# Fish Tank Capacity Calculator Fish tank capacity calculator: size your aquarium in gallons and liters, estimate total filled weight, and see how many fish your tank can actually hold. ## What this calculates Figure out exactly how much water your aquarium holds and how many fish it can support. This fish tank capacity calculator handles rectangular, cube, and cylindrical tanks, subtracts substrate displacement, and applies the right stocking rule for your water type. ## Inputs - **Tank Shape** — options: Rectangular, Cube, Cylindrical - **Length (or Diameter for Cylindrical)** (in) — min 1, max 300 - **Width (ignored for Cylindrical)** (in) — min 1, max 200 - **Height** (in) — min 1, max 200 - **Water Type** — options: Freshwater (community fish), Freshwater (cichlids / larger fish), Goldfish / Koi pond fish, Saltwater / Marine, Saltwater Reef - **Substrate & Decor Displacement** (%) — min 0, max 40 — Rocks, gravel, and decorations reduce water volume by 5-15 percent typically ## Outputs - **Gross Tank Volume** — Total internal volume before substrate and decor - **Actual Water Capacity** — Water volume after subtracting substrate and decor - **Water Capacity** — Actual water volume in liters - **Water Weight** — Weight of the water alone - **Estimated Filled Weight** — Water plus ~25 percent for substrate, glass, and rock - **Max Fish Stocking** — formatted as text — Stocking capacity using the adjusted inch-per-gallon rule - **Surface Area** — Water surface area (gas exchange drives stocking limit, not volume) - **Stocking Guide** — formatted as text — Practical stocking recommendation for this tank type ## Details How the fish tank capacity calculator works Tank volume is geometry. Measure in inches, then convert: - Rectangular: length x width x height / 231 = gallons - Cube: side^3 / 231 = gallons - Cylindrical: pi x (diameter/2)^2 x height / 231 = gallons - Liters: gallons x 3.785 - Water weight: 8.34 lb per gallon for freshwater, 8.56 lb per gallon for saltwater Gross volume is only half the story. Substrate, rocks, and decorations typically displace 5 to 15 percent of the water. A planted tank with 3 inches of aquasoil and heavy hardscape can lose 20 to 25 percent. This calculator subtracts whatever displacement you enter. Stocking: how many fish can the tank hold The traditional "1 inch of fish per gallon" rule only works for slim-bodied community fish in tanks larger than 10 gallons. Body shape, waste output, and activity matter more than raw length. Here is a more realistic stocking guide: - Community freshwater (tetras, rasboras, livebearers): 1 inch of adult fish per gallon - Cichlids and larger freshwater fish: 1 inch per 3 gallons - Goldfish and koi: 1 inch per 10-20 gallons (they produce 3-4x the waste of similar-sized tropicals) - Saltwater fish-only: 1 inch per 4-5 gallons - Reef tank: 1 inch per 5-7 gallons (invertebrates are sensitive to nitrate) Surface area matters more than depth Oxygen enters the water at the surface. A long, shallow tank of 55 gallons holds more fish comfortably than a tall narrow 55-gallon hex, because gas exchange depends on surface area. For high-oxygen species like goldfish or planted tank with heavy plant mass, prioritize a footprint over height. Weight and structural load Water weighs about 8.34 lb per US gallon, so a 75-gallon tank holds about 625 lb of water alone. Add glass (around 150 lb for a 75), substrate, rock, and stand, and the load reaches 900 lb or more. Anything above 55 gallons should sit on a load-bearing wall or be placed parallel to floor joists. Check with a structural engineer for tanks over 125 gallons on upper floors. Cycling a new tank The tank must go through the nitrogen cycle before heavy stocking. Add an ammonia source (fish food or pure ammonium chloride), test weekly, and wait for nitrites and ammonia to fall to zero and nitrates to appear. That usually takes 4 to 6 weeks. Add fish gradually after that, never more than 25 percent of final stocking at a time. ## Frequently Asked Questions **Q: How do I calculate the capacity of a fish tank?** A: Multiply length by width by height (all in inches) and divide by 231 to get US gallons. A standard 48 x 13 x 21 inch tank holds 13,104 cubic inches divided by 231 = 56.7 gallons gross. Subtract about 10 percent for substrate and decor to get the actual water volume. For cylindrical tanks, use pi times radius squared times height. Enter your dimensions above and the calculator does it automatically. **Q: How many fish can I put in a 20 gallon tank?** A: A 20-gallon freshwater community tank holds about 20 inches of slim-bodied adult fish. That could be 8 neon tetras (1.5 in), 5 platies (2 in), and 1 Bristlenose pleco (4 in), for 21 inches total. If you prefer cichlids, you'd only have room for a single 6-inch cichlid or a pair of dwarf species. For goldfish, 20 gallons fits one fancy goldfish. Always cycle the tank fully before stocking. **Q: How much does a filled fish tank weigh?** A: Water weighs 8.34 lb per US gallon (8.56 for saltwater). Add about 25 percent for the glass, substrate, rock, and stand. A 55-gallon tank holds roughly 460 lb of water and weighs about 575 lb fully set up. A 75-gallon tank weighs around 780 lb filled. Tanks over 55 gallons should sit on load-bearing walls or parallel to floor joists. Check with a structural engineer for anything over 125 gallons on upper floors. **Q: Why does the 1-inch-per-gallon rule not always work?** A: The rule assumes slim-bodied fish with moderate waste output in tanks over 10 gallons. It falls apart for bulky fish (a 10-inch Oscar does not belong in a 10-gallon tank), messy fish (goldfish produce 3x the waste of tetras), or schooling fish that need room to shoal. Use it as a starting point, then research each species' specific minimum tank size and social requirements. **Q: Do substrate and rocks reduce my tank's capacity?** A: Yes. A 2-inch gravel bed displaces about 5 to 10 percent of tank volume. Heavily planted tanks with 3 inches of aquasoil, large driftwood, and rockwork can lose 15 to 25 percent. That matters when dosing medications (overdosing is easy if you use gross volume) and when calculating stocking capacity. This calculator subtracts whatever displacement percentage you enter. **Q: What is the minimum tank size for common fish?** A: Betta: 5 gallons minimum. Neon tetras (school of 6): 10 gallons. Guppies (small group): 10 gallons. Goldfish (fancy): 20 gallons for the first plus 10 per additional. Angelfish: 29 gallons. Oscar: 75 gallons. Discus: 55 gallons for a group of 6. Common pleco: 75 gallons. Always check adult size, not juvenile size sold in pet stores. --- Source: https://vastcalc.com/calculators/health/fish-tank Category: Health & Fitness Last updated: 2026-04-08