Aquarium Volume Calculator
This aquarium volume calculator converts tank dimensions (inches) to US gallons and liters for rectangular, cylindrical, and bow-front tanks. A standard 55-gallon aquarium measuring 48×13×21 inches holds approximately 56.7 gallons. Enter your dimensions to get an instant calculation.
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Frequently Asked Questions
How many gallons is my fish tank?
For a rectangular aquarium, multiply length × width × height (all in inches) then divide by 231 (cubic inches per US gallon). Example: a standard 55-gallon tank is 48×13×21 inches = 13,104 cubic inches ÷ 231 ≈ 56.7 gallons. Nominal sizes are approximate; measure your actual tank for exact volume.
What is the formula for aquarium volume in gallons?
For rectangular tanks: Gallons = Length (in) × Width (in) × Height (in) ÷ 231. For cylindrical tanks: Gallons = π × radius² × Height ÷ 231. The constant 231 is the number of cubic inches in one US liquid gallon.
How many gallons is a 48×13×21 inch aquarium?
A 48×13×21 inch tank has a volume of 48 × 13 × 21 = 13,104 cubic inches ÷ 231 ≈ 56.73 US gallons. This is nominally sold as a '55-gallon' aquarium, but the actual measured volume using 231 cu in/gallon is approximately 56.7 gallons.
How do I convert aquarium gallons to liters?
1 US gallon = 3.785 liters. Multiply gallons by 3.785 to get liters. For example, a 55-gallon tank contains approximately 55 × 3.785 = 208.2 liters. This calculator shows liters automatically alongside gallons.
How many fish can I put in my aquarium?
A common starting guideline is 1 inch of adult fish per gallon of water. So a 55-gallon tank can hold about 55 inches of fish. This is a rough rule — some fish need more space due to territory, waste production, or swimming habits. Research each species' specific requirements.
What size filter do I need for my aquarium?
Aquarium filters are rated in GPH (gallons per hour). For most community fish tanks, choose a filter that cycles the full tank volume 5–10 times per hour. For a 55-gallon tank, look for a filter rated 275–550 GPH. For messy fish or heavily planted tanks, err on the higher side.
How much does a filled aquarium weigh?
Water weighs approximately 8.34 lbs per US gallon (1 kg per liter). A filled 55-gallon tank contains about 458 lbs of water alone — add glass/acrylic, substrate, and decorations for total weight. Always ensure your furniture or stand can support this load.
What is a bow-front aquarium?
A bow-front aquarium has a curved front panel that bows outward, adding volume beyond a simple rectangle. The extra volume is approximated as a half-cylinder with the bow depth as the radius. This calculator adds the semicylindrical extra volume to the rectangular base.
Aquarium Volume Formulas
Aquarium volume is measured in US gallons. Dimensions are entered in inches. 1 US gallon = 231 cubic inches = 3.785 liters.
Rectangular
Gallons = L × W × H / 231
Cylindrical
Gallons = π × r² × H / 231
Bow Front (approximation)
Gallons = (L × W × H + π × bowDepth² × H / 4) / 231
Standard Tank Sizes
| Size | Dimensions (L×W×H in) | Gallons |
|---|---|---|
| 10-gallon (standard) | 20 × 10 × 12 | 10.4 |
| 20-gallon (long) | 30 × 12 × 12 | 15.6 |
| 29-gallon | 30 × 12 × 18 | 23.4 |
| 40-gallon (breeder) | 36 × 18 × 16 | 44.9 |
| 55-gallon | 48 × 13 × 21 | 56.7 |
| 75-gallon | 48 × 18 × 21 | 77.0 |
| 125-gallon | 72 × 18 × 22 | 127.4 |
Why Tank Volume Matters
- Stocking levels — The "inch per gallon" guideline suggests 1 inch of adult fish per gallon of water.
- Filtration — Filters are rated in GPH (gallons per hour). Aim for 4–10× tank volume per hour.
- Water changes — A 25% water change means removing and replacing 25% of the calculated volume.
- Dosing chemicals — Medications, dechlorinators, and additives are dosed per gallon.
- Heater sizing — Aquarium heaters are rated in watts. A common rule is 3–5 watts per gallon for tropical fish.
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