in
Enter length > 0.in
Enter width > 0.in
Enter height > 0.Tank Volume
—
Was this calculator helpful?
Sources & Methodology
Volume formulas use standard geometric calculations. US gallon = 231 cubic inches exactly. 1 liter = 1,000 cubic centimeters. Fish stocking guidelines from established aquarium husbandry standards.
NIST — Unit Conversion Reference
National Institute of Standards and Technology reference for the exact US gallon definition (231 cubic inches) and liter (1,000 cm³) used in all volume conversions on this page.
Fishkeeping World — Aquarium Size & Stocking Guide
Reference for standard aquarium stocking guidelines including the 1 inch per gallon rule and recommended fish per gallon ratios used in the stocking recommendations section.
Rectangular: Volume (in³) = L × W × H • US Gallons = in³ ÷ 231 • Liters = cm³ ÷ 1000
Cylindrical: Volume (in³) = π × (diameter/2)² × H • US Gallons = in³ ÷ 231
Bow Front: Effective volume ≈ rectangular × 1.10 (bow adds ~10% more water)
Hexagonal: Volume (in³) = (3√3/2) × side² × H • US Gallons = in³ ÷ 231
Actual water: Subtract 10–20% for substrate and decorations.
Cylindrical: Volume (in³) = π × (diameter/2)² × H • US Gallons = in³ ÷ 231
Bow Front: Effective volume ≈ rectangular × 1.10 (bow adds ~10% more water)
Hexagonal: Volume (in³) = (3√3/2) × side² × H • US Gallons = in³ ÷ 231
Actual water: Subtract 10–20% for substrate and decorations.
⏱ Last reviewed: April 2026
How to Calculate Aquarium Tank Volume
Knowing your aquarium’s exact water volume is essential for proper fish stocking, medication dosing, fertilizer amounts, CO2 injection rates, and water change calculations. The formula varies by tank shape — rectangular is the simplest, while curved front and hexagonal tanks require geometric adjustments.
Volume Formulas by Tank Shape
Rectangular: Gallons = (L × W × H) ÷ 231 (dimensions in inches)
Cylindrical: Gallons = (π × r² × H) ÷ 231 (r = radius = diameter/2)
Hexagonal: Gallons = ((3√3/2) × s² × H) ÷ 231 (s = side length)
Metric: Liters = (L × W × H) ÷ 1000 (dimensions in cm)
231 cubic inches = 1 US gallon (exact). 1,000 cm³ = 1 liter (exact). Always use interior dimensions.
Standard US Aquarium Sizes Reference
| Tank Name | L × W × H (inches) | US Gallons | Liters |
|---|---|---|---|
| 10 Gallon | 20 × 10 × 12 | 10.4 | 39.3 |
| 20 Gallon Long | 30 × 12 × 12 | 18.7 | 70.8 |
| 29 Gallon | 30 × 12 × 18 | 28.1 | 106.3 |
| 40 Gallon Breeder | 36 × 18 × 16 | 45.2 | 171.2 |
| 55 Gallon | 48 × 13 × 21 | 56.7 | 214.6 |
| 75 Gallon | 48 × 18 × 21 | 78.5 | 297.2 |
| 90 Gallon | 48 × 18 × 24 | 89.7 | 339.6 |
| 125 Gallon | 72 × 18 × 22 | 160.5 | 607.5 |
Fish Stocking Guidelines
- The 1-inch-per-gallon rule: For small fish (under 1 inch), 1 inch of adult fish body length per gallon of water. This is a rough minimum guideline — more space is always better.
- Small fish (1–2 inches): Allow 1–2 gallons per fish as a starting point
- Medium fish (2–4 inches): Allow 3–5 gallons per fish
- Large fish (4+ inches): Allow 10+ gallons per fish; some species need much more
- Actual water volume: Subtract 10–20% from total volume for substrate and decorations
💡 Water Change Tip: For most freshwater tanks, change 25% of the water weekly. For reef tanks, change 10–15% twice weekly. Multiply your actual water volume by 0.25 to find the weekly change amount. Example: 55-gallon tank with ~48 gallons actual water → weekly change = 48 × 0.25 = 12 gallons.
