21 (number)

This article is about the number 21. For the year, see 21.

21 (twenty-one) is a number. It comes between twenty and twenty-two, and is an odd number. It is divisible by 1, 3, 7, and 21.

← 20 21 22 →
← 20 21 22 23 24 25 26 27 28 29 → List of numbers — Integers ← 0 10 20 30 40 50 60 70 80 90 →
Cardinal	twenty-one
Ordinal	21st (twenty-first)
Factorization	3 × 7
Divisors	1, 3, 7, 21
Greek numeral	ΚΑ´
Roman numeral	XXI
Binary	101012
Ternary	2103
Quaternary	1114
Quinary	415
Senary	336
Octal	258
Duodecimal	1912
Hexadecimal	1516
Vigesimal	1120
Base 36	L36

Twenty-one is also a name of a popular card game and is the age at which people in the United States can buy alcoholic drinks.

In Roman numerals, 21 is written as XXI.

In Greek numerals 21 is written as κα