Every calculator you need for a UK aquarium in one place. Work out true water volume, sensible stocking, heater wattage, salt and medication doses, unit conversions, and how much water to change to hit a nitrate target.
Six free calculators for UK aquarists. All work in litres and Celsius, with US-gallon and Fahrenheit conversions built in. No sign-up, nothing stored.
Aquarium Volume Calculator
Real water volume, with an optional deduction for substrate and hardscape.
Stocking Calculator
A sensible starting estimate for small community fish. Adult size and messiness matter more than any rule — treat this as a guide, not gospel.
Based on roughly 1 cm of adult fish per litre for small community species, adjusted for size and waste. Always research each species’ adult size and shoaling needs before buying.
Aquarium Unit Converter
Convert the units you’ll actually meet in the hobby.
Type in any box — the others update automatically.
Heater Wattage Calculator
Roughly 1 watt per litre in a heated room, 1.5 in an unheated one.
For tanks over 200 litres, two heaters at ~60% each protect against a stuck-on or stuck-off failure. See our best aquarium heater guide.
Aquarium Salt & Dosing Calculator
Work out aquarium-salt quantities and generic per-litre medication doses.
Salt is dissolved in a jug of tank water before adding. Never use salt with catfish, loaches or scaleless fish — see our ich treatment guide. Always follow the medication’s own label; this is a convenience calculation only.
Water Change / Nitrate Calculator
See how far a water change lowers nitrate, or how big a change you need to reach a target.
Assumes fresh water contains no nitrate. If your tap water has nitrate, the achievable minimum is higher — see how to lower nitrate.
How to use these tools
Tap a tab to switch calculator. Everything runs in your browser — nothing is stored or sent anywhere. The volume figure feeds every other calculation, so work that out first, then use it for stocking, heater sizing, salt doses and water changes.
For the reasoning behind each number, see our guides on setting up a fish tank, choosing a heater, lowering nitrate and stocking for beginners.
— Jaqline Smalling, FishForever