A fish stocking calculator should answer one question: how much waste can this water carry? Length alone can’t tell you that. Four neon tetras and one fancy goldfish add up to about the same inches, yet the goldfish outweighs all four by roughly 19 to 1. The tool below estimates each fish’s adult weight from its length and body shape, turns weight into waste production with the metabolism curve fisheries scientists use, adjusts for water temperature, filter turnover and weekly water changes, and then reports a stocking percentage. It also shows what the old inch per gallon rule and the surface rule would say, so you can see where they go wrong. Most free aquarium stocking calculator tools hide their math; this one shows each step.
Fish Stocking Calculator: Weigh the Fish, Not the Inches
Pick up to four fish species and the number of fish of each, then enter your tank’s gallons, length and width. Adult fish size is built in for 18 common species, from rasboras and the cherry barb to plecos. Anything else, a cichlid for example, goes in as “Other” with its adult length and a body shape. Don’t know the water volume? The fish tank volume calculator gets tank capacity from inside measurements.
Fish Stocking Calculator
Pick your fish and enter the tank, filter and weekly water change. The calculator estimates each fish's adult weight from its length and body shape, scales that to waste output with the fish metabolism curve and water temperature, and reports how full the tank is. It also shows the old inch-per-gallon and surface area readings for comparison.
Your Stocking Level
Estimate only. Adult weight uses the median form factor for each body shape (Froese 2006), waste scales with weight to the 0.8 power (Clarke and Johnston 1999), and 100 percent is the moderate stocking level from aquariumscience.org (900 g of fish per 100 gallons, which it equates with AqAdvisor 100 percent for four inch fish). It does not check temperament, schooling needs or water chemistry. Reviewed September 2026.
Read the headline number this way. Under 31 percent, the tank is lightly stocked. Up to 100 percent is moderate, the safe stocking level most keepers should aim for. Between 100 and 311 percent is high stocking: a heavily stocked tank works only with heavy filtration, strong aeration and disciplined water changes. Above 311 percent counts as overstock by any standard we found.
Why the Inch-per-Gallon Rule Breaks Down
The rule is simple: one inch of adult fish per gallon of water. It counts inches of fish, and its flaw is geometry. Weight grows with the cube of length, so a fish that doubles in length weighs about eight times as much. LiveAquaria makes the same point: full-bodied species put a higher bioload on the water than slender ones of the same length.
That error runs in both directions. For small schooling species the rule is too strict, because a 1.6 inch tetra weighs under a gram. For large species it is dangerously loose. Put a common pleco, sold at 5 cm and full grown at up to 60 cm, into a 20 gallon: the inch rule reads 118 percent, a little over. By weight it is nearly ten times over.
The surface area rule (12 square inches of water surface per inch of fish) has the same blind spot, since it also counts length rather than mass. It does capture something real, though: gas exchange happens at the surface, so a long, shallow tank holds more than a tall one of equal volume.
How the Calculator Works: Five Steps to a Percentage
It takes five steps to calculate the result, each tied to a published figure.
- Adult weight. Weight in grams equals a form factor times total length in centimeters, cubed. Froese’s 2006 meta-analysis of 3,929 weight-length relationships for 1,773 species gives the median form factor for each body shape (table below).
- Waste output. Resting metabolism in teleost fish scales with body mass to the power 0.79, per Clarke and Johnston’s 1999 review of 69 species. The calculator uses 0.8. Ammonia output tracks metabolism, so a 1 gram animal produces more waste per gram than a 60 gram one.
- Temperature. Across species, metabolism rises by a factor of 1.83 for every 10 C (Q10). The reference is 78 F. An unheated goldfish setup at 68 F runs at 0.71 of the tropical rate.
- Capacity. Aquariumscience.org gives three stocking densities for a 100 gallon stocked aquarium: about 280 grams of fish (light), 900 g (moderate) and 2,800 g (heavy). It says the moderate figure matches the 100 percent reading on aqadvisor.com for four inch fish. The calculator sets 100 percent there and carries it through steps 1 to 3 for every other size and shape. By default it also subtracts 15 percent of the volume for gravel, rock, equipment and the plants in a planted tank, as LiveAquaria recommends, which makes our 100 percent slightly stricter.
