Bow Front Aquarium Volume Calculator

1. Length: Measure the inside length along the flat back panel, from one side glass to the other.
2. Flat Depth: At one end of the tank, measure the inside depth along the straight side panel, from the back glass to the point where the front glass starts to curve.
3. Bow Depth: At the middle of the curve, measure the total inside depth from the back glass to the furthest point of the front glass, then subtract the flat depth.
4. 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 front glass is flat, with no outward curve, use the Rectangular calculator instead.

The calculator adds a rectangular body (length × flat depth) to a half-ellipse bow that is as wide as the tank and as deep as the bow depth. A bow with a different curve profile holds somewhat more or less than this estimate. In the worked example below the bow front holds 42.48 US gallons; a rectangular calculator would give 44.88 gallons using the 16-inch total depth or 33.66 gallons using only the 12-inch flat depth, and neither matches.

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 bow front aquarium with the following inside dimensions: 36 inches long, 12-inch flat depth, 4-inch bow depth, 18 inches tall.

1. Rectangular section: 36 × 12 × 18 = 7,776 cubic inches
2. Bow section: π × (36÷2) × 4 × 18 ÷ 2 = π × 18 × 4 × 18 ÷ 2 = 2,035.75 cubic inches
3. Total volume: 7,776 + 2,035.75 = 9,811.75 cubic inches
4. Convert to gallons: 9,811.75 ÷ 231 = 42.48 US gallons
5. Convert to liters: 42.48 × 3.785 = 160.79 liters

Result: This bow front aquarium holds approximately 42.48 US gallons (160.79 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 bow depth from the wrong reference point:Bow depth is how far the curve projects beyond the flat depth: measure from the plane of the flat back panel to the farthest point of the curve, then subtract the flat depth — do not measure from the front rim or average across the curve. Entering the total depth in the bow depth field overstates the curved section's contribution.
• Treating a bow front like a straight rectangular tank:Estimating a bow front's volume using only length × flat depth × height ignores the extra water the curved bow section holds, which can be several gallons on a large display tank. Always use the bow front formula, not the rectangular one, for a curved-front tank.