Aquarium Volume Calculator
Calculate the water volume of your aquarium from its dimensions.
Supports gallons and liters, imperial and metric measurements.
Fish tank water volume and stocking are the two fundamental calculations every aquarium keeper must understand. Volume determines filter size, heater wattage, and chemical dosages. Stocking determines how many fish can thrive without ammonia buildup.
Volume formulas: Rectangular tank: V = Length × Width × Height ÷ 231 (cubic inches to gallons) In liters: V = Length × Width × Height ÷ 1,000 (cm measurements)
In metric (cm to liters): V (liters) = L × W × H ÷ 1,000 Note: actual water volume is 80–90% of calculated volume (gravel, decorations, and headspace reduce usable volume).
Stocking density rules:
“Inch per gallon” rule (simplistic but widely used for beginners): Max fish length (inches) = Tank Gallons × 1
Surface area rule (more accurate): Max Inches of Fish = Surface Area (sq in) ÷ 12
Where surface area = tank length × tank width in square inches.
What each variable means:
- Why surface area matters — gas exchange (O₂ in, CO₂ out) happens at the water surface; a tall, narrow tank holds the same volume as a wide, short tank but has far less surface area and supports fewer fish
- Bioload vs. body size — a single Oscar (12 inches) produces more waste than 12 neon tetras (1 inch each) of equal total inches; large messy fish need more space and filtration
- Filtration rate — filter should turn over tank volume 4–10× per hour; for a 40-gallon tank, a 200–400 GPH (gallons per hour) filter
Reference: stocking guides by fish type:
- Neon tetras (1.5"): 1" per gallon rule = fine
- Goldfish (6–12"): Need 20 gallons for first fish, 10 gallons per additional
- Betta fish: 5-gallon minimum, solo housing
- Cichlids: 30–55 gallons minimum depending on species; territorial
- Discus: 50+ gallons, sensitive to water quality
Worked example: Rectangular tank: 36" (L) × 18" (W) × 18" (H).
- Volume = 36 × 18 × 18 ÷ 231 = 11,664 ÷ 231 = 50.5 gallons
- Actual water volume ≈ 50.5 × 0.85 = 42.9 gallons
- Surface area = 36 × 18 = 648 sq in → 648 ÷ 12 = 54 inches of fish maximum
- Stocking example: 20 neon tetras (30") + 2 dwarf gourami (4") + 6 corydoras catfish (9") + 1 bristlenose pleco (4") = 47" total — well within the 54" limit ✓