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40 Gallon Breeder Dimensions

A standard 40 gallon breeder aquarium measures about 36 × 18 × 16 in (91 × 46 × 41 cm) on the outside. Exact sizes vary by brand: Aqueon lists 363⁄16 × 18¼ × 1615⁄16 in and Marineland 365⁄16 × 189⁄32 × 16¾ in. Once it's set up with 2 inches of sand, it holds roughly 37–38 gallons of water and weighs about 440 lb in total.

40 gallon breeder tank calculator

Drawn to scale from your numbers: sand at the bottom, water up to your fill line.

Start from a real tank
Outside dimensions
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Glass, tank weight and rock
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37.5US gal of water

Once set up. 48.4 gal by outside dimensions · 43.0 gal before substrate

Total weight on the stand estimate
444 lb
Water
142 litres
Footprint
4.6 sq ft
Load on that footprint
97 lb/sq ft
Heater rule of thumb
113–188 W
Filter flow rule of thumb
150–225 gph
Substrate needed
73 lb
Water weight
313 lb
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Setting up a 40 breeder?

  • Plan your fish: the stocking calculator checks numbers and tank mates
  • Size your heater for your room temperature
  • Check another tank's volume, any shape

Not every 40 gallon breeder is the same size

The “40 gallon breeder” label is a class of tank, not an exact measurement. Here is what each source actually lists:

SourceL × W × H (in)EmptyFull
Aqueon size chart363⁄16 × 18¼ × 1615⁄1658 lb458 lb
Marineland ML10040 (dealer listing)365⁄16 × 189⁄32 × 16¾52 lb320 lb*
Petco product page (Aqueon)36¼ × 18¼ × 17––
SaltwaterAquarium.com size table36 × 18 × 1658 lb–

Checked 24 Sep 2026. Marineland's full weight is simply the label gallons × 8.34 lb plus the empty tank; Aqueon's is closer to the full outside volume filled with water. Neither includes sand, rock or the stand. That's why the two brands disagree. *Marineland's 320 lb looks like a listing error: its own method (label gallons × 8.34 lb + empty tank) gives about 386 lb.

Same stand, three tanks

Tanks built on a 36 × 18 footprint share stands. The breeder sits in the middle of the range:

TankL × W × H (in)Outside volume
30 gallon breeder36 × 18 × 1233.7 gal
40 gallon breeder363⁄16 × 18¼ × 1615⁄1648.4 gal
65 gallon36 × 18 × 2467.3 gal

Nominal sizes from SaltwaterAquarium.com (30 breeder), Aqueon (40 breeder) and Marineland dealer listing (65).

A stand rated for a 65 will hold a 40 breeder, but check the rating, because the weights differ a lot.

40 breeder vs 40 long

Same water, very different shapes:

TankL × W × H (in)FootprintOutside volume
40 breeder (Aqueon)363⁄16 × 18¼ × 1615⁄164.6 sq ft48.4 gal
40 long (Marineland, nominal)48 × 13 × 164.3 sq ft43.2 gal
  • Breeder: 18 in front to back, deep enough for layered aquascapes, territorial fish and bigger bodied species to turn. Fits a 36 in space.
  • Long: a full 4 ft of swimming length for active schooling fish, but only 13 in deep. It shares the 48 × 13 footprint with a 55 gallon.

How much water a 40 gallon breeder really holds

Multiply the outside dimensions of an Aqueon 40 breeder and you get 48.4 gallons. You never get that much water, because of three things:

  1. Glass. With about 1/4 in glass (an estimate; check your tank), the inside is roughly 35-11/16 × 17¾ in.
  2. The gap at the top. Most people keep the water about an inch below the rim.
  3. Substrate. Two inches of sand takes up about 21 litres of space.

That leaves about 43.0 gallons with a bare bottom and 37.5 gallons with 2 in of sand. Use the real figure, not the label, when you dose medication or water conditioner.

