What Is an Aquarium Fish Stocking Calculator?
This calculator applies the classic freshwater aquarium "1 inch of adult fish per gallon" guideline to help you sanity-check how many fish your tank can reasonably hold. Add up the adult length of every fish you plan to keep, and compare that total to your tank's gallon capacity.
It's a simplified starting point that beginner aquarists have used for decades — useful for a rough estimate, but not a substitute for researching each species' real space, filtration and temperament needs.
How to Read Your Results
Total Fish Inches
The sum of every fish's adult length multiplied by how many of that fish you're keeping.
Tank Capacity
Your tank's gallon count, treated as the "budget" of inches available under the 1-inch-per-gallon guideline.
Stocking Level
Total fish inches divided by tank capacity, shown as a percentage. Under 100% generally leaves margin for waste and growth; well over 100% is a warning sign of overcrowding.
Real-World Example
A 20-gallon tank stocked with 6 neon tetras (2 in. each) and 2 dwarf gouramis (3 in. each):
| Fish group | Count | Length | Subtotal |
|---|---|---|---|
| Neon tetras | 6 | 2 in. | 12 in. |
| Dwarf gouramis | 2 | 3 in. | 6 in. |
| Total | 18 in. | ||
18 inches in a 20-gallon tank is 90% of capacity — appropriately stocked, with a little room to spare.
Tips for Responsible Stocking
- The rule works best for small, slim-bodied community fish — it badly underestimates space needs for goldfish, large cichlids and other messy or deep-bodied fish.
- Filtration matters as much as gallons. A well-filtered, heavily planted tank can support slightly more fish than the raw ratio suggests.
- Always research each species' minimum tank size and social needs (schooling fish need groups, territorial fish need space) before adding them.
- Stock gradually and test your water parameters as you add fish, rather than adding everyone on day one.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team