Number dictionary · Exact arithmetic
21: factors and number facts
twenty-one is an odd number and is composite. Its 4 positive divisors form 2 factor pairs.
Number JD Editorial Team · Updated 2026-10-08
- In words
- twenty-one
- Roman numeral
- XXI
- Prime factorization
- 3 × 7
- Digital root
- 3
All factors of 21
A positive factor divides 21 with no remainder. In ascending order, the complete list is:
1, 3, 7, 21
Factor pairs
- 1 × 21 = 21
- 3 × 7 = 21
Each pair describes an arrangement of 21 items into equal rows. The pairs have different row and column counts; 21 cannot make a square array of whole items.
Divisor totals
- Positive divisors
- 4
- Sum of all divisors
- 32
- Sum excluding 21
- 11
- Classification
- deficient
21 is deficient because its proper divisors add to 11, which is less than 21.
Prime factorization of 21
21 = 3 × 7
Repeated division, step by step
- 21 ÷ 3 = 7
- 7 ÷ 7 = 1
Multiply the prime divisors in these steps to reconstruct 21. Each prime in this factorization occurs once. The divisor-count calculation is (1 + 1) × (1 + 1) = 4.
The preceding prime is 19. The next larger prime is 23. The factor list above explains why 21 is composite.
Learn the factorization method21 in words, Roman numerals and other bases
| Representation | Value |
|---|---|
| English (US style) | twenty-one |
| Roman numeral | XXI |
| Binary (base 2) | 10101 |
| Octal (base 8) | 25 |
| Decimal (base 10) | 21 |
| Hexadecimal (base 16) | 15 |
Check the binary conversion
Read 10101 from right to left using place values 1, 2, 4, 8 and successive powers of two. Keep the values whose bit is 1:
16 + 4 + 1 = 21
Check the hexadecimal conversion
1 × 16^1 + 5 × 16^0 = 21
Hexadecimal uses A through F for digit values 10 through 15. These notations represent the same quantity; changing the base does not change the integer.
Square, cube and square root of 21
- 21 squared
- 21 × 21 = 441
- 21 cubed
- 21 × 21 × 21 = 9,261
- Square root
- ≈ 4.582576
- Simplified radical
- √21
21 lies between 16 (4²) and 25 (5²), so it is not a perfect square. It is not a perfect cube.
21 is triangular: it is the sum of the integers from 1 through 6.
Distinguish a square from a square rootMultiplication table for 21
These are the first twelve positive multiples. Unlike the finite factor list, the sequence of multiples continues without an upper limit.
Divisibility checks
| Divide by | Whole quotient | Remainder | Divides exactly? |
|---|---|---|---|
| 2 | 10 | 1 | No |
| 3 | 7 | 0 | Yes |
| 4 | 5 | 1 | No |
| 5 | 4 | 1 | No |
| 6 | 3 | 3 | No |
| 7 | 3 | 0 | Yes |
| 8 | 2 | 5 | No |
| 9 | 2 | 3 | No |
| 10 | 2 | 1 | No |
| 11 | 1 | 10 | No |
| 12 | 1 | 9 | No |
For example, 21 = 7 × 3 + 0. A remainder of zero is what makes a divisor a factor.
Digit sum and digital root of 21
21 contains 2 decimal digits. Its digit sum is 2 + 1 = 3.
- 21: 2 + 1 = 3
The digital root is 3. Reversing its digits gives 12; it is not a decimal palindrome. Digit patterns depend on the notation being used.
Digital-root arithmetic always finishes at a single digit for a positive integer. Some numerology methods stop at 11, 22 or 33 instead; that is a separate interpretive convention.
Fractions and a worked comparison
As a fraction of 100, 21/100 simplifies to 21/100 by dividing both terms by their greatest common divisor, 1. Its decimal value is 0.21.
The greatest common divisor of 21 and 12 is 3. Their least common multiple is 84. The identity GCD × LCM = 21 × 12 = 252 checks both results.
Work through GCF and LCMQuestions about 21
What are the factors of 21?
The positive factors are 1, 3, 7, 21. There are 4 factors, including 1 and 21.
Is 21 a prime number?
No. 21 is composite: 3 × 7 = 21.
How do you write 21 in Roman numerals?
21 is XXI in standard subtractive Roman numeral notation.
What is the binary representation of 21?
21 in decimal is 10101 in base 2. Adding its active place values gives 16 + 4 + 1 = 21.
Methods and reference definitions
Number JD calculates each result from the integer on this page. Positive factors are enumerated in pairs; repeated division gives the prime factorization; place values verify base conversions. Decimal square roots are rounded to six places when irrational.
NUMBER JOURNAL / JD
A reflective note on 21
Meaning varies by tradition and personal experience. One sighting is not evidence of what will happen next.
The digits of 21 add to 3. That arithmetic result is separate from the cultural symbolism of the pattern. If this number matters to you, write down what you noticed and choose one grounded action rather than treating the sighting as a prediction.