the front panel, not to the outer edge of an angled wing section, which would overstate the front-to-back distance.
• Confusing front panel width with the back panel width:On a flat-back hexagon, the back panel is typically narrower than the front. The formula uses the front panel width specifically — measure the widest flat glass at the front, not the shorter back panel.