Fish Tank Size Calculator
Calculate your aquarium's volume in gallons and liters from its dimensions, plus a quick stocking estimate of how many small fish it can hold using the inch-per-gallon guide.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
How the Fish Tank Size Calculator Works
This calculator finds your aquarium’s volume in US gallons and liters from its dimensions, then gives a quick stocking estimate of how many small fish it can comfortably hold. Enter the tank’s length, width/depth and height in inches, plus the adult length of one fish, and it does the rest.
The Formula
Volume (in³) = length × width × height Gallons = Volume (in³) ÷ 231 Liters = Gallons × 3.785
There are 231 cubic inches in one US gallon. To estimate usable water, the tool subtracts about 10% for gravel, decor and the gap above the water line.
Worked Example
For a tank that is 36 in long, 18 in wide and 16 in tall:
- Volume = 36 × 18 × 16 = 10,368 in³
- Gallons = 10,368 ÷ 231 = ≈ 44.9 US gallons (≈ 170 L)
- Usable water ≈ 90% = ≈ 40 gallons
- With 2-inch fish: 40 ÷ 2 = ≈ 20 small fish
Common Tank Sizes
| Dimensions (L × W × H) | Approx. gallons |
|---|---|
| 20 × 10 × 12 in | ≈ 10 |
| 24 × 12 × 16 in | ≈ 20 |
| 36 × 18 × 16 in | ≈ 45 |
| 48 × 18 × 21 in | ≈ 78 |
How to Use the Stocking Estimate
The “one inch of fish per gallon” rule is a popular starting point for slim, small community fish such as tetras, danios and rasboras. Treat it as a rough guide only:
- Messy or large fish (goldfish, cichlids, plecos) need far more water per fish.
- Body shape matters — a chunky fish produces more waste than a slim one of the same length.
- Always check the minimum tank size for each species before buying.
Use the calculated volume for choosing equipment, dosing treatments and planning water changes, and use the stocking number only as a sanity check, not a hard limit.
Frequently asked questions
How do I calculate the volume of my fish tank in gallons?+
Multiply length x width x height in inches to get cubic inches, then divide by 231 to get US gallons. For example, a 36 x 18 x 16 inch tank holds 36 x 18 x 16 = 10,368 cubic inches, or about 44.9 gallons.
How many fish can I keep in my tank?+
A rough guide is about one inch of small community fish per gallon of usable water. This tool estimates the count for you, but it is only a starting point. Active, large or messy species need far more space than the simple rule suggests.
Why is the actual water volume less than the calculated volume?+
Gravel, rocks, plants, decorations and the gap above the water line all reduce usable water, typically by around 10 percent. This calculator factors in roughly 90 percent net water when estimating how many fish a tank can hold.
Is the inch-per-gallon rule reliable?+
It is a handy beginner guideline for small, slim community fish but it has real limits. Body shape, waste output, swimming activity and territory all matter. Always research each species' minimum tank size before stocking.
How do I convert aquarium gallons to liters?+
Multiply US gallons by 3.785 to get liters. For example, a 45-gallon tank holds about 170 liters. This calculator shows both gallons and liters automatically once you enter the tank dimensions.