AquaPulse Tools · Free web calculator
Aquarium Calculator
Work out your tank's water volume in litres, US gallons and UK gallons, and its approximate water weight, from a few simple measurements. Runs entirely in your browser — no login, no account, nothing saved or sent anywhere.
Aquarium volume and weight calculator
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Keeping this number useful over time
Now you know your tank's true volume, AquaPulse's Water Check can use it to put a water test in context — the same ammonia reading means something different in a small tank than a large one, and interpreting it against your actual volume, not a guess, is where this number keeps paying off.
AquaPulse provides informational guidance only, based on what you enter — it doesn't take automatic readings and isn't a substitute for testing your water with a reliable kit.
Why an accurate number actually matters
It's tempting to round to whatever's printed on the box, or skip measuring altogether once a tank is set up and running well. In practice, this figure feeds into most of the other decisions you'll make about the tank, and a number that's off by a meaningful margin can throw each of them off in turn.
- Medication dosing. Most medications, conditioners and fertilisers are dosed strictly against water volume. A tank that's actually smaller or larger than assumed means a "standard" dose runs stronger or weaker than the product's instructions intend — worth getting right before treating a sick fish, not after.
- Water changes. A percentage-based routine only removes what you think it does if the percentage is taken from a real figure. Our Water Change Assistant turns a percentage into an actual volume to remove and replace.
- Stocking decisions. Volume is a genuine starting point for working out what a tank can support, though it isn't the whole answer — see "How many fish can I keep in this tank?" below for the rest of what matters.
- Equipment sizing. Heaters and filters are both specified against a volume range, and sizing either from a guessed figure risks under- or over-shooting what the tank actually needs. Our Aquarium Heater Calculator and Filter Turnover Calculator both start from the same measurement as this one.
- Water parameter interpretation. The same amount of ammonia, medication or fertiliser produces a very different concentration in a small tank than a large one, so volume sits quietly behind almost every reading you take. See our ammonia and nitrate guides, or use Water Check to interpret a specific reading.
How many fish can I keep in this tank?
There's no reliable universal formula for this, and we won't pretend otherwise. Rules of thumb like "one inch of fish per gallon" are widely repeated but don't hold up: a single large or messy-eating fish can outweigh the bioload of a dozen small, efficient ones, and adult size — not the size a fish is sold at — is what actually matters over time.
Tank volume from the calculator above is a genuine starting point, but safe stocking for your specific tank depends on several things together:
- Adult size, not juvenile size. Plan around how large each species grows, not the size it is when you bring it home.
- Bioload, not just body length. A fish's waste output, appetite and messiness vary hugely by species and aren't proportional to size alone.
- Filtration capacity. A heavily stocked tank needs filtration matched to that bioload, not just to the tank's volume.
- Territory, temperament and swimming space. Footprint (length × width) often matters more than raw volume, especially for territorial or active-swimming species.
- Your maintenance routine. More heavily stocked tanks generally need more frequent testing and water changes to stay stable — see our Water Change Assistant for working out how much and how often.
If you're setting up a new tank, our starting a new aquarium guide covers cycling it safely before adding livestock. Once you know roughly how much your tank can hold, our guide to choosing tank mates covers the temperament, adult-size and predation factors that determine which species can actually share it safely.
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Related tools and guides
Common questions
Frequently asked questions
How do I calculate my aquarium's volume in litres or gallons?
Measure the tank's internal length, width and height (or diameter and height for a cylinder) in centimetres or inches, then multiply length by width by height to get the volume in cubic centimetres, and divide by 1000 for litres. The calculator above does this automatically and also converts to US and UK gallons.
Why is my actual water volume less than my tank's stated capacity?
A tank's stated capacity is usually its brim-full internal volume. In practice, substrate, decor, hardscape and equipment displace water, and most aquariums aren't filled all the way to the rim, so the real water volume is normally somewhat less than the full-tank figure.
How much does a full fish tank weigh?
Water itself weighs approximately 1 kilogram per litre for freshwater, or about 1.025 kg per litre for saltwater. The calculator above gives you the water weight for your tank, but the total weight a floor or stand needs to support also includes the tank and stand's own weight, plus substrate, rock, hardscape and equipment — check your tank's specification sheet or a specialist if you're unsure a floor can take the load.
How many fish can I keep in my aquarium?
There's no reliable universal formula — rules like "one inch of fish per gallon" don't hold up because bioload, adult size, temperament and filtration all vary enormously by species. Tank volume is only a starting point; see the guidance above for the factors that actually determine safe stocking for a given tank.
Is a bowfront tank's volume the same as a standard rectangular tank of the same outer dimensions?
Not exactly — a bowfront tank's curved glass bulges outward, so its true volume is slightly more than a plain rectangular box of the same outer length, width and height. Treat the rectangular-box estimate as a conservative approximation, and use your manufacturer's stated volume if one is available.
How this calculator works: volume is calculated directly from your entered dimensions using standard geometry, and unit conversions use exact published constants (1 US gallon = 3.785411784 litres, 1 UK gallon = 4.54609 litres). Water weight uses a standard approximate density of 1 kg per litre for freshwater and 1.025 kg per litre for saltwater. Nothing here is species-specific, and no data you enter is saved or sent anywhere.