Square of 21
21 squared equals 441. Squares appear in area calculations, the Pythagorean theorem, and statistics (variance uses squared distances).
Squares obey quiet rules. Divided by 9, a square only ever leaves 0, 1, 4 or 7 — 0 here. On the last digit, squares end only in 0, 1, 4, 5, 6 or 9, and 1 fits. Between its neighbors, 400 sits 41 below and 484 sits 43 above — the square ladder widens by exactly two at every step. An odd base keeps the square odd: 441 ends in an odd digit, as every odd square must. Two anchors most worth memorizing in this range are 12² = 144 and 15² = 225 — the odd-step rule above is why knowing them pays off, because from any anchor the next square is one step along this page's ladder.
| Expression | Value |
|---|---|
| 21² | 441 |
Frequently asked questions
What is 21 squared?
21 squared is 441, because 21 × 21 = 441. The exponent 2 is why it is written 21².
What is 22 squared, next to 21²?
22² = 484, which is 43 more than 21² = 441. Each step up adds double the base plus one.
Where do squares show up in real problems?
Area of a square with side 21, distance in the Pythagorean theorem, and variance in statistics all use squared values.