21
21 is the legal minimum drinking age in the United States, and the target hand total in blackjack.
Where 21 fits
Why 21 is special
21 is a rare case of a number whose primary public meaning is a specific legal threshold rather than a mathematical property — and a jurisdiction-specific one at that, not a universal age of majority.
Same number, different meanings
How 21 is understood across different contexts — never implying one meaning because of another.
- Mathematics
21 is a rare case of a number whose primary public meaning is a specific legal threshold rather than a mathematical property — and a jurisdiction-specific one at that, not a universal age of majority.
- Law & regulation
The US National Minimum Drinking Age Act of 1984 requires states to set 21 as the minimum legal drinking age to receive full federal highway funding — a US-specific legal threshold, not a global standard.
- Media & popular culture
21 is the target hand total in blackjack (also called 'twenty-one'), one of the most widely played casino card games — reaching exactly 21 with two cards is a 'blackjack', the strongest starting hand.
21 in Numbers Around Us
21 is part of
Factors & digit properties
- Divisors (4)
- 1, 3, 7, 21
- Prime factorisation
- 3 × 7
- Digit sum
- 3
- Digital root
- 3
Representations
- Decimal
- 21
- Binary
- 10101
- Octal
- 25
- Hexadecimal
- 15
- Roman numeral
- XXI
- Scientific notation
- 2.100 × 10^1
Other mathematical patterns
- Triangular number
- Deficient number
- Semiprime
- Harshad (Niven) number
- Lucky number (mathematical sieve)
Related numbers & connections
Some daisy and aster species commonly show 21 petals, a documented phyllotaxis pattern.
Numbers Around Us
- appears inPhyllotaxis petal patterns
why?
Strength 0.50, confidence "high".
Law & regulation
The US National Minimum Drinking Age Act of 1984 requires states to set 21 as the minimum legal drinking age to receive full federal highway funding — a US-specific legal threshold, not a global standard.
DocumentedLaw & regulationGeography: United StatesPeriod: 1984 – present
Media & popular culture
21 is the target hand total in blackjack (also called 'twenty-one'), one of the most widely played casino card games — reaching exactly 21 with two cards is a 'blackjack', the strongest starting hand.
DocumentedMedia & popular culture
Common questions
Is 21 a prime number?
No, 21 is not prime.
What are the factors of 21?
The factors of 21 are 1, 3, 7, 21.
What is 21 in Roman numerals?
21 in Roman numerals is XXI.
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