Quick answer: Enter your tank size and each fish species with its quantity and adult length, and this calculator totals the adult fish length against your tank's inch-per-gallon capacity. With the defaults, a 20-gallon tank with 6 neon tetras and 4 guppies totals 17.0 of 20.0 inches, near capacity. It is free, with no signup, and you can also try the Epoxy Resin Calculator and Fence Material Calculator.
Fish Tank Stocking Calculator
How many fish can you keep? Enter your tank size and your fish, and check the result against the simple inch-per-gallon guideline for small freshwater community tanks.
Stocking a tank is about adult fish size, not the size of the fish when you buy them. This calculator adds up the adult length of every fish you plan to keep and compares it with your tank capacity, so you can see at a glance whether you have room, are near capacity, or are likely overstocked.
If you switch to Liters, the number you entered stays the same. Capacity in inches is then your liters divided by 3.785.
Your fish
Enter each species, how many you keep, and its adult length in inches. You can change any adult length if your fish grows larger than the default.
Total adult fish length vs. tank capacity:
17.0 / 20.0 inches
Near capacity — fine for now, but leave room for growth and avoid adding more.
How this calculator works
This tool uses the common inch-per-gallon guideline: about 1 inch of adult fish per gallon of water, for small freshwater community tanks roughly 10 to 55 gallons. Total length is the sum of quantity times adult length for every row. Capacity in inches equals your tank size in gallons, or your tank size in liters divided by 3.785 when Liters is selected.
The result is a guideline, not a hard rule. Up to 80% of capacity is shown as within the guideline. Above 80% and up to 100% is shown as near capacity. Above 100% is shown as likely overstocked.
Tips for not overstocking your aquarium
- Stock for adult size. Fish that look small in the shop may grow several times larger.
- Add fish gradually, so the filter and water chemistry can adjust to the extra load.
- Upgrade filtration before adding more fish, rather than after problems appear.
- Research schooling and territorial needs. Some fish need groups, and some need their own space.
- Test water monthly for ammonia, nitrite and nitrate, and act early if readings rise.
This calculator is a planning guide only. It does not account for body shape, waste load, aggression, plants, or filtration strength. Always research each species before buying.
How this calculator works
The calculator uses the common inch-per-gallon guideline for small freshwater community tanks, roughly 10 to 55 gallons: about 1 inch of adult fish per gallon of water. It adds up the adult length of every fish you plan to keep, then compares that total with the tank's capacity in inches. Always use adult size, not the size of the fish when you buy them, because fish that look small in the shop may grow several times larger.
Total adult length = sum of (quantity x adult length) for every fish row. Capacity in inches = tank size in gallons, or tank size in liters divided by 3.785 when Liters is selected. Up to 80% of capacity is shown as within the guideline, above 80% and up to 100% is shown as near capacity, and above 100% is shown as likely overstocked.
Worked example: the default setup
Use the defaults: a 20-gallon tank, 6 neon tetras at 1.5 inches adult length, and 4 guppies at 2 inches adult length.
- Neon tetras: 6 x 1.5 = 9.0 inches.
- Guppies: 4 x 2 = 8.0 inches.
- Total adult length = 9.0 + 8.0 = 17.0 inches.
- Capacity = 20 gallons = 20.0 inches, so 17.0 divided by 20.0 = 85%, which the tool shows as near capacity, with 3.0 inches of room left.
Near capacity means the tank is fine for now, but you should leave room for growth and avoid adding more fish. The result is a planning guide only: it does not account for body shape, waste load, aggression, plants, or filtration strength, so research each species before buying.
Quick reference: guideline capacity by tank size
Capacity is 1 inch per gallon, and the 80% column is the point up to which the tool still shows the tank as within the guideline.
| Tank size | Capacity in inches | 80% of capacity |
|---|---|---|
| 10 gallons | 10 inches | 8 inches |
| 20 gallons | 20 inches | 16 inches |
| 30 gallons | 30 inches | 24 inches |
| 50 gallons | 50 inches | 40 inches |
If you work in liters, divide by 3.785 first: for example, a 75-liter tank gives about 19.8 inches of capacity, close to a 20-gallon tank.
Frequently Asked Questions
What is the inch-per-gallon rule?
It is a simple guideline for small freshwater community tanks: about 1 inch of adult fish per gallon of water. This calculator adds up the adult lengths of your fish and compares the total with your tank size in gallons, or your tank size in liters divided by 3.785.
Should I use adult size or current size?
Always use adult size. Fish that look small in the shop may grow several times larger, and stocking for their size now can leave the tank overstocked later.
What does near capacity mean?
It means your total is between 80% and 100% of the guideline capacity. The tank is fine for now, but you should leave room for growth and avoid adding more fish.
What does this calculator not account for?
It is a planning guide only. It does not account for body shape, waste load, aggression, plants, or filtration strength, so always research each species before buying.