29
29 is a prime number, the atomic number of copper, and forms a twin-prime pair with 31.
- Odd
- Prime
Where 29 fits
Why 29 is special
29's mathematics (prime, twin with 31) coincides with its everyday claim to fame: it's the extra day added to February in a leap year.
Same number, different meanings
How 29 is understood across different contexts — never implying one meaning because of another.
- Mathematics
29's mathematics (prime, twin with 31) coincides with its everyday claim to fame: it's the extra day added to February in a leap year.
- History
A lunar month — one full cycle of Moon phases — runs about 29.53 days, which no whole number matches. Calendars that track the Moon directly resolve this by alternating month lengths: in the Hijri calendar months run 29 or 30 days, and 29 is the short one. That alternation, not a fixed rule, is what keeps the months in step with the phases.
- Science
Copper and gold are the two familiar metals that are not grey or silver, and the reason is the same in both: electrons in the filled d-band absorb light from the blue end of the spectrum when they jump to the conduction band, so the reflected light is weighted towards red. In copper the absorption edge sits slightly further into the visible range than in gold, which is why one looks reddish and the other yellow.
- Everyday life
February has 29 days only in a leap year, which occurs roughly once every four years to keep the calendar aligned with Earth's orbital period.
Not just a mathematical number
29is also used in ways where ordinary arithmetic doesn't apply — each listed below with what genuinely does and doesn't follow from it.
- Mathematical number
A genuine quantity — its mathematical properties (prime, square, factors, and so on) are real facts about it.
A genuine quantity that ordinary arithmetic applies to — it can be prime, even, a perfect square, added to another number meaningfully, and so on.
- Measurement
Also used as a measurement — Calendar years.
A quantity expressed against a defined unit or standard (24 hours in a day, 60 seconds in a minute) — arithmetic is meaningful, but the specific value is a fixed convention of the measuring system, not a discovered mathematical property.
Factors & digit properties
- Divisors (2)
- 1, 29
- Prime factorisation
- 29
- Digit sum
- 11
- Digital root
- 2
Representations
- Decimal
- 29
- Scientific notation
- 2.900 × 10^1
Roman numerals
X X I X
XXIX uses the subtractive pair IX: one before ten — ten minus one.
Binary
1 1 1 0 1
1 × 16 + 1 × 8 + 1 × 4 + 0 × 2 + 1 × 1 = 29
Octal
3 5
3 × 8 + 5 × 1 = 29
Hexadecimal
1 d
1 × 16 + d × 1 = 29
Read as powers of two: 16 + 8 + 4 + 1 = 29.
In historical numeral systems
Egyptian
2 hobbles, 9 strokes
2 × 10 + 9 × 1 = 29. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
2 corner wedges + 9 vertical wedges
29 fits in a single sexagesimal place: 2 corner wedges + 9 vertical wedges.
Greek (Ionic)
kappa, then theta, marked as a numeral
kappa-theta + keraia
kappa + theta written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
kaf, then tet
kaf-tet
kaf (20) + tet (9) = 29, written in descending order.
Chinese numerals
èr shí jiǔ
èr shí jiǔ
二 before 十 multiplies it: 2 tens. 九 after it adds 9. The written form mirrors the arithmetic — "èr shí jiǔ". The financial (anti-fraud) form is 貳拾玖.
Maya
1 dot (upper, twenties place) over 1 bar + 4 dots (lower, units place)
1 × 20 + 9 = 29. Places stack vertically with the smallest at the bottom, so the upper group is worth twenty times the lower.
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
- Deficient number
- Lucas number
- Sophie Germain prime
- Twin prime
Related numbers & connections
29 is a factor of 87 (29 × 3 = 87).
View 87 →29 and 31 are twin primes — two primes exactly two apart.
View 31 →February has 29 days only in a leap year — the correction, applied roughly every fourth year, that keeps the calendar aligned with Earth's orbital period.
History
A lunar month — one full cycle of Moon phases — runs about 29.53 days, which no whole number matches. Calendars that track the Moon directly resolve this by alternating month lengths: in the Hijri calendar months run 29 or 30 days, and 29 is the short one. That alternation, not a fixed rule, is what keeps the months in step with the phases.
DocumentedHistoryCulture: Islamic (Hijri) calendar
Science
Copper and gold are the two familiar metals that are not grey or silver, and the reason is the same in both: electrons in the filled d-band absorb light from the blue end of the spectrum when they jump to the conduction band, so the reflected light is weighted towards red. In copper the absorption edge sits slightly further into the visible range than in gold, which is why one looks reddish and the other yellow.
DocumentedScience29 is the atomic number of copper. It conducts electricity better than any metal except silver and costs far less, which is why it carries most of the world's wiring; alloyed with tin it gives bronze, with zinc brass.
DocumentedScience
Everyday life
February has 29 days only in a leap year, which occurs roughly once every four years to keep the calendar aligned with Earth's orbital period.
DocumentedEveryday life
Mathematics
29 = 2² + 3² + 4² — the smallest prime that is the sum of three consecutive squares. It is also prime and forms a twin pair with 31, which is why the leap-day date and the following month's length sit on two primes two apart.
DocumentedMathematics
Common questions
Why does February get a 29th day every four years?
Because a year is about 365.2422 days, not 365, and the leftover quarter-day accumulates. Adding a day every four years overcorrects slightly, so the Gregorian rule skips the leap year in century years not divisible by 400 — 1900 was not a leap year, 2000 was. That refinement is the whole difference between the Gregorian and Julian calendars.
Is 29 a prime number?
Yes, 29 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 29?
The factors of 29 are 1, 29.
What is 29 in Roman numerals?
29 in Roman numerals is XXIX. IX is subtractive: one before ten — ten minus one. Written additively it would be XXVIIII, which Roman inscriptions used freely — the strict subtractive form is a later convention.
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