🎯

Fish Stocking (Inch per Gallon) Calculator

Estimate how many fish your freshwater tank can hold using the classic inch-per-gallon rule, adjusted for fish size, plus a rough count for a chosen adult length.

Tank volume

Stocking rule

Average adult fish length

Loading calculator…

How it works

Total inches = gallons × ratio · Fish count = floor(total inches ÷ adult length)

The inch-per-gallon rule is a beginner starting point, not a law — it works passably for small, slender community fish but breaks down for fat-bodied or messy species, which is why a 10-inch goldfish never belongs in 10 gallons. Bioload (waste output) grows faster than length because it tracks body mass, so a single large fish pollutes far more than several small ones of the same total length. Use the conservative "1 inch per 2 gallons" ratio for anything chunky or high-waste, always plan for the adult size rather than the juvenile you buy, and let filtration and surface area — not just a headline number — guide the final stocking.

Worked example

A 20-gallon tank under the standard 1 inch-per-gallon rule holds about 20 inches of fish. Stocking 2-inch tetras, that is 20 ÷ 2 = 10 fish — a comfortable small school with room for the filter to keep up.

Frequently asked questions

Is the inch-per-gallon rule accurate?

Only loosely, and only for small slim fish. It ignores body mass, waste output, territory needs and swimming space, so treat it as a sanity check rather than a hard limit. Research each species’ real requirements before stocking.

Should I use the fish’s current size or adult size?

Always its full adult size. Fish grow, and a tank stocked to the limit with juveniles becomes badly overstocked within months, spiking ammonia and stunting the fish.

Why is one big fish worse than several small ones?

Waste output scales with body mass, which rises far faster than length. A single fish twice as long is roughly eight times the volume and produces vastly more waste, so total inches understates the load big fish place on your filter.

Related calculators

Aquarium Stocking Calculator

Check whether your fish tank is understocked, balanced or overstocked using the inch-per-gallon guideline, with remaining capacity in inches.

Aquarium Volume Calculator

Calculate an aquarium's water volume in gallons and litres from its dimensions — rectangular or cylinder — with a realistic allowance for substrate and décor.

Aquarium Heater Wattage Calculator

Size the aquarium heater you need from your tank volume and how far you have to raise the water above room temperature — with the next standard heater size to buy.

Aquarium CO2 Bubble Rate Calculator

Get a safe starting CO2 injection rate in bubbles per second and per minute for a planted aquarium, based on tank volume and how heavily planted it is.

This calculator is for educational purposes. Double-check important results before acting on them.