Number reference
65 explained.
65: sixty-five, composite; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- sixty-five
- Classification
- composite
- Factors
- 1, 5, 13, 65
- Prime factorization
- 5 × 13
- Square root
- 8.062258
- Cube root
- 4.020726
- Binary
- 1000001
- Octal
- 101
- Hexadecimal
- 41
- Roman numeral
- LXV
- Digit sum
- 11
- Digital root
- 2
What makes 65 notable
- Semiprime. 65 = 5 × 13, a product of exactly two primes. That gives it just four divisors, so it splits into equal groups in only two non-trivial ways.
65 in everyday use
Sixty-five has been the standard retirement age in many countries since Bismarck's Germany and the 1935 US Social Security Act, though most are now raising it. It is the atomic number of terbium, a rare earth used in green phosphors and solid-state devices. On many American highways 65 miles per hour is the rural speed limit. A 65th anniversary is blue sapphire.
Sixty-five is 5 × 13, and it is the second number after 50 that can be written as the sum of two squares in two ways: 1 + 64 and 16 + 49. It is also the magic constant of the 5 × 5 magic square, the sum of every row when the numbers 1 to 25 are arranged correctly. Sixty-five is the hypotenuse of several right triangles with whole-number sides, including 16-63-65 and 33-56-65.
Nearby numbers
How to read this page
The factor list shows every way 65 splits into equal whole-number groups — 4 divisors in total. The prime factorisation is the compact form of that same information, and the base conversions restate 65 for contexts that do not use decimal.
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