15
15 is a triangular number, the atomic number of phosphorus, and the size of a rugby union team.
- Odd
- Composite
Where 15 fits
Why 15 is special
15's mathematical identity (triangular, the sum of 1 through 5) sits alongside a genuine sporting distinction: it's exactly two more players than a rugby league team.
Same number, different meanings
How 15 is understood across different contexts — never implying one meaning because of another.
- Mathematics
15's mathematical identity (triangular, the sum of 1 through 5) sits alongside a genuine sporting distinction: it's exactly two more players than a rugby league team.
- Culture
The quinceanera marks a girl's fifteenth birthday across much of Latin America and in Latino communities elsewhere, functioning as a formal coming-of-age celebration rather than simply a birthday party.
- Science
The temperature at the centre of the Sun is about 15 million kelvin (the Solar System). Against roughly 5,800 kelvin at the visible surface. The core is where fusion happens; the surface is merely what the heat has to escape through.
- Sport
A rugby union team fields 15 players, compared to 13 in rugby league — one of the clearest numeric distinctions between the two codes of rugby.
15 is part of
Factors & digit properties
- Divisors (4)
- 1, 3, 5, 15
- Prime factorisation
- 3 × 5
- Digit sum
- 6
- Digital root
- 6
Representations
- Decimal
- 15
- Scientific notation
- 1.500 × 10^1
Roman numerals
X V
Binary
1 1 1 1
1 × 8 + 1 × 4 + 1 × 2 + 1 × 1 = 15
Octal
1 7
1 × 8 + 7 × 1 = 15
Hexadecimal
f
f × 1 = 15
Read as powers of two: 8 + 4 + 2 + 1 = 15.
In historical numeral systems
Egyptian
1 hobble, 5 strokes
1 × 10 + 5 × 1 = 15. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
1 corner wedge + 5 vertical wedges
15 fits in a single sexagesimal place: 1 corner wedge + 5 vertical wedges.
Greek (Ionic)
iota, then epsilon, marked as a numeral
iota-epsilon + keraia
iota + epsilon written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
tet, then vav
tet-vav
The ordinary descending rule would give יה (yod 10 + he 5), which spells a form of the divine name, so 15 is written טו — tet (9) + vav (6) — instead.
Chinese numerals
shí wǔ
shí wǔ
十 is ten. Adding 五 after it gives 15 — "ten wǔ". Writing 一十 is not wrong, but nobody writes eleven that way on its own. The financial (anti-fraud) form is 壹拾伍.
Maya
3 bars
15 fits in the bottom place: 3 bars, since a bar is five and a dot is one.
Systems that simply cannot reach this value are left out. Where a script needs a font most readers do not have, the reading is given instead of the glyphs — see number systems for each system in full.
Other mathematical patterns
- Triangular number
- Deficient number
- Hexagonal number
- Semiprime
- Lucky number (mathematical sieve)
Related numbers & connections
15 is a factor of 60 (15 × 4 = 60).
View 60 →A rugby union team fields 15 players per side, against 13 in rugby league.
Mathematically related
Numbers Around Us
- appears inStandard team size in a field sport
why?
Strength 0.50, confidence "high".
- appears inSpace and astronomy
why?
Strength 0.50, confidence "high".
Culture
The quinceanera marks a girl's fifteenth birthday across much of Latin America and in Latino communities elsewhere, functioning as a formal coming-of-age celebration rather than simply a birthday party.
DocumentedCultureCulture: Latin AmericanGeography: Latin America and Latino communities elsewhere
Science
The temperature at the centre of the Sun is about 15 million kelvin (the Solar System). Against roughly 5,800 kelvin at the visible surface. The core is where fusion happens; the surface is merely what the heat has to escape through.
DocumentedScienceGeography: the Solar System15 is the atomic number of phosphorus, essential to DNA and to ATP, the molecule cells use to store and transfer energy.
DocumentedScience
Sport
A rugby union team fields 15 players, compared to 13 in rugby league — one of the clearest numeric distinctions between the two codes of rugby.
DocumentedSport
Mathematics
In the 3x3 magic square — the arrangement of 1 to 9 in which every row, column and diagonal sums to the same total — that total is always 15. There is only one such square up to rotation and reflection, and it appears in Chinese tradition as the Lo Shu diagram.
DocumentedMathematics
Common questions
Why do all the rows of a 3×3 magic square add to 15?
Because they have to. The numbers 1 to 9 total 45, and three rows share that total equally, so each must be 15. That also fixes 5 in the centre — it is the only number that can sit in all four lines through the middle and keep them balanced. There is exactly one such square, up to rotation and reflection.
What is a quinceañera?
A girl's fifteenth-birthday celebration across much of Latin America and in Latino communities elsewhere, marking a formal transition to adulthood rather than simply a birthday. The form varies by country and family — a Mass, a father-daughter dance and a change of shoes are common elements, none universal.
Is 15 a prime number?
No, 15 is not prime.
What are the factors of 15?
The factors of 15 are 1, 3, 5, 15.
What is 15 in Roman numerals?
15 in Roman numerals is XV.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 5 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026