40 Gallon Tank Dimensions (40 breeder and 40 long) and Weight
A 40 gallon tank (40 breeder) measures 36" × 18" × 16", the 40 long 48" × 13" × 16". It holds about 151.4 liters and weighs about 467 lb full: 58 lb of glass, 334 lb of water and 75 lb of gravel.
Volume
28.05 gal
Liters
106.2 L
This is the full water volume; actual capacity is a bit lower once you add substrate, rocks and the fill line.
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What a 40 gallon tank actually measures
The 40 breeder 40 gallon footprint is 36 × 18 × 16 inches inside the glass. Run those through the rectangular formula and you get 44.88 gallons of geometric volume, which is not the same as the 40 on the box. Nominal sizes are round numbers picked for the label; the glass is cut to whatever the mould is. Your real water volume is lower again, because substrate, rock and the inch or two you leave below the rim all come out of it. Budget roughly 10 to 15 percent for a planted tank with a two-inch substrate bed.
Watch the footprint, not just the gallons. A 40 long holds the same nominal 40 gallons at 48 × 13 × 16 inches. Same water, different tank: the longer one gives more swimming length and more surface area for gas exchange, which matters more to active schooling fish than the gallon count does.
How much a 40 gallon tank weighs, empty and full
Water weighs 8.34 lb per US gallon, so 40 gallons is about 334 lb before you add anything. A standard glass 40 gallon weighs about 58 lb empty, and a 2 inch bed of gravel or sand over its 36 × 18 inch floor adds about 75 lb (at 100 lb per cubic foot). All in, that is about 467 lb resting on 4.5 square feet, or 104 lb per square foot. That number, not the gallons, is what decides whether the furniture you were planning to use is a stand or a mistake.
| Component | 40 breeder | 40 long |
|---|---|---|
| Empty glass tank | 58 lb | 55 lb |
| Fresh water | 334 lb | 334 lb |
| 2" gravel or sand | 75 lb | 72 lb |
| Total, freshwater | 467 lb | 461 lb |
| Total, saltwater | 475 lb | 469 lb |
| Load on the footprint | 104 lb/sq ft | 106 lb/sq ft |
Empty weights are typical retail figures for standard glass; check the label on yours. Live rock and hardscape are extra, often 1 to 1.5 lb per gallon on a reef.
What fits in 40 gallons
Each row is the largest single-species group the stocking calculator still rates as comfortable (at or below 85 percent of capacity) in 40 gallons, with adult size and waste output weighted. Species whose minimum tank size is larger than this tank are not listed at all.
| Species | Comfortable group |
|---|---|
| Neon Tetra | up to 38 |
| Guppy | up to 29 |
| Corydoras Catfish | up to 23 |
| Angelfish | up to 5 |
| Fancy Goldfish | up to 1 |
Mixing species changes these numbers; run your actual list through the stocking calculator.
Other standard tank sizes
- 5 gallon tank16" × 8" × 10.5"
- 10 gallon tank20" × 10" × 12"
- 20 gallon tank24" × 12" × 16"
- 29 gallon tank30" × 12" × 18"
- 55 gallon tank48" × 13" × 21"
- 75 gallon tank48" × 18" × 21"
- 90 gallon tank48" × 18" × 24"
- 125 gallon tank72" × 18" × 21"
Volume by tank shape
- Rectangular tankslength × width × height ÷ 231
- Cube tanksedge³ ÷ 231
- Cylinder tanksπ × (diameter ÷ 2)² × height ÷ 231
- Bowfront tankslength × width × height × 1.06 ÷ 231
- Hexagonal tanks(3√3 ÷ 2) × edge² × height ÷ 231
What to do with the number
- What bioload actually isGallons is the denominator, not the answer
- Cycle the tank firstFishless cycling, step by step
Frequently asked questions
What are the dimensions of a 40 gallon tank?
The 40 breeder footprint is 36 × 18 × 16 inches. A 40 long is also sold at 48 × 13 × 16 inches, same nominal gallons, different footprint.
How many liters is a 40 gallon tank?
About 151.4 liters (US gallons × 3.785).
How much does a 40 gallon fish tank weigh?
About 467 lb set up: 58 lb for a standard empty glass tank, 334 lb of water (40 gal × 8.34 lb/gal) and about 75 lb for a 2 inch gravel bed. Saltwater adds 8 lb. Rock, the stand and the lid come on top.
How much does an empty 40 gallon tank weigh?
About 58 lb for standard glass, and about 55 lb for the 40 long. Rimless low-iron glass and acrylic tanks differ: acrylic of the same size is roughly half the weight of glass.
Is a 40 gallon tank really 40 gallons?
Not exactly. Those inside dimensions work out to 44.88 gallons of geometric volume, and the real water volume lands below that once substrate, rock and the gap below the rim are subtracted.
Keep going with the same pet
Got your gallons? Check how many fish that really supports.
Your dog's age in human years, breed-size adjusted, not the ×7 myth.
Your cat's age in human years using the real feline curve, not the ×7 myth.
How much to feed: daily calories and cups by weight, activity and life stage.
Bioload-based stocking guidance, not the oversimplified 1-inch-per-gallon rule.