Aquarium surface area rule
Multiply tank length by width, then divide by 12 sq in per inch of tropical fish or 30 for coldwater fish like goldfish.
- 30 × 12 in
- tank top
- 360 sq in
- of surface
- 30 in
- of tropical fish
- 12 in
- of goldfish
of tropical fish by surface area
30 × 12 in of water surface
- 360 sq in ÷ 12 sq in per inch = 30 in.
- For coldwater fish (30 sq in per inch): 12 in.
A rule of thumb, not science. Use the smaller of this and the gallon rule below.
| Tank | L × W (in) | Surface (sq in) | Tropical (12 sq in/in) | Goldfish (30 sq in/in) | Gallon rule, 1 gal/in |
|---|---|---|---|---|---|
| 5 gal | 16 × 8 | 128 | 10.5 in | 4 in | 4.5 in |
| 10 gal | 20 × 10 | 200 | 16.5 in | 6.5 in | 9 in |
| 20 gal high | 24 × 12 | 288 | 24 in | 9.5 in | 17.5 in |
| 20 gal long | 30 × 12 | 360 | 30 in | 12 in | 16 in |
| 29 gal | 30 × 12 | 360 | 30 in | 12 in | 24.5 in |
| 40 gal breeder | 36 × 18 | 648 | 54 in | 21.5 in | 39 in |
| 55 gal | 48 × 13 | 624 | 52 in | 20.5 in | 49.5 in |
| 75 gal | 48 × 18 | 864 | 72 in | 28.5 in | 69 in |
Why a 20 long beats a 20 high
Gas exchange happens at the water surface, so it depends on length times width, not on height. A 20 long is 30 × 12 in, which is 360 sq in and supports 30 in of tropical fish. A 20 high is 24 × 12 in, or 288 sq in, which supports 24 in. After 12% for substrate and decor the 20 long holds 16.5 gal of water and the 20 high 17.6 gal, so the gallon rule barely tells them apart. Where the surface rule gives the lower number, use that one. The 20 long also has more floor and swimming length.
Check the same tank by gallons
24.7 gal net · max 24 fish · comfortable 19
most adult fish this tank holds by the rule
comfortable: 19 (80%), at 1 in each
- One-inch-per-gallon rule: 1 in of fish per 1 gal.
- 28.1 gal gross, minus 3.4 gal (12%) for substrate and decor = 24.7 gal of water.
- Surface-area check: 30 × 12 = 360 sq in, about 30 in of tropical fish at 12 sq in per inch.
A rule of thumb, not science. Adult size, temperament, swimming space, and each species' minimum tank matter more than any inch rule.
How to use this calculator
Measure the top
Length times width in inches gives square inches of water surface.
Choose tropical or coldwater
Tropical fish get 12 sq in per inch of fish. Coldwater goldfish get 30.
Compare with the gallon rule
Use the smaller of the two numbers.
The rule we use
For this page, the calculator applies the rule that matches the fish, and takes 12% of the gross gallons off by default for substrate and decor. The three rules are 1 gal per inch for small slim community fish, 3 gal per inch for goldfish and fish of 5 in or more, and 5 gal per inch for saltwater. See aquarium stocking rules explained for the formula, filtration factors, and the origin of each.
These are rules of thumb used by hobbyists, not science. Surface area controls gas exchange: oxygen goes in and carbon dioxide leaves through the surface. Height does not add any.
Common mistakes
Counting height
Two tanks with the same gallons can have very different surface areas. A tall tank has less surface.
Covering the surface with plants
Floating plants shade the water and also reduce the open surface for gas exchange.
Ignoring the filter's surface agitation
Gentle ripples increase gas exchange. A still surface does less.
Frequently asked questions
What is the aquarium surface area rule?
Allow 12 sq in of water surface per inch of tropical fish, and 30 sq in per inch for coldwater fish like goldfish.
Why is a 20 long better than a 20 high?
A 20 long (30 × 12 in) has 360 sq in of surface, against 288 sq in for a 20 high (24 × 12 in).
Does the surface area rule replace the gallon rule?
No. Use both and take the smaller number.
How much surface does a 29 gallon have?
30 × 12 in is 360 sq in, so 30 in of tropical fish or 12 in of goldfish.
Does an air stone add surface area?
No, but it increases gas exchange through surface agitation and lets you stock a little closer to the limit.