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Aquariums: real water volume, not the box rating

Rated gallons are brim-full; your fish live in the working volume. Charts for every standard size, calculators for customs, and the substrate-aware measuring method.

The gallons on the box are brim-full nominal — the water you actually keep is 10–15% less once substrate and the air gap are accounted for, and that working volume is what drives stocking, heater wattage and dosing. Measuring from the inside glass bottom to the real water line (or from the top of the substrate) gives the number that matters. Below: a rectangular calculator for any custom build, charts for every standard trade size from the 10 to the 125, and the sizing guide that explains why bigger tanks are actually easier to keep stable than small ones.

Tools

Rectangular tank calculator

Litres-first calculator

Charts

55 gallon

40 breeder

29 gallon

All aquarium charts

Guides

Choosing a size (and what gallons mean)

Reference

What water weighs — floor loading

55 gal dimensions