Reef Lab
Reef tank calculators
The calculators built into the Reef Lab app, free on the web. No sign-up, no ads, nothing to install — and the same arithmetic the app uses, so the numbers agree.
Salinity calculator
Specific gravity to ppt and back, corrected for sample temperature and your device's calibration. The correction most online converters leave out is worth well over a ppt.
Salt mix calculator
How much salt for the batch you are mixing, in litres or gallons and by brand — plus how to move a running tank's salinity up or down without shocking it.
Dosing calculator
How much to dose to reach your alkalinity, calcium or magnesium target — and across how many days, so the correction stays inside a safe daily rate.
Consumption calculator
Two readings a few days apart become the daily dose your pump should run — including the correction almost everyone forgets when a pump is already going.
Water parameters
Every target range in one table, with what each parameter does, how often to test it, and what actually moves it when it drifts.
Why these are free
They are part of Reef Lab, a logbook app for reef tanks. Putting them on the web costs us nothing and saves you the search for a converter that handles temperature properly — and if you end up wanting somewhere to keep the readings, the app is there.
Nothing you type here is sent anywhere. The calculations run entirely in your browser, there is no account and no tracking cookie.
Same numbers as the app
These pages share the app's arithmetic rather than reimplementing it: the seawater density model follows EOS-80, the standard the printed conversion tables come from, and the product ratings come from the manufacturers' own figures.
That matters if you use both. A web tool that disagreed with the app in your hand would be worse than no web tool at all.
Somewhere to keep all this
Reef Lab logs water parameters, dosing, care and inventory — with charts, a traffic-light rating on every reading, and voice input for when your hands are wet. Free, works offline, no account to start.