**Thirteen** (13) is the natural number following twelve and preceding fourteen. It is a Fibonacci number.

Thirteen is the 6th smallest prime number, the next is seventeen.

13 goes into 999,999 exactly 76,923 times, so fractions with 13 in the denominator have six digit repeating sequences in their decimal expansions.

In base fourteen and higher bases (such as hexadecimal), thirteen is represented as D.

**Thirteen** is also:

- The atomic number of aluminum.
- ASCII and Unicode code point for carriage return.
- Regarded as an unlucky number in many cultures. Unreasoned fear of the number 13 is termed triskaidekaphobia. Due to this fear, some tall buildings have resorted to skipping the thirteenth floor or having a 12a-th floor instead. The thirteenth of a month is likewise ominous, at least when it falls on a Friday. The 13th falls on a Friday more often than any other day. In fact it occurs more often than any other day/number combination except those that coincide with it (Thursday the 12th, Saturday the 14th, etc).
- In the Christian religion, the number is considered significant since there were thirteen present at the Last Supper. Also, if both Judas and Matthias are counted, there were thirteen apostles. Thirteen was also once associated with the Epiphany by Christians, the Christ child having received the Magi on his thirteenth day of life.
- Regarded as a lucky number in China because it is synonymous with 實生 which means "must be alive". However 13 is regarded as unlucky by those Chinese under the influence of foreign cultures.
- The age of maturity in Jewish tradition. Boys get their bar mitzvah on the Shabbat after their thirteenth birthdays.
- The number of original colonies the United States was founded from. The original flag had thirteen stars, one for each state. New stars have since been added whenever a new state joins the union, but the idea of adding stripes for new states was soon dropped, so the American flag to this day has thirteen horizontal stripes, six white ones and seven red ones.
- The number of guns in a gun salute to U.S. Army, Air Force and Marine Corps Major Generals, and Navy and Coast Guard Rear Admirals Upper Half.
- The number of loaves in a baker's dozen.
- In bingo it is called Unlucky for some.
- The number of players in a rugby league team.

The "13" on a Hell's Angel's patch or tattoo refers to the thirteenth letter of the alphabet, M, for "marijuana".

There are 13 steps on the pyramid on the back of the American 1 Dollar Bill

Some theories of relativity state that there are 13 dimensions.

There were 13 Plutonium slugs in Fat Man, the atomic bomb dropped on Nagasaki.

This article is about the number. For the year AD 13, see 13.