odd, composite, perfect square · 81st positive integer
The number 81
81 is an odd composite number. It is the 81st positive integer.
Quick reference
| Prime factorization | 3 × 3 × 3 × 3 |
| Divisors | 1, 3, 9, 27, 81 |
| Binary | 1010001 |
| Hexadecimal | 0x51 |
| Roman numeral | LXXXI |
| Digit sum | 9 |
| Properties | odd, composite, perfect square |
Reading 81 in NumGrid context
For 81, 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 81’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 81 a prime number?
No. 81 is composite. Its prime factorization is 3 × 3 × 3 × 3.
What are the factors of 81?
The factors (divisors) of 81 are 1, 3, 9, 27, 81.
What is 81 in binary?
81 in binary is 1010001. In hexadecimal it's 51.
What is 81 as a Roman numeral?
81 in Roman numerals is LXXXI.
What's the digit sum of 81?
Adding the digits of 81: 8 + 1 = 9.