Fishkeeping runs on a handful of numbers: how much water you have, how many fish it holds, how much heat, light, substrate and CO2 it needs. These free aquarium calculators work them out in seconds so you can set up, stock and maintain a tank with confidence instead of guesswork.
Setup and stocking
Aquarium volume calculator — start here. Nearly every other decision depends on your real water volume, and the label on the box is not it. Get gallons, liters and filled weight for rectangle, bow front, cylinder and hexagon tanks.
Aquarium stocking calculator — how many fish your volume can actually support, weighed by adult size and bioload instead of the inch-per-gallon myth.
Aquarium substrate calculator — the weight of gravel, sand or aqua soil to buy for your footprint and depth.
Equipment
Aquarium heater size calculator — heater wattage for your volume and the temperature gap in your room, with a flag for when to run two heaters.
Planted aquarium lighting calculator — a lumen target and LED wattage for your tank size and plant demand.
Water quality and planted tanks
Water change calculator — the percentage and gallons to swap to bring nitrate from where it is to where you want it.
CO2 calculator — dissolved CO2 in ppm from your pH and KH, checked against the planted safe range.
Nitrate to phosphate calculator — your NO3:PO4 ratio against the balanced Redfield range.
Where to go next
New to all of this? The fish tank setup guide ties these numbers together into a working tank, and the cycling guide keeps it safe once it is running.