How is this calculated?
The rule of thumb "1–2 gallons of water per inch of adult fish length" is a rough guideline for peaceful schooling fish in a community tank — nothing more. The calculator sums, for each species entered (count × adult size), the calculated space requirement and compares it to your tank volume.
This rule of thumb has significant limits. It accounts for neither swimming-space needs (bottom-dwelling vs. free-swimming species), territorial behavior, filtration capacity, nor actual compatibility between species. Large, territorial, or particularly sensitive fish often need considerably more room than the rule suggests. Always research the specific requirements of each species before buying.
What the rule of thumb does NOT replace
Filtration capacity, regular water-parameter monitoring during acclimation, and adequate hiding spots matter at least as much as the raw gallon count. Even stocking that fits on paper can overwhelm a tank through the wrong species combination or insufficient filtration.