What Is the Fish Tank Stocking Calculator?
This calculator applies the popular "one inch of fish per gallon" rule of thumb to help hobbyist aquarists estimate how heavily their tank is stocked and how many more fish they can safely add. It works for any unit system as long as you keep tank volume in US gallons and fish length in inches. The rule is a starting guideline only — it is universal in the aquarium hobby and not tied to any country or product.
How to Use It
Enter your tank's volume in gallons, the combined adult length (in inches) of fish you already keep, and the typical adult length of the new fish you want to add. The calculator returns the maximum fish length your tank supports, the remaining inches available, roughly how many more fish fit, and what percentage of capacity is already used.
The Formula Explained
Maximum fish length equals tank gallons divided by one, so a 20-gallon tank supports about 20 inches of fish. Remaining capacity is that maximum minus the inches of fish you currently keep. Dividing the remaining inches by the average length of each new fish gives a rough count of additional fish.
$$\text{New Fish} = \left\lfloor \frac{\text{Tank (gal)} - \text{Stocked (in)}}{\text{Length per Fish (in)}} \right\rfloor$$where:
$$\begin{gathered} \text{New Fish} = \left\lfloor \frac{R}{\text{Length per Fish (in)}} \right\rfloor \\[1.5em] \text{where}\quad \left\{ \begin{aligned} R &= \max\!\left(\text{Tank (gal)} - \text{Stocked (in)},\ 0\right) \\ \text{Used} &= \frac{\text{Stocked (in)}}{\text{Tank (gal)}} \times 100\% \end{aligned} \right. \end{gathered}$$
Worked Example
For a 20-gallon tank already holding 8 inches of fish, the maximum is 20 inches and the remaining capacity is \(20 - 8 = 12\) inches. If each new fish averages 2 inches, you can add about \(12 \div 2 = 6\) more fish, with 40% of capacity currently used.
FAQ
Is the inch-per-gallon rule accurate? It is a rough beginner guideline. Body shape, waste output, swimming space, and filtration all matter, so heavy-bodied fish need more room than the rule suggests.
Should I use adult or current size? Always plan with the expected adult length so the tank does not become overcrowded as fish grow.
Does this account for filtration? No. Strong filtration and frequent water changes can support slightly more stock, while poor maintenance supports less.
