125 Gallon Tank: Dimensions and Weight
A 125 gallon tank (standard) measures 72" × 18" × 21". It holds about 473.2 liters and weighs about 1399 lb full: 206 lb of glass, 1043 lb of water and 150 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.
Advertisement
What a 125 gallon tank actually measures
The standard 125 gallon footprint is 72 × 18 × 21 inches inside the glass. Run those through the rectangular formula and you get 117.82 gallons of geometric volume, which is not the same as the 125 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.
How much a 125 gallon tank weighs, empty and full
Water weighs 8.34 lb per US gallon, so 125 gallons is about 1043 lb before you add anything. A standard glass 125 gallon weighs about 206 lb empty, and a 2 inch bed of gravel or sand over its 72 × 18 inch floor adds about 150 lb (at 100 lb per cubic foot). All in, that is about 1399 lb resting on 9 square feet, or 155 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 | standard |
|---|---|
| Empty glass tank | 206 lb |
| Fresh water | 1043 lb |
| 2" gravel or sand | 150 lb |
| Total, freshwater | 1399 lb |
| Total, saltwater | 1425 lb |
| Load on the footprint | 155 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 125 gallons
Each row is the largest single-species group the stocking calculator still rates as comfortable (at or below 85 percent of capacity) in 125 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 60 |
| Guppy | up to 60 |
| Corydoras Catfish | up to 60 |
| Angelfish | up to 17 |
| Fancy Goldfish | up to 5 |
| Oscar | up to 3 |
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"
- 40 gallon tank36" × 18" × 16"
- 55 gallon tank48" × 13" × 21"
- 75 gallon tank48" × 18" × 21"
- 90 gallon tank48" × 18" × 24"
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 125 gallon tank?
The standard footprint is 72 × 18 × 21 inches.
How many liters is a 125 gallon tank?
About 473.2 liters (US gallons × 3.785).
How much does a 125 gallon fish tank weigh?
About 1399 lb set up: 206 lb for a standard empty glass tank, 1043 lb of water (125 gal × 8.34 lb/gal) and about 150 lb for a 2 inch gravel bed. Saltwater adds 26 lb. Rock, the stand and the lid come on top.
How much does an empty 125 gallon tank weigh?
About 206 lb for standard glass. Rimless low-iron glass and acrylic tanks differ: acrylic of the same size is roughly half the weight of glass.
Is a 125 gallon tank really 125 gallons?
Not exactly. Those inside dimensions work out to 117.82 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.