Operator math, worked
How to calculate the volume of a rectangular tank in gallons
A rectangular or square tank holds water in a box shape. You find how many gallons it holds by getting the cubic feet first, then turning cubic feet into gallons. This is one of the most common operator-math problems.
Last reviewed 2026-08-04
The formula
gallons = length x width x depth x 7.48
The steps
Get the cubic feet
Multiply length by width by water depth, all in feet. That gives the volume in cubic feet.
Turn cubic feet into gallons
Multiply the cubic feet by 7.48, because one cubic foot of water holds 7.48 gallons.
Worked example
A tank is 20 feet long, 10 feet wide, and 8 feet of water deep. How many gallons?
- Cubic feet: 20 x 10 x 8 = 1,600 cubic feet
- Gallons: 1,600 x 7.48 = 11,968 gallons
Answer: 11,968 gallons
Common questions
- Why multiply by 7.48?
- One cubic foot of water is 7.48 gallons, so it converts cubic feet to gallons.
- What depth do I use?
- The water depth, not the tank height, unless the tank is full.
- What units go in?
- Feet for length, width, and depth. The answer comes out in gallons.