Quick answer: gallons = length × width × depth (inches) ÷ 231, or in feet, l × w × depth × 7.4805. A rectangular tank is the one shape where half the depth is exactly half the volume — the cross-section never changes, so fill level and volume rise together.
Why rectangular tanks are the simple case
Every other tank shape complicates partial fills. A cylinder lying on its side is narrow at the bottom, wide in the middle and narrow at the top, so a dipstick reading of 25% depth is not 25% of the volume. A rectangular tank has none of that — its cross-section is the same rectangle from floor to rim, so the volume is simply length × width × whatever depth of liquid is in it.
That makes a rectangular tank the easiest to gauge by eye or dipstick: read the depth, and the fraction of full is exactly the fraction of the height. If you have a round or oval tank instead, the main tank volume calculator handles the circular-segment maths those shapes need.
Common rectangular tank sizes
| Tank | Inside dimensions | US gallons | Litres |
|---|---|---|---|
| 10-gallon aquarium | 20 × 10 × 12 in | 10.4 | 39 |
| 55-gallon aquarium | 48 × 13 × 21 in | 56.7 | 215 |
| IBC tote | 45 × 37 × 40 in | 275 | 1,040 |
| Stock / water trough | 96 × 24 × 24 in | 239 | 905 |
| Small cistern | 60 × 36 × 36 in | 337 | 1,275 |
| Sump / catch basin | 24 × 18 × 30 in | 56.1 | 212 |
| Septic tank (1,000 gal) | 90 × 51 × 51 in | 1,013 | 3,835 |
Worked example
A rectangular water tank 48 in long, 24 in wide, 30 in tall inside, filled to 24 in:
- Volume of liquid: 48 × 24 × 24 = 27,648 in³
- Gallons: 27,648 ÷ 231 = 119.7 US gallons
- Full capacity: 48 × 24 × 30 = 34,560 in³ ÷ 231 = 149.6 gallons
- Percent full: 24 ÷ 30 = 80% — exactly the depth fraction
- Water weight at this level: 119.7 × 8.34 ≈ 998 lb
Common mistakes & pro tips
- Outside dimensions. Capacity is the inside cavity; subtract the wall thickness.
- Dividing by 128 instead of 231. 128 is fluid ounces per gallon; 231 is cubic inches per gallon.
- Forgetting freeboard. Tanks are rarely filled to the very brim — leave headroom for splashing and expansion.
- Ignoring the outlet height. Usable volume is between the outlet and the overflow, not floor to rim.
- Pro tip — mark a dipstick in gallons. Because depth and volume are proportional here, you can mark a stick linearly: full at the top, half at the middle, and so on.
Frequently asked questions
How do I calculate the volume of a rectangular tank in gallons?
Multiply length × width × height in inches, then divide by 231 for US gallons. A tank 48 × 24 × 24 inches holds 27,648 in³ ÷ 231 ≈ 119.7 gallons. In feet, multiply length × width × height and then by 7.4805.
How do I find the volume at a partial fill level?
For a rectangular tank it is simple: the volume is directly proportional to depth, because the cross-section never changes. A tank filled to half its height holds exactly half its capacity, unlike a cylindrical tank on its side. Just use the liquid depth in place of the full height.
How many gallons is an IBC tote?
A standard IBC tote holds 275 US gallons, roughly 1,040 litres or one cubic metre of usable capacity. Its internal dimensions are about 45 × 37 × 40 inches. A larger 330-gallon version is also common.
Should I use inside or outside tank dimensions?
Inside dimensions, because capacity is set by the internal cavity. Wall thickness is subtracted from every dimension, and on a thick-walled poly or fibreglass tank the difference between inside and outside volume is significant.
How much does a full rectangular water tank weigh?
About 8.34 pounds per gallon of water, plus the tank itself. A 120-gallon tank holds roughly 1,000 pounds of water — half a ton — which is why the stand or base under any sizeable tank needs to be rated for the full load.