78
78 is a triangular number (the sum of 1 through 12) and the atomic number of platinum.
- Even
- Composite
Where 78 fits
Why 78 is special
78's triangular-number identity ties it neatly to the twelve months of the calendar year, purely as a mathematical curiosity.
78 connects to:
- 39 — has factor
- vinyl record speeds — appears in
Same number, different meanings
How 78 is understood across different contexts — never implying one meaning because of another.
- Mathematics
78's triangular-number identity ties it neatly to the twelve months of the calendar year, purely as a mathematical curiosity.
- Everyday life
Most of the world's platinum ends up in exhaust pipes. Element 78 catalyses the conversion of carbon monoxide and unburnt hydrocarbons into carbon dioxide and water without being consumed, which is what a catalytic converter does, and it tolerates the heat and the sulfur that would poison cheaper catalysts. That single application is why converters are worth stealing, and why platinum demand tracks vehicle emissions regulation rather than jewellery fashion.
- Media & popular culture
Records described as 78 rpm ran at speeds varying with the motor and the region, converging near 78 only after electrical recording. The figure derives from a 3600 rpm motor with a 46-tooth gear.
- Science
78 is the atomic number of platinum. It resists corrosion almost completely, which suits it to jewellery and laboratory ware, and it is the principal catalyst in most vehicle catalytic converters.
78 is part of
Factors & digit properties
- Divisors (8)
- 1, 2, 3, 6, 13, 26, 39, 78
- Prime factorisation
- 2 × 3 × 13
- Digit sum
- 15
- Digital root
- 6
Representations
- Decimal
- 78
- Scientific notation
- 7.800 × 10^1
Roman numerals
L X X V I I I
Binary
1 0 0 1 1 1 0
1 × 64 + 0 × 32 + 0 × 16 + 1 × 8 + 1 × 4 + 1 × 2 + 0 × 1 = 78
Octal
1 1 6
1 × 64 + 1 × 8 + 6 × 1 = 78
Hexadecimal
4 e
4 × 16 + e × 1 = 78
Read as powers of two: 64 + 8 + 4 + 2 = 78.
In historical numeral systems
Egyptian
7 hobbles, 8 strokes
7 × 10 + 8 × 1 = 78. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
1 vertical wedge (sixties place), 1 corner wedge + 8 vertical wedges (units place)
1 × 60 + 18 = 78. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
omicron, then eta, marked as a numeral
omicron-eta + keraia
omicron + eta written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
ayin, then het
ayin-het
ayin (70) + het (8) = 78, written in descending order.
Chinese numerals
qī shí bā
qī shí bā
七 before 十 multiplies it: 7 tens. 八 after it adds 8. The written form mirrors the arithmetic — "qī shí bā". The financial (anti-fraud) form is 柒拾捌.
Maya
3 dots (upper, twenties place) over 3 bars + 3 dots (lower, units place)
3 × 20 + 18 = 78. 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
- Triangular number
- Abundant number
Related numbers & connections
39 is a factor of 78 (39 × 2 = 78).
View 39 →Discs described as 78 rpm ran at speeds that only converged near that figure after electrical recording.
Mathematically related
- has factor39
why?
Strength 0.50, confidence "high".
Numbers Around Us
- appears inVinyl record playback speeds
why?
Strength 0.50, confidence "high".
Everyday life
Most of the world's platinum ends up in exhaust pipes. Element 78 catalyses the conversion of carbon monoxide and unburnt hydrocarbons into carbon dioxide and water without being consumed, which is what a catalytic converter does, and it tolerates the heat and the sulfur that would poison cheaper catalysts. That single application is why converters are worth stealing, and why platinum demand tracks vehicle emissions regulation rather than jewellery fashion.
DocumentedEveryday lifePeriod: 1975 – present
Media & popular culture
Records described as 78 rpm ran at speeds varying with the motor and the region, converging near 78 only after electrical recording. The figure derives from a 3600 rpm motor with a 46-tooth gear.
DocumentedMedia & popular cultureGeography: InternationalPeriod: c.1900–1950sDiscs described as 78 rpm ran at speeds varying by manufacturer and region until electrical recording standardised them near 78. The figure derives from a 3,600 rpm motor geared down by a 46-tooth wheel.
DocumentedMedia & popular cultureGeography: InternationalPeriod: c.1900–1950s
Science
78 is the atomic number of platinum. It resists corrosion almost completely, which suits it to jewellery and laboratory ware, and it is the principal catalyst in most vehicle catalytic converters.
DocumentedScience
Mathematics
78 is a triangular number (the sum of 1 through 12) and factors as 2 × 3 × 13.
DocumentedMathematics
Common questions
How many cards are in a tarot deck?
78 in the standard deck: 22 Major Arcana and 56 Minor Arcana across four suits. The deck began as a fifteenth-century Italian card game, and the divinatory use is a later, eighteenth-century French development — the cards are older than the practice they are now best known for.
Is 78 a prime number?
No, 78 is not prime.
What are the factors of 78?
The factors of 78 are 1, 2, 3, 6, 13, 26, 39, 78.
What is 78 in Roman numerals?
78 in Roman numerals is LXXVIII.
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