- Maintenance. A filter moving less than 4 tank volumes per hour, or changing less than 25 percent of the water weekly, shrinks capacity in proportion. The weaker of the two sets the limit. Going beyond either earns no bonus; a huge filter can’t remove nitrate, and big water changes don’t build more biofilter.
| Body shape | Form factor (Froese 2006) | Weight of a 4 inch adult | Examples in the list |
|---|---|---|---|
| Eel-like | 0.00131 | 1.4 g | Kuhli loach (use Other) |
| Elongated | 0.00838 | 8.8 g | Slim loaches (use Other) |
| Fusiform (torpedo) | 0.0137 | 14.4 g | Tetras, rasboras, livebearers, betta, plecos |
| Short and deep | 0.0187 | 19.6 g | Gouramis, corydoras, angelfish, discus, fancy goldfish |
A deep-bodied animal weighs about a third more than a torpedo shape of the same length. An eel-like one weighs a tenth as much. The inch rule treats all four rows as identical.
Four Tanks Run Through Three Rules
Each scenario below is reproduced exactly by the calculator and sits in its automated test file. Unless stated, the setup is a 20 gallon, 24 by 12 inches, 78 F, an 80 gph filter and 25 percent weekly water changes.
| Setup | Adult weight | Weight method | Inch rule | Surface rule |
|---|---|---|---|---|
| 10 neon tetras + 6 corydoras, 20 gal | 47 g | 58% | 161% | 135% |
| 1 betta, 5 gal, 16 by 8 in, 20 gph | 3.5 g | 18% | 50% | 23% |
| 1 fancy goldfish, 20 gal long (30 by 12 in), 68 F, 100 gph | 66 g | 33% | 30% | 20% |
| 1 common pleco, 20 gal | 2,959 g | 974% | 118% | 98% |
Neons and corydoras in a 20 gallon
The inch rule calls this peaceful community 61 percent over. It isn’t. Sixteen small animals weigh 47 grams combined, and the weight method puts them at 58 percent, moderate stocking. The corydoras carry most of the load; six of them are 6.4 g each against 0.9 g for a neon. Charterhouse Aquatics recommends groups of at least six corydoras, so this is the smallest sensible group, not a trimmed one.
A betta in a 5 gallon
One 2.5 inch betta weighs about 3.5 grams (published adult weights are 3 to 4 g) and uses 18 percent of a 5 gallon. Waste isn’t what sets the tank size here. The swimming length check needs 15 inches, the tank has 16, and the 5 to 10 gallon tank in the setup list below is the usual starting size. Keep one male per tank; the species is sold as the Siamese fighting fish for a reason.
A fancy goldfish at room temperature
At 6 inches and a short, deep body, the goldfish weighs 66 grams, more than the whole neon and corydoras group. Cooler water trims its metabolism to 0.71 of the tropical rate, so the weight method lands at 33 percent. The real problem shows up in the length check: 6 times 6 inches is 36, and a 20 long is 30 inches. Build Your Aquarium calls 20 gallons the absolute minimum for one adult ryukin, and 30 to 40 gallons safer for one that reaches 8 to 10 inches.
A common pleco sold as a juvenile
This is the case the inch rule gets most wrong. Full grown at 60 cm, the calculator estimates about 2,959 grams, and the tank needs 119 gallons and roughly 12 feet of length by the checks. The inch rule reads 118 percent. The weight method reads 974 percent. A bristlenose pleco tops out at 15 cm, grazes algae well, and lists a 90 litre (24 gallon) minimum.
How Filtration and Water Changes Move the Number
Filter flow, water changes and temperature change the answer after the species list is set. Take the neon and corydoras tank at 58 percent:
- Filter at half the baseline. A 40 gph filter turns 20 gallons over twice an hour. Capacity halves and the level jumps to 116 percent, heavy stocking.
- Water changes of 10 percent a week. Nitrate levels off at roughly the weekly input divided by the share replaced, so 10 percent changes support 0.4 of the load that 25 percent changes do. Result: 145 percent. LiveAquaria’s target for freshwater is nitrate below 20 ppm.
- Warmer water. Raising the heater from 78 to 88 F lifts metabolism by about 40 percent, and the reading with it.
- A bigger filter and 50 percent changes. Still 58 percent. The calculator never rewards maintenance beyond the baseline, which keeps it honest for most keepers.
Heater size and gravel depth don’t change the stocking math, but both come up during the same planning session. The aquarium heater size calculator and the fish tank gravel calculator handle them.
