When it was time to fertilize my yard recently, I saw that all the fertilizer bags were marked for 5000 square feet or 10,000, etc. Yes, the stinking yards here are small. I also had no idea what the conversion from acres to square feet is. That format shows the acres to square feet conversion factors. So I knew I'd need the 10,000 sq ft bag of fertilizer.
Units is a fun program, with an incredible number of conversions.
Read the man page for lots of great examples. You can edit it and add anything that somehow isn't already in there. If I'd had units about 20 years ago, I could have told Mr. Ford, my high school chemistry teacher that one furlong per fortnight is exactly 0.00037202455 miles per hour. If you're a developer, and you've ever had to do your own memory management, you've probably had some type of memory access problem as well. You can probably see the problem here in a second.