How this calculator works
Multiply length × width × height in inches to get cubic inches; one US gallon is 231 cubic inches. The water level percentage accounts for the gap at the top. Fresh water weighs about 8.34 lb per gallon.
Gallons = L × W × H (in) ÷ 231 × Fill%
Liters = Gallons × 3.785
Weight = Gallons × 8.34 lb
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Length | 48 in |
| Width (front to back) | 13 in |
| Height | 21 in |
| Water level | 90 % of height |
| Result | Value |
|---|---|
| Water volume | 51.1 gallons |
| Water volume | 193.3 liters |
| Nominal tank size (brim full) | 56.7 gallons |
| Water weight | 426 lb |
Step by step:
- Cubic inches = 48 × 13 × 21 = 13,104
- Gallons = ÷ 231 = 56.73; at 90% = 51.05
- Water weight = × 8.34 = 426 lb
Standard aquarium sizes
Outside dimensions, 90% water level.
| Tank | Water volume | Water weight |
|---|---|---|
| 10 gal (20 × 10 × 12 in) | 9.4 gallons | 78 lb |
| 20 long (30 × 12 × 12 in) | 16.8 gallons | 140 lb |
| 29 gal (30 × 12 × 18 in) | 25.2 gallons | 211 lb |
| 40 breeder (36 × 18 × 16 in) | 40.4 gallons | 337 lb |
| 75 gal (48 × 18 × 21 in) | 70.7 gallons | 590 lb |
Common mistakes to avoid
- Forgetting that substrate and rock reduce real water volume — dose medication on actual water.
Frequently asked questions
How many gallons is a 48 × 13 × 21 tank?
About 56.7 gallons brim-full, sold as a 55-gallon tank; about 51 gallons at a 90% water level.