Is the inch-per-gallon rule reliable?

Not on its own. It treats every centimetre of fish as equivalent, so it badly under-counts large-bodied species and ignores filtration entirely. This calculator keeps the length-based idea but adds a weighting for body mass and adjusts capacity for turnover, plants and water type.

Does filtration change how much I can stock?

Yes. Capacity is scaled by how your turnover compares with the target for your water type — four times the tank volume per hour for freshwater, five for brackish and six for saltwater. The adjustment is capped between 0.65x and 1.6x, because a very large filter does not make a small tank limitless. Protein skimmers and UV clarifiers are excluded: they treat water but do not provide aquarium turnover.

Do live plants increase capacity?

A little, in freshwater only. Plants take up some of the waste nitrogen, so each one adds one percent of capacity up to a maximum of ten percent. Marine and brackish tanks get no plant allowance.

Why does my juvenile fish count as a full-grown adult?

Because it will become one. Stocking advice based on today's size would call a tank of twenty juveniles comfortable right up until it is badly overstocked. Where you mark fish as juveniles or fry, the calculator also shows what the tank is carrying today alongside the fully grown figure.

What water volume should I enter?

The actual water volume, not the tank's printed capacity. Substrate, hardscape and the gap below the rim commonly account for ten to fifteen percent of a tank's stated size, and that difference matters at the margins.

Is this a hard limit?

No — it is a planning estimate. An established, heavily planted tank with generous water changes can sit above the guide comfortably, and a neglected one can struggle below it. Treat a reading over 100% as a prompt to check your maintenance and test results rather than as a verdict.

Aquarium bioload calculator

Describe your tank and get a stocking estimate that accounts for what your fish will grow into, how much water your filter actually moves, and whether the tank is fresh, brackish or marine.

Your tank

These are the same fields the Studio Scaped tracker uses, so nothing needs re-entering if you carry this setup over.

Water type

FreshwaterSaltwaterBrackish

Marine tanks are held to roughly half the stocking of freshwater, and brackish to three-quarters.

Water volumeUnit

Use the water volume, not the tank’s printed capacity — substrate, hardscape and the gap below the rim typically take 10–15% of it.

Fish and invertebrates

Search the Studio Scaped catalogue. Stocking is judged on adult size, so a tank of juveniles is measured by what they will become.

Add a species

Filtration (optional)

Turnover changes how much waste the tank can process. Leave this empty and we assume standard filtration rather than penalising you for not knowing.

Add a filter

Plants (optional)

Live plants take up some of the waste nitrogen, so they earn a small capacity allowance — up to 10%, no matter how many you add.

Add a plant

Stocking context

Estimated bio-load

Add water volume

Add your water volume to create an estimate.

–Not assessed

Fish: 0
Water: Not set
Filtration: Standard filtration assumed
Plants: No plant allowance

This is a planning estimate, not a stocking guarantee. Adult size, body mass, feeding, filter media, maintenance and fish behaviour still matter.

Keep this tank

Carry this setup into the free Studio Scaped tracker — it arrives as a new tank with your fish, plants and filtration already filled in, ready to log water tests against. You’ll be asked to create an account, and nothing you already have there is changed.

Continue in the tracker

Add your water volume to enable this.

How the estimate works

Every fish contributes its adult length multiplied by how many you keep, with an extra weighting that grows for fish over 10cm. That weighting matters: a single 30cm fish produces far more waste than five 6cm ones, even though the lengths add up the same. The inch-per-gallon rule misses this entirely, which is why it fails so badly on large species.

That fish total is then compared against the tank’s capacity, which starts from your water volume and is adjusted three ways:

Water type. Marine tanks are held to roughly half the stocking of freshwater and brackish to three-quarters, reflecting how much less forgiving they are of a swing in water quality.
Filter turnover. Capacity scales with how your turnover compares to the target for your water type — four times the tank volume per hour for freshwater, five for brackish, six for saltwater. The adjustment is capped between 0.65x and 1.6x, because an enormous filter does not make a small tank limitless. Protein skimmers and UV clarifiers are deliberately excluded: they treat water, but they are not turnover.
Live plants. In freshwater only, each plant adds one percent of capacity up to a maximum of ten percent, reflecting the waste nitrogen they take up.

Fish marked as juveniles or fry are still planned at their adult size, because they will get there. Where that applies, the result also shows what the tank is carrying today, so you can see how much headroom you are actually working with.

Adult sizes come from the Studio Scaped fish knowledge base. Where we have no size on file for a species, the calculator says so and asks you for the number rather than guessing one.

Common questions

How is aquarium bioload calculated?: Each fish contributes its adult length multiplied by its quantity, with a weighting that grows for fish over 10cm — a 30cm fish produces far more waste than five 6cm ones, even though the lengths add up the same. That total is compared against the tank's capacity, which starts from the water volume and is then adjusted for water type, filter turnover and live plants.
Is the inch-per-gallon rule reliable?: Not on its own. It treats every centimetre of fish as equivalent, so it badly under-counts large-bodied species and ignores filtration entirely. This calculator keeps the length-based idea but adds a weighting for body mass and adjusts capacity for turnover, plants and water type.
Does filtration change how much I can stock?: Yes. Capacity is scaled by how your turnover compares with the target for your water type — four times the tank volume per hour for freshwater, five for brackish and six for saltwater. The adjustment is capped between 0.65x and 1.6x, because a very large filter does not make a small tank limitless. Protein skimmers and UV clarifiers are excluded: they treat water but do not provide aquarium turnover.
Do live plants increase capacity?: A little, in freshwater only. Plants take up some of the waste nitrogen, so each one adds one percent of capacity up to a maximum of ten percent. Marine and brackish tanks get no plant allowance.
Why does my juvenile fish count as a full-grown adult?: Because it will become one. Stocking advice based on today's size would call a tank of twenty juveniles comfortable right up until it is badly overstocked. Where you mark fish as juveniles or fry, the calculator also shows what the tank is carrying today alongside the fully grown figure.
What water volume should I enter?: The actual water volume, not the tank's printed capacity. Substrate, hardscape and the gap below the rim commonly account for ten to fifteen percent of a tank's stated size, and that difference matters at the margins.
Is this a hard limit?: No — it is a planning estimate. An established, heavily planted tank with generous water changes can sit above the guide comfortably, and a neglected one can struggle below it. Treat a reading over 100% as a prompt to check your maintenance and test results rather than as a verdict.

Track this tank over time

A stocking estimate is a snapshot. The free Studio Scaped tracker keeps the same bioload figure updated as your fish grow, alongside 22 water parameters, your maintenance history and your test results.