in
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in
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in
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Reduces gross volume for actual water estimate
🐠 Rule of thumb: 1 inch of adult fish per gallon for tropical fish. Planted tanks can support more.
Tank Volume
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Sources & Methodology
Volume formulas use exact NIST conversion values. 1 US gallon = 231 cubic inches exactly. Bow front and corner tank shape factors based on standard aquarium industry approximations.
NIST — Unit Conversions
Exact conversion: 1 US gallon = 231 cubic inches; 1 litre = 61.0237 cubic inches
The Aquarium Guide — Tank Volume Reference
Industry reference for standard tank sizes and shape correction factors for bow front and corner aquariums
Methodology: Rectangular: V(cu in) = L x W x H. Cylinder: V = π x (d/2)² x H. Bow front: V = rectangular x 0.9. Corner/pentagon: V = rectangular x 0.6. Gross US gal = cu in / 231. Gross litres = cu in x 0.016387. Actual volume = gross x (1 - substrate%/100). Weight = actual litres x 1.0 kg (or US gal x 8.34 lbs).
⏱ Last reviewed: April 2026
How to Calculate Aquarium Volume in Gallons in 2026
Knowing your aquarium volume precisely is essential for correct dosing of water conditioners, medications, fertilisers, and beneficial bacteria starter cultures. It also determines how many fish you can safely keep. For a rectangular tank, simply multiply the internal length, width, and height in inches, then divide by 231 (the exact number of cubic inches in one US gallon).
The Formula
US Gallons = Length (in) x Width (in) x Height (in) / 231
Litres = Length (cm) x Width (cm) x Height (cm) / 1,000
Cylinder: V = π x (diameter/2)² x height / 231
Cylinder: V = π x (diameter/2)² x height / 231
Standard Aquarium Sizes Reference
| Tank Name | Dimensions (L x W x H in) | US Gallons | Litres |
|---|---|---|---|
| 5 gallon | 16 x 8 x 10 | 5.5 | 20.8 |
| 10 gallon | 20 x 10 x 12 | 10.4 | 39.4 |
| 20 gallon long | 30 x 12 x 12 | 18.7 | 70.8 |
| 29 gallon | 30 x 12 x 18 | 28.0 | 106.0 |
| 40 gallon breeder | 36 x 18 x 16 | 44.8 | 169.5 |
| 55 gallon | 48 x 13 x 20 | 54.0 | 204.4 |
| 75 gallon | 48 x 18 x 21 | 78.5 | 297.0 |
| 125 gallon | 72 x 18 x 22 | 122.9 | 465.0 |
Gross Volume vs. Actual Water Volume
Gross volume is the maximum capacity assuming the tank is filled to the brim with no substrate or decorations. Actual water volume is lower because gravel, rocks, wood, and plant roots displace water. Typical reductions: light gravel only ≈ 10%; typical gravel and artificial decor ≈ 15%; heavy rock and driftwood ≈ 20%. Always use actual water volume for chemical dosing calculations.
Aquarium Weight
Water weighs 8.34 lbs per US gallon (1 kg per litre). A filled 55-gallon tank with 50 gallons of actual water weighs 50 × 8.34 = 417 lbs of water, plus about 60 lbs of glass, 50 lbs of gravel, and stand weight — often exceeding 600 lbs total. Before setting up a large aquarium, verify your floor can support the load, especially on upper floors or above older joists.
💡 Water change tip: For a weekly 25% water change on a 55-gallon tank with 50 gallons actual water: remove 50 × 0.25 = 12.5 gallons. Use the calculator above to find your actual volume, then multiply by your change percentage to know exactly how much water to prepare.
Frequently Asked Questions
Rectangular tank: length (in) x width (in) x height (in) / 231 = US gallons. For a 48 x 13 x 20 inch tank: 48 x 13 x 20 / 231 = 54.0 US gallons.
A standard 55-gallon tank (48 x 13 x 20 in) holds 54.0 gallons gross. Actual water volume with typical gravel and decor is approximately 45-50 gallons.
In centimetres: length x width x height / 1,000 = litres. For a 60 x 30 x 36 cm tank: 60 x 30 x 36 / 1,000 = 64.8 litres. In inches: cubic inches x 0.016387 = litres.
A 40-gallon breeder (36 x 18 x 16 in) holds 44.8 gallons gross. Actual water volume is typically 36-40 gallons after accounting for substrate and decor.
No. Gravel, rocks, decorations, and fill level all reduce usable water volume by 10-20% typically. Always subtract substrate displacement when calculating chemical doses.
The traditional guideline is 1 inch of adult fish per gallon. Modern advice considers adult size, filtration quality, species bioload, and whether the tank is planted. A heavily filtered, planted tank can support more fish than the 1-inch rule suggests.
Water weighs 8.34 lbs per US gallon (1 kg per litre). A 55-gallon tank with water alone weighs 458 lbs. Add tank glass, gravel, and stand for total weight often exceeding 600 lbs. Check floor load capacity before setup.
A standard 75-gallon tank (48 x 18 x 21 in) holds 78.5 gallons gross. Actual water volume with substrate and decor is approximately 65-72 gallons.
V = pi x (diameter/2) squared x height. Divide by 231 for US gallons. For 18 inch diameter, 24 inch height: pi x 81 x 24 / 231 = 26.4 US gallons.
10 US gallons = 37.85 litres. A standard 10-gallon tank (20 x 10 x 12 in) is a common starter size for tropical fish and bettas.
Standard guideline: 25-30% water change weekly. For a 55-gallon tank (50 gallons actual): remove 12-15 gallons weekly. Heavily stocked tanks may need 10-15% twice weekly. Always use dechlorinated water at the correct temperature.
Always dose based on the volume of new water you are adding, not the full tank. For a 25% water change on a 50-gallon actual volume: add conditioner for 12.5 gallons of new water. Follow the product label dose rate per gallon.
A single betta requires a minimum 5 gallons (19 litres); 10 gallons is recommended for stable water quality. Small bowls and vases cause stress and health problems for bettas despite being commonly sold for this purpose.
Measure the internal dimensions — from inside glass to inside glass — not the outer frame. Length is the longest horizontal dimension; width is the front-to-back measurement; height is the water height, not the outer frame height. Subtract any overflow boxes or inside equipment that displaces water.
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