The USGS Water Science School

How much is it?

If you start browsing through our water-use topics you'll see that water-use data are usually expressed in million of gallons of water used per day (Mgal/d). You'll understand the data better if you can visualize how much a million gallons is. A good-sized bath holds 50 gallons, so a million gallons would be 20,000 baths. But, it might be easier to take a look at the pool and cube below:

If you were a swimming-pool builder and a customer asked you to build a pool that would hold a million-gallons, then they had better have a big yard! You would need to build a pool about 267 feet long (almost as long as a football field), 50 feet wide, and 10 feet deep. The cube on the right side shows you that 1 million gallons would form a cube that is 51.1 feet on each side.

Some conversions:

- 1 Mgal/d = 1.547 cubic feet per second
- 1 Mgal/d = 694.4 gallons per minute
- 1 Mgal/d = 1.121 thousand acre-feet per year
- 1 million gallons = 3.07 acre feet

How about a little "pop-up quiz?" How many pounds do you think a million gallons of water weighs?