How many fish can I put in my tank?
This fish tank stocking calculator works out your aquarium’s volume and compares the fish you want with three stocking rules: the classic inch-per-gallon rule, the surface-area rule and a bioload check that gives chunky fish more weight than slim ones. It uses the adult size of each fish — from FishBase, the scientific fish database — because the tiny fish in the shop will grow.
Every stocking rule is crude. Filtration, water changes, plants, feeding and the temperament of each species matter as much as the numbers. Use the result to spot an obviously overstocked plan before you buy, not as proof that a tank is safe.
It works for tropical and coldwater aquariums, marine tanks and garden ponds, in centimetres or inches, with volumes in litres and US or UK gallons.
How to use the Fish Tank Stocking Calculator
- Choose cm or inches, the water type and how much space decor takes (10% is a common allowance).
- Enter the tank length, width front to back and water depth.
- Add each fish species and how many; the adult length fills in, and you can edit it.
- Read the stocking percentage — the highest of the rules — and the three gauges.
- Check any size warnings: a fish as long as the tank is deep cannot turn comfortably.
Fish Tank Stocking Calculator features
Three rules side by side
Volume (inch per gallon), surface area and a bioload-weighted check, each as a gauge.
Adult sizes from FishBase
28 common aquarium fish with their published maximum lengths.
Marine and pond settings
Practical Fishkeeping’s figures for fish-only marine, reef and pond stocking.
Litres and gallons
Volume in litres, US gallons and UK gallons; measure in cm or inches.
Size warnings
Flags fish that will outgrow the tank’s depth.
The stocking rules used
Source: Practical Fishkeeping, “Frequently asked questions on stocking densities” (Matt Clarke; checked October 2026). The gallon in these figures is the UK gallon (4.55 L), which makes the rule slightly stricter than when it is quoted per US gallon. Volume is worked out after taking off the decor allowance.
The surface-area rule — about 1 inch of fish per 12 square inches of water surface — is a hobby rule of thumb based on oxygen exchange at the surface. It favours long, shallow tanks over tall ones; Practical Fishkeeping argues volume and filtration matter more. The bioload weighting multiplies each fish’s length by 1 (slim), 1.5 (medium) or 2 (deep or heavy-bodied); it is this tool’s own assumption to reflect that a 10 cm angelfish produces far more waste than ten 1 cm fry.
| Water | Rule (Practical Fishkeeping) | Per litre |
|---|---|---|
| Tropical or coldwater | 1 in per gallon (2.5 cm per 4.55 L) | 0.55 cm |
| Marine, fish only | 1 in per 2 gallons (2.5 cm per 9 L) | 0.28 cm |
| Marine with invertebrates | 1 in per 4 gallons (2.5 cm per 18 L) | 0.14 cm |
| Garden pond | 10 in per 100 gallons (25 cm per 455 L) | 0.055 cm |
A worked example
An 80 × 35 × 40 cm tank holds 112 litres, or about 101 L after 10% for decor. By the volume rule that allows 101 × 2.5 ÷ 4.55 ≈ 55 cm of adult fish. Ten neon tetras (2.5 cm each), six bronze corydoras (7.5 cm) and a dwarf gourami (9.5 cm) add up to 79.5 cm — about 144% of the volume rule — so that plan is overstocked; the ten neons alone use about 45%.
Tips for stocking a new aquarium
Pets on land have calculators too: see the toxic foods for pets checker and the horse weight calculator.
- Cycle the tank before adding fish, then add a few at a time so the filter bacteria can catch up.
- Plan for adult size: common plecos, clown loaches, oscars and goldfish all outgrow typical starter tanks.
- Shoaling fish such as tetras and corydoras need groups; check temperament and temperature before mixing species.
- Test ammonia, nitrite and nitrate regularly — they are the real measure of overstocking.
Fish Tank Stocking Calculator FAQ
How many fish can I put in a 20 gallon tank?
By the inch-per-gallon rule, about 20 inches of slim adult fish per US gallon basis — fewer after allowing for decor and fewer still for large or heavy-bodied fish. Enter your tank’s size and species in the fish tank stocking calculator for a specific answer.
Is the one inch per gallon rule accurate?
Only roughly. It assumes small, slim fish and good filtration. It breaks down for big or deep-bodied fish, which is why this calculator also shows surface-area and bioload checks.
Should I use adult or current size?
Adult size. Fish bought at 2 cm can reach several times that, and the stocking level is about the grown fish.
How many fish for a pond?
Practical Fishkeeping gives 10 inches of fish per 100 UK gallons (25 cm per 455 L) as a basic pond guide. Choose “Garden pond” and enter the pond’s length, width and average depth.
What does SL and TL mean?
FishBase gives standard length (SL, nose to the base of the tail) or total length (TL, including the tail). The calculator uses the figure as published; you can edit any length.
Can the calculator tell me which fish get along?
No. It only checks space. Ask an experienced aquatic shop or an aquatic vet about compatibility and water conditions.