√21 ≈ 4.5826
√21 is an irrational number, so its decimals never end or repeat. It sits between 4 and 5 because 4² = 16 and 5² = 25.
| Decimal places | Value |
|---|---|
| 0 | 5 |
| 1 | 4.6 |
| 2 | 4.58 |
| 3 | 4.583 |
| 4 | 4.5826 |
| 5 | 4.58258 |
| 10 (digits) | 4.5825756950 |
21 has no perfect-square factor other than 1, so √21 is already in simplest radical form.
4.5826 × 4.5826 ≈ 21.0002, which is very close to 21.
Need another number? Use the square root calculator (also cube roots and other roots).
| Number | Square root |
|---|---|
| 17 | 4.1231 |
| 18 | 4.2426 |
| 19 | 4.3589 |
| 20 | 4.4721 |
| 22 | 4.6904 |
| 23 | 4.7958 |
| 24 | 4.8990 |
| 25 | 5 |