Volume calculator
The volume calculator converts your tank's dimensions into water capacity using standard geometric formulas, then applies a fill-line adjustment so the result reflects usable water rather than the tank's rim-to-rim capacity.
Rectangular tanks
231 is the number of cubic inches in one US gallon. For centimeter input, the tool computes liters directly and converts to gallons:
Cylindrical tanks
Radius is half the entered diameter. The same 1000 cm³-per-liter conversion applies for metric input.
Fill-line adjustment
Selecting 90% or 80% fill simply multiplies the rim-to-rim result by that fraction, modeling the fact that tanks are never filled completely to the top in practice.
Known limitations
- Bow-front, hexagon, corner, and other non-rectangular, non-cylindrical shapes are approximated using the rectangular formula and will run 5–10% high, since the calculator doesn't model curved or angled panels.
- Glass or acrylic wall thickness isn't subtracted, so very large tanks with thick panels will show a slightly higher volume than their true interior capacity.
- Substrate, decorations, and equipment are not deducted — the fill-line setting approximates this, but doesn't measure it directly.
Stocking calculator
The stocking calculator estimates a safe upper bound using an adjusted version of the traditional "inch of adult fish per gallon" guideline, scaled by tank type and filtration level.
Base formula (non-goldfish tank types)
Type rates
| Tank type | Rate (inches per gallon) | Reasoning |
|---|---|---|
| Freshwater community | 1.0 | Standard baseline guideline |
| Semi-aggressive / cichlid | 0.6 | These species need roughly 1.7× the space of a community fish of the same length, due to territory and higher bioload |
| Heavily planted / nano | 1.15 | Live plants improve oxygenation and help process waste, allowing a slightly higher density |
Filtration multiplier
| Setting | Multiplier |
|---|---|
| Standard (matches tank size) | 1.0 |
| Overfiltered | 1.15 |
| Underfiltered | 0.85 |
Goldfish formula (calculated separately)
Goldfish are excluded from the inch-per-gallon model because they produce substantially more waste and grow far larger than their store size. The result is then multiplied by the same filtration factor as above.
Known limitations
- The calculator doesn't know your tank's footprint (surface area), only its volume — a long, shallow tank can safely support more fish than a tall, narrow tank of the same volume.
- It has no species database, so it can't flag specific incompatibilities, territorial behavior, or an individual species' unusually high or low bioload.
- It assumes a fully cycled tank with an established beneficial-bacteria colony — a new, uncycled tank should be stocked far more slowly than this number suggests.
Why these numbers can differ from other calculators
Other aquarium calculators use similar formulas but may apply different rounding, different type-rate assumptions, or different fill-line defaults. A difference of a few percent between tools is expected and doesn't mean either result is "wrong" — for any dosing or equipment decision, use your own tank's actual measured volume rather than a manufacturer's rounded label.