The number 13

Established fact In maths and on the calendar

• 13 is prime: its only divisors are 1 and 13.
• 13 is a Fibonacci number, part of the sequence where each number is the sum of the two before it.
• Its digits add up to 4.
• 13 is the long side of the 5-12-13 right triangle (5² + 12² = 13²).
• Every month has a 13th, so it comes round twelve times a year.