Number reference
40 explained.
40: forty, composite; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- forty
- Classification
- composite
- Factors
- 1, 2, 4, 5, 8, 10, 20, 40
- Prime factorization
- 2 × 2 × 2 × 5
- Square root
- 6.324555
- Cube root
- 3.419952
- Binary
- 101000
- Octal
- 50
- Hexadecimal
- 28
- Roman numeral
- XL
- Digit sum
- 4
- Digital root
- 4
What makes 40 notable
- Factor structure. 40 breaks down into 2 distinct primes (2, 5) and has 8 divisors in total, so it can be split into equal whole-number groups 6 ways beyond 1 and itself.
40 in everyday use
Forty is the atomic number of zirconium and the standard working week in hours across much of the world. The word quarantine comes from the Venetian quaranta giorni, the forty days ships waited offshore during plague years. Minus 40 is the one temperature at which the Celsius and Fahrenheit scales agree. Forty appears repeatedly in scripture — forty days of flood, forty years in the desert, forty days in the wilderness — and Ali Baba faced forty thieves.
Forty is the only number whose English name has its letters in alphabetical order: f, o, r, t, y. It factors as 2³ × 5, has eight divisors, and is abundant, since its proper divisors sum to 50. It is an octagonal number, following 1, 8, and 21, and it can be written as the sum of two squares, 4 + 36.
Nearby numbers
How to read this page
The factor list shows every way 40 splits into equal whole-number groups — 8 divisors in total. The prime factorisation is the compact form of that same information, and the base conversions restate 40 for contexts that do not use decimal.
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