40 Gallon Tank Dimensions: Breeder vs Long, Size and Weight
Published: 2026-11-10 · FishyKart Blog
There are two standard 40 gallon tanks, and they are different shapes. A 40 gallon breeder is about 36 x 18 x 17 inches (Aqueon: 36 3/16 x 18 1/4 x 16 15/16 in, 58 lb empty, 458 lb filled). A 40 gallon long is about 48 x 13 x 17 inches (Marineland: 48 15/32 x 12 27/32 x 16 7/8 in, 52 lb empty). Same water, same height: the breeder trades length for depth, the long trades depth for length. Which one you need depends on your fish and on the stand space you have.
Photo: Wikimedia Commons, public domain
40 Gallon Breeder vs 40 Long: Specs
| Measure | 40 gallon breeder (Aqueon) | 40 gallon long (Marineland) |
|---|---|---|
| Length | 36 3/16 in (91.9 cm) | 48 15/32 in (123.1 cm) |
| Width (front to back) | 18 1/4 in (46.4 cm) | 12 27/32 in (32.6 cm) |
| Height | 16 15/16 in (43.0 cm) | 16 7/8 in (42.9 cm) |
| Footprint | 660 sq in (4.59 sq ft) | 623 sq in (4.32 sq ft) |
| Empty weight | 58 lb (26 kg) | 52 lb (24 kg) |
| Filled weight (maker's figure) | About 458 lb (208 kg), with gravel and decor | About 386 lb (175 kg), close to glass plus water |
| Nominal volume | 40 US gallons = 151 liters = 33.3 UK gallons | |
Sources: Aqueon product catalog, "Critical dimensions for all aquariums", checked November 2026: aqueon.com (Aqueon does not list a 40 long). Marineland 40 gallon long, model ML10400, manufacturer specs as listed by a Marineland dealer: bluefishaquarium.com. Metric values, footprints and UK gallons calculated by FishyKart. Other brands differ by a fraction of an inch.
Real Water Volume
Outside dimensions include the glass and frame, so they multiply out to more than the label: 48.4 gallons gross for the breeder and 45.5 gallons for the long. Take away a 2 inch gravel bed and keep the water 1 inch below the rim, and a 40 breeder holds about 40 gallons (151 liters) of water, a 40 long about 37 gallons (142 liters). Use those numbers for water conditioner and medication; a 25% water change is about 9 to 10 gallons. To check your own tank, use the aquarium volume calculator.
Weight and Where to Put It
At about 8.3 lb per gallon (derived from the USGS water density table), 40 gallons of water weigh about 333 lb. Aqueon's 458 lb for the filled breeder includes gravel and decor; Marineland's 386 lb for the long is close to glass plus water, so add 40 to 60 lb for a typical gravel bed and rock.
Spread over the base, a filled 40 breeder puts about 100 lb per square foot on the floor, a 40 long about 90 to 100 lb, against about 146 lb for a 55 gallon on its narrow base. Still, place it against a wall on a stand rated for the tank, run the long side across the floor joists, and ask a contractor about old or weak floors. This is general information, not a structural assessment.
40 Gallon vs 29, 55 and 75
| Tank | L x W x H (inches) | Footprint | Empty / filled |
|---|---|---|---|
| 29 gallon | 30 1/4 x 12 1/2 x 18 3/4 | 2.63 sq ft | 40 / 330 lb |
| 40 gallon breeder | 36 3/16 x 18 1/4 x 16 15/16 | 4.59 sq ft | 58 / 458 lb |
| 40 gallon long (Marineland) | 48 15/32 x 12 27/32 x 16 7/8 | 4.32 sq ft | 52 / 386 lb |
| 55 gallon | 48 1/4 x 12 3/4 x 21 | 4.27 sq ft | 78 / 625 lb |
| 75 gallon | 48 1/2 x 18 1/2 x 21 1/8 | 6.23 sq ft | 98 / 785 lb |
Dimensions and weights: Aqueon "Critical dimensions" table, except the 40 long (Marineland). Footprints calculated by FishyKart.
- 40 breeder vs 29: 6 inches longer, nearly 6 inches deeper and 75% more floor area, while about 2 inches shorter. It is the usual upgrade when a 29 gets crowded.
- 40 long vs 55: almost the same 4 foot by 13 inch footprint, so they share lids and many stands, but the 55 is 4 inches taller. The 40 long is lighter (386 vs 625 lb filled) and easier to reach into.
- 40 breeder vs 75: the 75 keeps the same 18 inch depth but adds a foot of length and 4 inches of height. Fish that need a 4 foot tank as adults belong in the 75, not the breeder.
Breeder or Long: Which to Choose
Pick the breeder for fish that hold territory or live on the bottom, and for aquascapes: the 18 inch depth gives room for rock caves, plant groups and separate zones. A 40 breeder is the size we suggest for two breeding pairs of convict cichlids, for a single adult axolotl with room to spare, for blue gouramis and for sailfin mollies, whose males need space to grow full sails. It is also the easier tank to light and to reach into.
Pick the long for active schooling fish that swim end to end, such as danios, rainbowfish and larger tetra groups, and for fish that patrol a long glass front. The extra foot of length matters more to them than depth. The narrow 13 inch base also fits shallow alcoves and shelves where a breeder will not.
For fancy goldfish, either shape works for about three fancy goldfish; the breeder's larger surface area helps oxygen exchange, which goldfish need.
Before You Buy a 40
- Check the stand: a 40 breeder needs a 36 x 18 inch top, a 40 long a 48 x 13 inch top. A 30 or 29 gallon stand is too small for either.
- Measure lids and lights by both length and depth. 48 inch fixtures for a 55 usually fit a 40 long; a breeder needs 36 inch fixtures and an 18 inch deep lid.
- Compare filled weight, not the label: the two 40s differ by about 70 lb as listed, mostly because of how each maker counts gravel.
- Empty the tank completely before moving it, and carry it with two people.