Aquarium calculators that show their working
Free tools for freshwater, planted and reef tanks. Every calculator explains its formula, gives a worked example and cites where its numbers come from, so you can check the math before you dose, buy or build.
Tank setup
Tank volume
Gallons and liters for rectangle, cylinder, bow-front and hexagon tanks.
Substrate & gravel
How many pounds or bags of gravel, sand or aquasoil to buy.
Heater size
Heater wattage from tank volume, room temperature and target temperature.
Filter flow rate
Recommended GPH or L/h turnover for your tank type.
Tank weight
Total weight of water, glass, substrate and rock, plus floor load.
Glass thickness
Safe glass thickness for building or checking a DIY tank.
Water quality & stocking
Water change & nitrate
How much water to change to bring nitrate down to your target.
Stocking & bioload
Estimate how fully stocked your tank is before you add more fish.
Medication dosage
Scale a product's label dose to your tank's real water volume.
Planted & reef
Fertilizer dosing (EI)
Dry fertilizer grams for target ppm and Estimative Index schedules.
CO2 from pH & KH
Estimate dissolved CO2 and check you are in the safe range.
Salinity & salt mix
Convert specific gravity and ppt, and work out how much salt to add.
Why TankMath?
Most aquarium numbers are passed around as rules of thumb, and many calculators online give a number without telling you where it came from. We do it differently:
- Every formula is shown. You can see exactly what the calculator does and check it by hand.
- Sources are cited. Unit conversions, chemistry constants and engineering formulas come from published references, listed on each page.
- Real-world corrections. Where it matters, we account for things like substrate displacement, so your doses are based on the water you actually have.
- Fast and private. Everything runs in your browser. Nothing you type is sent anywhere.