Frequently Asked Questions
For rectangular tanks: multiply length × width × height (in inches), then divide by 231 for US gallons. Example: 48 × 13 × 21 = 13,104 cubic inches. 13,104 ÷ 231 = 56.7 gallons. Use centimeters and divide by 1,000 for liters.
A standard 4-foot (48-inch) aquarium is typically a 55-gallon tank with dimensions 48” × 13” × 21”. The exact volume is 56.7 gallons. A 4-foot tank with an 18-inch width becomes a 75-gallon (78.5 gallons actual). The name refers to approximate capacity, not exact volume.
Measure all dimensions in centimeters. Multiply L × W × H to get cubic centimeters. Divide by 1,000 to get liters. Example: 120 cm × 40 cm × 50 cm = 240,000 cm³ ÷ 1,000 = 240 liters. Alternatively, multiply US gallons by 3.785 to convert.
Actual usable water volume is typically 80–90% of the calculated total. Substrate (gravel/sand) takes 10–15% of volume; decorations and rocks take 5–10%; you should not fill to the very brim. A 55-gallon tank typically holds about 45–50 gallons of actual water. Always use actual water volume for medication and stocking calculations.
The general 1-inch-per-gallon rule applies to small fish (under 1 inch). For medium fish (1–3 inches), allow 1 fish per 2–3 gallons. For larger fish (3+ inches), provide 5–10 gallons per fish. These are minimum guidelines — filtration capacity, fish species compatibility, and oxygen levels all matter as much as volume.
Volume = π × radius² × height, then divide by 231 for gallons. Example: 24-inch diameter (12-inch radius), 24 inches tall: π × 144 × 24 = 10,857 in³. 10,857 ÷ 231 = 47 gallons. Use the Cylinder tab in the calculator above for instant results.
Water weighs 8.34 lbs per US gallon. A 55-gallon tank with ~48 gallons of water weighs 400 lbs of water alone. Add 50–80 lbs for the glass tank, plus substrate and equipment. A fully set-up 55-gallon weighs 500–600 lbs. Always ensure your floor and stand can support the total weight before filling.
Multiply actual water volume by 0.25. For a 55-gallon tank with 48 gallons actual water: 48 × 0.25 = 12 gallons to replace weekly. Use 0.10 for 10% changes and 0.15 for 15% changes. Always pre-treat replacement water with dechlorinator and match temperature before adding to the tank.
Common US sizes: 10 gal (20”×10”×12”), 20 gal long (30”×12”×12”), 29 gal (30”×12”×18”), 40 gal breeder (36”×18”×16”), 55 gal (48”×13”×21”), 75 gal (48”×18”×21”), 90 gal (48”×18”×24”), 125 gal (72”×18”×22”). Actual volumes are slightly higher than the name suggests.
Measure interior dimensions (inside the glass, not outside). Measure length, width, and height to the nearest 0.25 inch. For bow front tanks, measure the maximum front depth at the bow center, then use 75% of that as effective depth. For hex tanks, measure one flat side. Always measure usable height, not to the brim.
36 × 18 × 18 = 11,664 in³. 11,664 ÷ 231 = 50.5 gallons (approximately 50 gallons). In liters: about 191 liters. This is a common 50-gallon breeder tank size. Actual water volume is approximately 42–45 gallons after accounting for substrate and decorations.
Related Calculators
Popular Calculators
Dog Years Calculator
Everyday Life
Hours From Calculator
Everyday Life
Military Time Converter
Everyday Life
Speed Distance Time Calculator
Everyday Life
Stripe Fee Calculator
Finance
Pain & Suffering Calculator
Legal
TDEE Calculator
Health
Roofing Calculator
Construction
GPA Calculator
Education
Mortgage Refinance Calculator
Finance
CPM Calculator
Marketing
Markup Calculator
Finance