First,
select an area that is covered with a paved layer like
a parking lot. Any rain that falls on this surface
will not soak in, it will be water runoff.

Next,
multiply the length by the width to get the area. To
get the volume, multiply the area by the feet of rain.

To get
the weight, multiply the volume by 62.5 pounds and then
convert to tons.

For example,
an area that is 50 feet by 100 feet equals 5000 square
feet. Multiplying 5000 square feet by 4.2 feet of
rain (a hypothetical yearly total) gives you a volume of
21,000 cubic feet of water. Then, multiply the 21,000
cubic feet by 62.5 pounds (the weight of one cubic foot
of water) to get a weight of 1,312,500 pounds. Finally,
divide that weight by 2000 pounds to get 656.25 tons of
water. So, on that rather tiny 50 x 100 parking lot,
over 656 TONS of rain falls in a year! That’s
more than the weight of 100 elephants!