Tank volume
Litres, UK/US gallons and usable-water scenarios from real internal dimensions.
Use calculator →Most-used tools
Popular aquarium calculators
Each tool shows the formula, the assumptions and a worked explanation instead of hiding the maths behind a result box.
Substrate
Turn tank footprint and bed depth into litres and an estimated product weight.
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Heater reference
See a labelled manufacturer reference band and the limitations around it.
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Why actual water volume matters
The number on the box is not always the water in the tank.
Glass thickness, the normal waterline, substrate, rock, wood and internal equipment all reduce the water present. That matters when you calculate percentage water changes, compare filter flow, estimate water mass or interpret equipment instructions.
The site keeps gross geometric volume and estimated usable water separate. You can see which value is measured, which is calculated and which is only a scenario assumption.
Practical reading
Guides that make the calculators more useful
The articles focus on measurement, assumptions, worked examples and the point where arithmetic stops and fishkeeping judgement begins.
Measure an aquarium properly
Internal dimensions, waterline, substrate and the numbers worth writing down once.
Open guide →Gross vs actual water volume
Why nominal tank size and the water actually present can both be valid but different numbers.
Open guide →How much does a full aquarium weigh?
Build a component ledger rather than counting the water and forgetting the rest.
Open guide →Choose a filter turnover target
Understand volume, rated flow, real installation conditions and species-sensitive current.
Open guide →UK vs US gallons
Avoid one of the easiest aquarium unit mistakes when reading international advice.
Open guide →Running-cost maths
Model heater duty cycle, pumps, lighting hours and your own electricity tariff.
Open guide →What makes the site different
Useful publisher content around the tools — not a calculator farm.
Formula shown
Every core result can be checked by hand.
Assumptions exposed
Displacement, duty cycle and turnover targets are visible inputs.
Care context sourced
Welfare and manufacturer claims link to recognised sources rather than being invented.
Boundaries stated
Geometry does not pretend to diagnose disease, certify a floor or replace species-specific care.
For husbandry and welfare context, the site links to recognised organisations such as the RSPCA and OATA. For product-specific examples, the manufacturer source is identified. The calculators themselves use transparent arithmetic that can be reproduced independently.
Start here
If you know the tank dimensions, begin with volume.
Real water volume is the input that feeds several other useful calculations. Measure the internal length, width and actual water height, then use the result for water-change arithmetic, filter-flow comparison and water-mass estimates.