Weight and stand

A set-up Aqueon 40 breeder comes to about 444 lb: 58 lb of glass, about 313 lb of water and about 73 lb of sand, before rock and the stand itself. That sits on 4.6 square feet.

  • A set-up 40 breeder weighs about 444 lb. Use a stand made for 36 × 18 tanks.
  • Place it across the floor joists where you can, especially on upper floors.

General guidance, not structural advice.

Equipment for a 40 gallon breeder

ItemStarting pointHow we got it
Heater113–188 W3–5 W per gallon of real water; use the higher end in a cold room.
Filter150–225 gphTurning over the water 4–6 times an hour
Sand, 2 in deep≈ 73 lbInside floor area × depth at about 1.6 kg per litre (estimate)
Light36 inMatch the tank length
Lid36 × 18 inCheck the maker's fit list

Rules of thumb, not guarantees. Fish choice, room temperature and filter type all change the right answer.

What a 40 gallon breeder is good for

The 18 in depth is the point of a breeder: room for aquascaping and for fish that need space to turn. Check any mix in the stocking calculator.

How the numbers are worked out

Outside volume (gal) = L × W × H ÷ 231 Inside length/width = outside − 2 × glass Water height = H − glass (bottom) − gap below rim − substrate depth Water (gal) = inside L × inside W × water height ÷ 231 Water weight (lb) = gallons × 8.34 (fresh water) Substrate (lb) = inside L × inside W × depth × 0.016387 L/in³ × 1.6 kg/L × 2.2046

Worked examples

Aqueon 40 breeder, 1/4 in glass, 1 in gap, 2 in sand: inside 35.69 × 17.75 in, water height 16.9375 − 0.25 − 1 − 2 = 13.69 in. 35.69 × 17.75 × 13.69 = 8,670 in³ ÷ 231 = 37.5 gal (142.1 L).

Marineland 40 breeder, 1/4 in glass, 1 in gap, 2 in sand: inside 35.81 × 17.78 in, water height 16.75 − 0.25 − 1 − 2 = 13.5 in. 35.81 × 17.78 × 13.5 = 8,597 in³ ÷ 231 = 37.2 gal (140.9 L).

Bare-bottom Aqueon 40 breeder, 1/4 in glass, 1 in gap, 0 in sand: inside 35.69 × 17.75 in, water height 16.9375 − 0.25 − 1 − 0 = 15.69 in. 35.69 × 17.75 × 15.69 = 9,937 in³ ÷ 231 = 43.0 gal (162.8 L).

FAQ

Are all 40 gallon tanks the same size?

No. A 40 breeder is about 36 × 18 × 16 in; a 40 long is about 48 × 13 × 16 in. They hold similar water but need different stands.

What are the dimensions of an Aqueon 40 gallon breeder?

Aqueon's size chart lists 36 3/16 × 18¼ × 16 15/16 in, including the trim.

How many gallons is 36 × 18 × 18?

36 × 18 × 18 in is 11,664 cubic inches, or 50.5 US gallons measured from the outside. That's taller than a standard 40 breeder.

How much does a 40 gallon breeder weigh full?

Aqueon lists 458 lb. Our estimate with 2 in of sand is about 444 lb before rock.

Why is it called a breeder?

The name comes from its wide, shallow shape, which gives more floor area for its volume than a standard tank.

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Sources

  • Aqueon aquarium size and weight chart, aqueon.com (PDF): www.aqueon.com
  • Marineland standard aquariums, Bluefish Aquarium (dealer): bluefishaquarium.com
  • Marineland 40 Gallon Breeder ML10040, Bluefish Aquarium: bluefishaquarium.com
  • Aqueon Standard Glass Aquarium Tank 40 Gallon Breeder, Petco: www.petco.com
  • Aquarium Weights & Sizes, SaltwaterAquarium.com: www.saltwateraquarium.com
  • All accessed 24 Sep 2026. Calculations by TankRatio; “estimate” and “rule of thumb” figures are ours.

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