Stocking calculator: every published surface-area rule, side by side
"How many fish can I put in this tank?" has no single honest answer — it has several published ones, and they disagree. Rather than hide behind one number, this calculator computes your tank against all of the widely published rules: the classic surface-area rule (12 square inches per inch of fish), its metric phrasing (30 cm² per cm), INJAF's 6× length rule, and the legacy "1 inch per gallon" shown for comparison only. Expect a range, not a verdict.
Key takeaways
- Surface area beats volume as a proxy: gas exchange happens at the surface, so a long tank stocks better than a tall one.
- Use ADULT fish length — the juvenile size in the shop is not the number any rule was designed for.
- All published length rules assume slim-bodied fish; they break for goldfish and other deep-bodied species.
- The result is a starting point for species research, not a stocking permission slip.
The calculator
| Surface area | — |
| Classic: 12 in² per inch (INJAF, Spruce Pets, LiveAquaria) | — |
| Metric: 30 cm² per cm (INJAF) | — |
| Legacy: 1 inch per US gallon (comparison only) | — |
| Surface-rule range | — |
| 6× length rule (INJAF) | — |
Length means ADULT total length of a slim-bodied fish. Deep-bodied fish (goldfish, many cichlids) need substantially more surface per fish than any of these rules grant them.
Methodology — the variants, honestly
The classic surface-area rule appears across decades of aquarium literature and in modern care guides: "12 square inches of surface for every 1 inch of fish." The Spruce Pets and LiveAquaria publish it in their stocking guidelines, and INJAF (It's Not Just A Fish) states it as "12 square inches for every 1 inch of fish / 30 square cm for every 1 cm of fish". In metric that is 77.42 cm² of surface per 2.54 cm of fish — 30.48 cm² per cm.
The metric phrasing of the same rule — 30 cm² per cm — is what most European hobbyists know. It is nominally the same rule, but rounding gives it a slightly more permissive result (a 60 × 30 cm tank: 59 cm of fish classic vs 60 cm metric), which is exactly why we show both rather than averaging them away.
The 6× rule (INJAF): the tank's length should be at least six times the adult length of the fish. A 10 cm species wants 60 cm of tank length regardless of volume. It captures the swimming-room dimension the area rules ignore.
The legacy "1 inch per gallon" is shown for comparison, clearly labelled, because you will still meet it in shops and old forums. It ignores surface area, body depth, adult size and behaviour, and INJAF's own worked examples show it failing badly for big fish. If your legacy number differs wildly from the surface numbers, believe the surface numbers — then research the species.
Worked example (the defaults): a 60 × 30 × 40 cm tank has 1800 cm² (279 in²) of surface and holds 72 L (19.0 US gal). The classic rule supports 23.25 inches (59 cm) of fish; the metric rule 60 cm; the legacy rule 19 inches (48 cm). For 4 cm adult fish that is 14–15 fish by surface area, and the 6× check passes at 15×. Add filtration, behaviour and schooling reality on top — see why inch-per-gallon fails for how these rules diverge.
Sources
- INJAF — Understanding fish stocking guides: surface rule (both phrasings), 6× rule, inch-per-gallon critique (checked 2026-11-16).
- The Spruce Pets — stocking guidelines: 12 in² per inch phrasing (checked via their stocking articles 2026-11-16).
- AqAdvisor: an example of modern calculators that refuse inch-per-gallon and model filter capacity and species behaviour instead (checked 2026-11-16).
FAQ
How many neon tetras fit in a 60 cm tank?
By surface rules, 14–15 at ~4 cm adult total length. Seriously Fish — whose profiles we cite throughout this site — recommend a 60 × 30 cm base tank and a group of at least 8–10 for welfare, not just physics. Schooling fish benefit from bigger groups than the arithmetic demands.
Why surface area instead of volume?
Oxygen diffuses in (and CO₂ out) across the air–water interface, so the footprint sets the gas-exchange budget. Two tanks of equal volume can differ enormously: a 60 cm-long tank has roughly double the surface of a cube-shaped tank of the same litres.
Do these rules work for goldfish?
No length rule works well for deep-bodied fish. A goldfish at 15 cm carries many times the mass — and produces many times the waste — of fifteen slim 1 cm fish. See our guide to tank sizes for the classic mis-stocked fish for goldfish-specific numbers.
What is the 6× rule?
INJAF's rule of thumb that tank length should be at least 6× the fish's adult length. It is a floor, not a target: active swimmers like danios benefit from far more length than 6× their body size.
Example: aquarium filters (affiliate concept)
Stocking lightly only works with filtration to match. This is where we would list filter lines sized to real (not rated) tank volume.
Check price →Heuristics, not limits
Every rule on this page predates modern filtration and none of them knows anything about your fish's temperament, territory needs, shoaling minimums or waste output. Use the range to sanity-check a plan, then research each species — Seriously Fish profiles are our default starting point — and read the stocking mistakes guide before you buy.