30
April, June, September and November each have 30 days, and 30 mph is the default speed limit on lit roads in Great Britain. 30 degrees is also a twelfth of a full turn, which is why consecutive hour marks on a clock sit that far apart.
- Even
- Composite
Where 30 fits
Why 30 is special
30 sits at the short end of the calendar's month-length split — 30 or 31 days, with February the sole exception — and, in a different everyday context, is Great Britain's default speed limit on roads with street lighting.
30 connects to:
- 15 — has factor
- numbers on earth — appears in
- numbers in culture and belief — appears in
- calendar years — is associated with
Same number, different meanings
How 30 is understood across different contexts — never implying one meaning because of another.
- Mathematics
30 sits at the short end of the calendar's month-length split — 30 or 31 days, with February the sole exception — and, in a different everyday context, is Great Britain's default speed limit on roads with street lighting.
- History
"Thirty days hath September, April, June and November" is the standard English mnemonic for month lengths, and versions of it are recorded in English from the fifteenth century onwards — long enough that the wording varies considerably between them. Its persistence is a fair indication that the pattern of 30- and 31-day months is genuinely arbitrary: a rule anyone could derive would not need a verse.
- Religion & belief
In Islam, the Quran is divided into thirty roughly equal parts, so that reciting one each night completes it across the month of Ramadan. The division is for recitation and does not follow the boundaries of the 114 suras.
- Everyday life
April, June, September and November each have 30 days — the shortest of the standard (non-February) calendar months.
- Culture
A 30-year anniversary is named for pearl. One of the more stable middle entries, agreed across most national lists.
- Law & regulation
Thirty is the minimum age for the United States Senate, set by Article I of the Constitution.
- Geography
The temperature inside a lightning channel is about 30 thousand kelvin. Roughly five times the surface of the Sun, for a few tens of microseconds. The thunder is the air that was heated expanding faster than sound.
- Science
30 is the atomic number of zinc. Most of it goes to galvanising — coating steel so it corrodes instead of the iron beneath — and it is an essential dietary trace element for enzyme function.
Not just a mathematical number
30is 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.
30 is part of
Factors & digit properties
- Divisors (8)
- 1, 2, 3, 5, 6, 10, 15, 30
- Prime factorisation
- 2 × 3 × 5
- Digit sum
- 3
- Digital root
- 3
Representations
- Decimal
- 30
- Scientific notation
- 3.000 × 10^1
Roman numerals
X X X
Binary
1 1 1 1 0
1 × 16 + 1 × 8 + 1 × 4 + 1 × 2 + 0 × 1 = 30
Octal
3 6
3 × 8 + 6 × 1 = 30
Hexadecimal
1 e
1 × 16 + e × 1 = 30
Read as powers of two: 16 + 8 + 4 + 2 = 30.
In historical numeral systems
Egyptian
3 hobbles
3 × 10 = 30. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
3 corner wedges
30 fits in a single sexagesimal place: 3 corner wedges.
Greek (Ionic)
lambda, marked as a numeral
lambda + keraia
A single letter, lambda, marked with a keraia to show it is a number rather than a word.
Hebrew numerals
lamed
lamed
Chinese numerals
sān shí
sān shí
三 before 十 multiplies it: 3 tens. The written form mirrors the arithmetic — "sān shí". The financial (anti-fraud) form is 參拾.
Maya
1 dot (upper, twenties place) over 2 bars (lower, units place)
1 × 20 + 10 = 30. 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
- Abundant number
- Harshad (Niven) number
Related numbers & connections
15 is a factor of 30 (15 × 2 = 30).
View 15 →30 appears in Numbers on Earth, the topic covering numbers that carry a documented earth-science or weather fact.
April, June, September and November each have 30 days — the shortest of the standard, non-February calendar months.
Mathematically related
- has factor15
why?
Strength 0.50, confidence "high".
Numbers Around Us
- appears inEarth
why?
Strength 0.50, confidence "high".
- appears inCulture and belief
why?
Strength 0.50, confidence "high".
Other connections
- is associated withCalendar years
why?
Strength 0.50, confidence "high".
History
"Thirty days hath September, April, June and November" is the standard English mnemonic for month lengths, and versions of it are recorded in English from the fifteenth century onwards — long enough that the wording varies considerably between them. Its persistence is a fair indication that the pattern of 30- and 31-day months is genuinely arbitrary: a rule anyone could derive would not need a verse.
DocumentedHistoryPeriod: 15th century – present
Religion & belief
In Islam, the Quran is divided into thirty roughly equal parts, so that reciting one each night completes it across the month of Ramadan. The division is for recitation and does not follow the boundaries of the 114 suras.
DocumentedReligion & beliefCulture: Islam
Everyday life
April, June, September and November each have 30 days — the shortest of the standard (non-February) calendar months.
DocumentedEveryday life
Culture
A 30-year anniversary is named for pearl. One of the more stable middle entries, agreed across most national lists.
DocumentedCultureGeography: United Kingdom, United States and much of Europe
Law & regulation
Thirty is the minimum age for the United States Senate, set by Article I of the Constitution.
DocumentedLaw & regulationGeography: United States30 mph (48 km/h) is the default speed limit on roads with street lighting in Great Britain, unless signposted otherwise — the country's most common urban speed limit.
DocumentedLaw & regulationGeography: Great Britain
Geography
The temperature inside a lightning channel is about 30 thousand kelvin. Roughly five times the surface of the Sun, for a few tens of microseconds. The thunder is the air that was heated expanding faster than sound.
DocumentedGeographyGeography: worldwide
Mathematics
Thirty degrees is a twelfth of a full turn, so consecutive hour marks on a clock face sit exactly 30 degrees apart. The 30-60-90 triangle formed by halving an equilateral triangle has sides in the fixed ratio 1 : root 3 : 2, which is why the angle recurs in drawing and construction.
DocumentedMathematics
Science
30 is the atomic number of zinc. Most of it goes to galvanising — coating steel so it corrodes instead of the iron beneath — and it is an essential dietary trace element for enzyme function.
DocumentedScience
Common questions
Which months have 30 days?
April, June, September and November — the four the rhyme names. The pattern is genuinely arbitrary, inherited from repeated Roman adjustments rather than designed, which is why English has needed a mnemonic for it since at least the fifteenth century.
Is 30 a prime number?
No, 30 is not prime.
What are the factors of 30?
The factors of 30 are 1, 2, 3, 5, 6, 10, 15, 30.
What is 30 in Roman numerals?
30 in Roman numerals is XXX.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 9 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026