Enter your tank's inside dimensions to get total capacity, filled volume and usable water in US gallons, UK gallons and litres — instantly.
Measure inside the glass. Get total capacity, filled volume and usable water in gallons (US & UK) and litres.
Results will show US gallons, UK gallons, litres and water weight.
Rectangular / cube: length × width × depth | Vertical cylinder: π × radius² × depth | Horizontal cylinder: circular segment area × length
Conversions: 1 US gallon = 3.785411784 L | 1 UK gallon = 4.54609 L | Measure inside the glass, not the outer frame.
Water weight excludes the tank, stand, substrate, rocks and equipment. Consult a qualified professional if floor or stand loading is a concern.
Compare physical volume with a conservative species-based space estimate. Planning aid only — not a stocking guarantee.
Results show minimum and recommended tank size with care notes.
A practical guide to getting the most accurate aquarium gallon and litre estimates from your inside dimensions.
The aquarium size calculator shows both US gallons (3.785 L each) and UK Imperial gallons (4.546 L each). If you are in the United Kingdom, use the UK gallon figure — UK fish-tank labels and many UK fish-keeping guides quote Imperial gallons, not US gallons. A 40 US gallon tank is only about 33 UK gallons.
Litres are the most widely used unit in Europe, Australia and most of Asia. This fish aquarium size calculator converts all measurements automatically — no manual conversion needed. Usable litres are shown after deducting the displacement percentage you chose for substrate and decor.
Tank labels print gross volume from outside dimensions. Actual usable water is always less. Measure inside the glass, enter the real water depth, and choose a displacement allowance for your gravel, rocks and equipment. The aquarium water size calculator then gives a conservative, realistic estimate of what your fish actually swim in.
Glass panels, silicone seams and external trim can reduce effective volume by 5–15% compared with outside dimensions. Enter length, width and height measured from the inner glass surfaces. For a cube aquarium enter only one side — the calculator handles the geometry automatically.
Volume alone does not tell you how many fish can live safely. Use the species planner below the volume section to connect gallons to species-specific minimums. Adult size, territory, group requirements, filtration and oxygen all affect the stocking limit — gallons are the starting point, not the conclusion.
Standard aquarium sizes: 10 US gal (37.9 L / 8.3 UK gal) · 20 US gal (75.7 L) · 29 US gal (109.8 L) · 55 US gal (208.2 L) · 75 US gal (283.9 L). Use the calculator to verify your actual usable volume if substrate, equipment or a lower fill line reduces what is shown on the label.
Each species has its own space requirements. These calculators give you a minimum and recommended size — not a generic rule of thumb.
Answers about US gallons, UK gallons, litres, displacement and how to interpret your result.
Use these alongside the aquarium size calculator to plan a complete, healthier setup.
Check whether selected fish can share a tank — temperament, temperature, pH and size rules.
Open Tool →Enter pH, ammonia, nitrite, nitrate and temperature for an instant water-quality score.
Open Tool →Estimate daily feeding frequency and portion rules to avoid overfeeding and water-quality problems.
Open Tool →Build a complete equipment, cycling and stocking checklist before buying animals.
Open Tool →Full-featured calculator with five tools on one page — volume, species planner, compatibility, water and feeding.
Open Tool →Expert guide to pH, ammonia, nitrite, nitrate, temperature and hardness for freshwater tanks.
Read Guide →