How It Is Calculated
This calculator estimates the usable water volume and weight for a rectangular fish tank. The calculation follows these steps:
- Total volume in cubic inches: Multiply the inside length (L), width (W), and height (H): Vin³ = L × W × H.
- Convert to gallons: Divide by 231 (cubic inches per gallon): Vgal total = Vin³ / 231.
- Apply displacement: Subtract the percentage (D) occupied by gravel, decorations, and unfilled space: Vgal = Vgal total × (1 − D / 100). This is the primary output.
- Convert to cubic feet: Divide by 1728 (cubic inches per cubic foot): Vft³ = (Vin³ / 1728) × (1 − D / 100).
- Water weight: Multiply the usable gallons by the density of water: for freshwater (F) use 8.34 lb/gal; for saltwater (S) use 8.55 lb/gal. Weight = Vgal × (8.34 if freshwater, 8.55 if saltwater).
Vgal = (L × W × H / 231) × (1 − D / 100)
Weight = Vgal × ρ (where ρ = 8.34 for freshwater, 8.55 for saltwater)
Assumptions
- The tank is rectangular with flat, parallel sides.
- All dimensions are inside measurements (from glass to glass).
- The displacement percentage accounts for gravel, decorations, and the air gap at the top.
- Water density is constant: 8.34 lb/gal for freshwater, 8.55 lb/gal for saltwater.
- The calculator does not include the volume of the glass itself or any internal equipment (e.g., filters, heaters).
Worked Example
For a tank with Length = 48 in, Width = 12 in, Height = 20 in, Displacement = 10%, and Water type = Freshwater:
- Vin³ = 48 × 12 × 20 = 11,520 in³.
- Vgal total = 11,520 / 231 = 49.87 gal (approx).
- After 10% displacement: Vgal = 49.87 × 0.9 = 44.88 gal.
- Vft³ = (11,520 / 1728) × 0.9 = 6.67 × 0.9 = 6.00 ft³.
- Water weight (freshwater): 44.88 × 8.34 = 374.3 lbs.
Practical Tips
- Always measure the inside dimensions of your tank for the most accurate results. Outside dimensions will overestimate volume.
- Displacement of 10–15% is typical for a lightly decorated tank; heavily decorated tanks may need 20–25%.
- Use the water weight to ensure your stand or floor can support the total load. A filled 55-gallon tank can weigh over 450 lbs.
- If you need to calculate the volume at a specific fill level (e.g., 75% full), use the tank volume with fill level calculator.
- For sizing a pump or filter, consider the pump power calculator to match flow rate and head.
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