20
20 is the atomic number of calcium and the benchmark figure for normal visual acuity ("20/20 vision").
- Even
- Composite
Where 20 fits
Why 20 is special
20 pairs an essential biological element (calcium, the mineral behind bones and teeth) with a clinical vision standard almost everyone has heard of.
Same number, different meanings
How 20 is understood across different contexts — never implying one meaning because of another.
- Mathematics
20 pairs an essential biological element (calcium, the mineral behind bones and teeth) with a clinical vision standard almost everyone has heard of.
- Science
20 is the atomic number of calcium, the primary mineral in bones and teeth.
- Everyday life
20/20 is the standard clinical benchmark for normal visual acuity; separately, 'a score' is a traditional English unit meaning twenty, as in Lincoln's Gettysburg Address opening 'four score and seven years ago'.
Factors & digit properties
- Divisors (6)
- 1, 2, 4, 5, 10, 20
- Prime factorisation
- 2^2 × 5
- Digit sum
- 2
- Digital root
- 2
Representations
- Decimal
- 20
- Binary
- 10100
- Octal
- 24
- Hexadecimal
- 14
- Roman numeral
- XX
- Scientific notation
- 2.000 × 10^1
Other mathematical patterns
- Abundant number
- Harshad (Niven) number
Related numbers & connections
20 is a factor of 80 (20 × 4 = 80).
View 80 →Science
20 is the atomic number of calcium, the primary mineral in bones and teeth.
DocumentedScience
Everyday life
20/20 is the standard clinical benchmark for normal visual acuity; separately, 'a score' is a traditional English unit meaning twenty, as in Lincoln's Gettysburg Address opening 'four score and seven years ago'.
Widely documentedEveryday life
Common questions
Is 20 a prime number?
No, 20 is not prime.
What are the factors of 20?
The factors of 20 are 1, 2, 4, 5, 10, 20.
What is 20 in Roman numerals?
20 in Roman numerals is XX.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 2 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026