100 x 100 = 10000
100 times 100 is 10000. Both numbers are powers of ten, so the zeros simply add up, and the answer is the square of one hundred as well as the product of the pair.
100 × 100 = 10,000
Adding 100 to itself 100 times gives the same answer.
Why the answer carries four zeros
100 is 10 x 10, so 100 x 100 is 10 x 10 x 10 x 10. That is ten to the fourth, written 10000. The shortcut people use is to count zeros: two on the left, two on the right, four in the answer. It works here only because both numbers are a one followed by zeros. It fails the moment a different digit leads, which is why 100 x 15 needs a different move.
10000 is 100 squared
A number multiplied by itself is a square, so 10000 is 100 squared and the square root of 10000 is exactly 100. Read it as area and it is a square 100 units along each side. A spreadsheet 100 rows deep and 100 columns wide holds 10000 cells. A 100 by 100 pixel icon is 10000 pixels.
Things that are 10000 of something
A plot 100 metres by 100 metres covers 10000 square metres, an area with its own name: one hectare. In imperial units 10000 square feet is about 0.23 of an acre, since an acre is 43560 square feet. Finance runs the same pair in reverse. One percent is 100 basis points, so 100 percent is 10000 basis points.
Calculations this gets confused with
- 100 + 100 = 200. Addition, not multiplication.
- 100 x 10 = 1000, one zero fewer and a tenth of the size.
- 100 x 1000 = 100000, one zero more.
- 100 percent of 100 is 100, because taking all of a number leaves it alone.
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