Betta Fish Setup Cost in 2026
Bettas are the most common first purchase in fishkeeping, so here is the betta fish setup cost, itemized from The Vet Desk’s January 2026 price guide:
| Item | Price |
|---|---|
| Aquarium, 5 to 10 gallons | $25 |
| Filter | $15 |
| Heater | $15 |
| Light | $15 |
| Gravel vacuum or siphon | $15 |
| Plants | $10 |
| Substrate | $10 |
| Air pump | $10 |
| Dechlorinator | $5 |
| Food | $5 |
| Airline tubing and air stone | $4 |
| Equipment total | $129 |
The animal itself is the cheap part. A standard male costs about $8, a plakat or koi about $12, rare colors up to $25, and the average female $5. That puts day one at $137 with a standard male. Skip the air pump, tubing and air stone ($14) when the filter already moves the surface and it’s $123. Ongoing costs run about $20 a month for food, dechlorinator and the occasional medication, or $240 a year, so the first year comes to roughly $377.
Two items on that list aren’t optional. Bettas are tropical fish, so the heater stays. And a filter is what makes the 5 gallon work: at 18 percent stocking the tank has waste capacity to spare, but only if bacteria in the filter are processing it.
Where This Estimate Falls Short
We built the method to be better than inches, not to be the last word. Known gaps:
- Tiny species read very low. Fifteen chili rasboras in a 10-gallon tank come to 8 percent. Waste isn’t the limit in small aquariums; space, groups and the minimum sizes are. That is why the minimum and length checks sit beside the percentage.
- Minimum sizes are one retailer’s list. The calculator flags 8 neons in a 10 gallon because Charterhouse lists 60 litres (about 16 gallons) as the minimum, even though their waste reads only 23 percent.
- Body shape is a median. Froese’s table has wide ranges within each shape. A well-fed fancy goldfish can outweigh the estimate.
- Temperament isn’t checked. There is no species compatibility, aggression or schooling logic. Always research tank mates separately, and watch for aggression among community fish after each addition.
- The anchor is one source. Our 100 percent rests on aquariumscience.org’s moderate level. AqAdvisor never published its formula.
- Adults, not juveniles. Presets use the top of each adult range. A tank of young fish reads high now and correct later.
Whatever the percentage says, stock conservatively and add fish slowly. LiveAquaria advises adding no more than 25 percent of the planned livestock at one time, then testing water parameters before the next group.
Stocking Questions From New Fishkeepers
How many fish can I put in a 10 gallon tank?
Fewer than the inch rule suggests for large fish, and the minimum tank size for each species matters more than waste. Fifteen chili rasboras read 8 percent by weight, and their listed minimum is 30 litres (about 8 gallons), so they fit. Neon tetras list 60 litres, so a 10 gallon is too small for them even though their waste would fit.
Is the one inch per gallon rule accurate?
Only roughly, for small slim species. It over-restricts schools of little tetras and badly under-restricts big fish, because weight grows with the cube of length. A common pleco scores 118 percent by inches and 974 percent by weight in a 20 gallon.
How much does a betta fish setup cost?
About $129 for equipment from The Vet Desk’s 2026 list, plus $5 to $25 for the betta, so $137 with a standard male. Plan on about $20 a month after that, roughly $377 for the first year.
How do I know if my aquarium is overstocked?
Test the water. Any ammonia or nitrite reading means the biofilter isn’t keeping up, and nitrate that climbs past 20 ppm between weekly changes means the load exceeds your maintenance. Gasping at the surface points to low oxygen. All of these are signs of poor water quality. The calculator’s heavy band (100 to 311 percent) is where these problems start for average setups.
Does tank shape change how many fish fit?
Yes. A long, shallow tank has more surface for gas exchange and more swimming length than a tall one of equal volume. That’s why the calculator asks for length and width and checks that the tank is at least six times the longest adult’s length.
Proper stocking starts with adult sizes. Use the fish stocking calculator with every species you plan to stock, then read all three checks, not only the headline. If any one of them fails, choose smaller species or a bigger tank before you buy. The rest of the pet cost calculator covers what the animals cost to keep once they’re home.
This page is general information, not veterinary advice. Weight estimates use the Froese 2006 form factors (Journal of Applied Ichthyology), waste scaling and Q10 from Clarke and Johnston 1999 (Journal of Animal Ecology), the moderate stocking level from aquariumscience.org, stocking rules from LiveAquaria, adult sizes from Charterhouse Aquatics, Fish Tank Advisor and Build Your Aquarium, and betta prices from The Vet Desk. Last reviewed: September 2026.