odd, composite, Fibonacci, triangular · 21st positive integer
The number 21
21 is an odd composite number. It's known as the legal drinking age in the United States and the perfect blackjack score. It is the 21st positive integer.
Quick reference
| Prime factorization | 3 × 7 |
| Divisors | 1, 3, 7, 21 |
| Binary | 10101 |
| Hexadecimal | 0x15 |
| Roman numeral | XXI |
| Digit sum | 3 |
| Properties | odd, composite, Fibonacci, triangular |
Why 21 is notable
The legal drinking age in the United States and the perfect blackjack score.
Reading 21 in NumGrid context
For 21, the underlying data on this page comes from classical number theory plus NumGrid solver-derived puzzle statistics. The breakdown above is the federal record; the paragraphs below add the per-entity context that makes the headline numbers usable for a real decision rather than just a data lookup.
The methodology behind every numeric value on this page is publicly documented on the classical number theory plus NumGrid solver-derived puzzle statistics portal and described in detail on this site’s methodology page. Refresh cadence varies by underlying series; the page surfaces the as-of date for each number so readers can trace any figure back to the source release.
Practical use of this page is in combination with the comparison and ranking pages elsewhere on the site, which surface the same data for 21’s peers within positive integers within the NumGrid target pool. A single-entity reading without peer context can be misleading when an entity is an outlier on one axis but typical on another.
Frequently asked
Is 21 a prime number?
No. 21 is composite. Its prime factorization is 3 × 7.
What are the factors of 21?
The factors (divisors) of 21 are 1, 3, 7, 21.
What is 21 in binary?
21 in binary is 10101. In hexadecimal it's 15.
What is 21 as a Roman numeral?
21 in Roman numerals is XXI.
What's the digit sum of 21?
Adding the digits of 21: 2 + 1 = 3.