Calculate aquarium tank volume, fish inches, or estimated max fish count from tank dimensions, adult fish size, and stocking rate.
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Aquarium Stock Calculator Formula
The aquarium stock calculator uses tank volume, decor displacement, and an inches-of-fish-per-gallon stocking rate. The basic tab uses a fixed rate of 2 inches of adult fish per gallon. The fish count tab lets you choose 2, 1.5, 1, or a custom inches-per-gallon rate.
G = L * W * H * 0.004329
EG = G * (1 - D)
F = EG * R
N = (EG * R) / S
For solving a missing tank dimension in the basic tab:
L = (F / R) / (W * H * 0.004329 * (1 - D))
W = (F / R) / (L * H * 0.004329 * (1 - D))
H = (F / R) / (L * W * 0.004329 * (1 - D))
- L = tank length, converted to inches
- W = tank width, converted to inches
- H = tank height, converted to inches
- G = tank volume in US gallons before displacement
- EG = effective water volume after decor and substrate displacement
- D = displacement rate, such as 0.10 for 10%
- R = stocking rate in inches of adult fish per gallon
- F = total inches of adult fish supported by the tank
- S = average adult fish size in inches
- N = estimated maximum number of fish
In the basic stock mode, you enter any three values among length, width, height, and inches of fish. The calculator converts the measurements to inches, applies the gallon conversion factor, subtracts the selected displacement, and solves for the missing value.
In the fish count mode, you enter the tank volume, average adult fish size, stocking guideline, and displacement. The calculator estimates the total adult fish length the tank can support, then divides by the average adult size to estimate fish count.
Stocking Rate and Displacement Reference
These values help you interpret the inputs used by the calculator. Use adult fish size, not juvenile size.
| Stocking rate | Meaning | Best used for |
|---|---|---|
| 2 in/gal | Less conservative estimate | Small, peaceful fish with good filtration and maintenance |
| 1.5 in/gal | Moderate estimate | Community tanks with mixed small fish |
| 1 in/gal | Conservative estimate | Beginner tanks, heavier-bodied fish, or lower maintenance schedules |
| Displacement setting | Volume removed | Typical setup |
|---|---|---|
| None | 0% | Bare tank or very little hardscape |
| Light | 5% | Thin substrate and a few decorations |
| Moderate | 10% | Common planted or decorated aquarium |
| Heavy | 20% | Deep substrate, large rocks, driftwood, or dense aquascape |
Example Aquarium Stock Calculations
Example 1: Estimate total inches of fish
You have a tank that measures 30 in long, 12 in wide, and 12 in high. Displacement is set to none.
G = 30 * 12 * 12 * 0.004329 = 18.7013
F = 18.7013 * 2 = 37.4026
The basic tab returns about 37.4 inches of adult fish.
Example 2: Estimate fish count
You have a 29 gallon tank, 10% displacement, an average adult fish size of 2 inches, and a stocking rate of 1.5 inches per gallon.
EG = 29 * (1 - 0.10) = 26.1
N = (26.1 * 1.5) / 2 = 19.575
The fish count tab returns about 19.6 fish. In practice, you would round down and also consider species behavior and filtration.
Aquarium Stock Calculator FAQ
Should you use the fish's current size or adult size?
Use the adult size. Many aquarium fish are sold as juveniles, but stocking based on current size can lead to an overcrowded tank later. If a fish is 1 inch now but usually grows to 3 inches, enter 3 inches as the size.
Is inches of fish per gallon always accurate?
No. It is a rough stocking rule. It works best for small, slim-bodied community fish. It is less useful for large fish, messy fish, territorial fish, schooling fish, or fish with special swimming-space needs. Tank shape, filtration, water changes, oxygen level, and compatibility also matter.
Why does displacement reduce the stocking estimate?
Substrate, rocks, driftwood, and decorations take up space that would otherwise hold water. Less water means less dilution for waste, so the calculator reduces the usable volume by the selected displacement percentage.
