Water and wastewater · Worked step by step
How many gallons are in a 20 by 10 by 8 foot tank?
If math has never been your thing, you are in the right place. Try it on paper first, then we write it out with you, one small step at a time, and explain every step in plain words so you never lose the thread.
The problem
A tank is 20 feet long, 10 feet wide, and holds water 8 feet deep. How many gallons does it hold?
Grab a pen and paper first
Give it a real try on paper before you watch. If you get stuck, take a hint. If you go blank, that is fine, press play and we will walk it through together.
Watch it solved, with you
Every step is written out slowly and explained in plain words, so you always know what just happened. Nobody skips ahead and leaves you behind.
Press play and I’ll write it out with you, one small step at a time.
Every step gets a plain-words explanation here, so you always know exactly what just happened and why.
The written version, step by step
- 1
Write down the three sizes
length = 20 ft • width = 10 ft • depth = 8 ft
- Write the three measurements first so none of them slip away.
- Length is 20, width is 10, and the water is 8 feet deep.
- They are all already in feet, so we don't have to convert anything. Good.
Quick trick: If any size were in inches, divide it by 12 first to make it feet.
- 2
Find the cubic feet (the space inside)
cubic feet = 20 × 10 × 8 • cubic feet = 1,600
- The space inside a box is length × width × depth.
- Do two at a time: 20 × 10 = 200. That is the floor.
- Then 200 × 8 (the depth) = 1,600 cubic feet of space.
Quick trick: Multiplying three numbers? Do two, then the third. Less to juggle in your head.
- 3
Turn cubic feet into gallons
gallons = 1,600 × 7.48 • gallons = 11,968
- Cubic feet is not gallons yet. One cubic foot of water is 7.48 gallons.
- So we multiply our 1,600 cubic feet by 7.48.
- 1,600 × 7.48 = 11,968 gallons. Done.
Quick trick: Quick check: 1,600 × 7.48 is a little under 1,600 × 8 = 12,800. So 11,968 makes sense.
Remember this for good
For a box-shaped tank: length × width × depth gives cubic feet. Then × 7.48 turns cubic feet into gallons.
Imagine filling the tank with 1-foot sugar cubes. Length × width × depth counts the cubes (cubic feet). Each cube holds 7.48 gallons, so multiply by 7.48 to get gallons.
Solve it fast next time
- It is always two steps: first cubic feet, then gallons.
- The × 7.48 is the LAST step for gallons. Never skip it, never do it twice.
- Use the water depth, not the tank's full height, unless it says the tank is full.
The answer: 11,968 gallons.