400
400 metres is one lap of a standard outdoor running track, and 400 is a perfect square (20²).
Where 400 fits
Why 400 is special
400 is the number that defines the shape of athletics: a standard outdoor track measures 400 m in lane 1, which makes it the reference distance every other track event is built around — 800 m is two laps, 1500 m is nearly four. It is also cleanly 20².
Same number, different meanings
How 400 is understood across different contexts — never implying one meaning because of another.
- Mathematics
400 is the number that defines the shape of athletics: a standard outdoor track measures 400 m in lane 1, which makes it the reference distance every other track event is built around — 800 m is two laps, 1500 m is nearly four. It is also cleanly 20².
- Science
The tensile strength of structural steel is about 400 million pascals (worldwide, common grades). 400 megapascals. Concrete is strong in compression and weak in tension, which is why the two are used together.
- Technology & computing
CWE-400 is uncontrolled resource consumption, the parent of the more specific CWE-770. Regular expression backtracking and unbounded recursion are common instances.
- Sport
A standard outdoor athletics track measures 400 metres in lane 1, making it the reference distance from which other track events are derived — 800 m is two laps and the 4 × 400 m relay one lap per runner.
Not just a mathematical number
400is 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.
- Code
Also used as a code — HTTP status codes.
An assigned identifier from a fixed registry or standard — its digits don't represent an amount, and arithmetic on it is meaningless (HTTP status 404 + 1 is not a 'better' status; it's a different, unrelated code).
400 is part of
Factors & digit properties
- Divisors (15)
- 1, 2, 4, 5, 8, 10, 16, 20, 25, 40, 50, 80, 100, 200, 400
- Prime factorisation
- 2^4 × 5^2
- Digit sum
- 4
- Digital root
- 4
Representations
- Decimal
- 400
- Scientific notation
- 4.000 × 10^2
Roman numerals
C D
CD uses the subtractive pair CD: a hundred before five hundred — five hundred minus a hundred.
Binary
1 1 0 0 1 0 0 0 0
1 × 256 + 1 × 128 + 0 × 64 + 0 × 32 + 1 × 16 + 0 × 8 + 0 × 4 + 0 × 2 + 0 × 1 = 400
Octal
6 2 0
6 × 64 + 2 × 8 + 0 × 1 = 400
Hexadecimal
1 9 0
1 × 256 + 9 × 16 + 0 × 1 = 400
Read as powers of two: 256 + 128 + 16 = 400.
In historical numeral systems
Egyptian
4 coils of rope
4 × 100 = 400. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
6 vertical wedges (sixties place), 4 corner wedges (units place)
6 × 60 + 40 = 400. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
upsilon, marked as a numeral
upsilon + keraia
A single letter, upsilon, marked with a keraia to show it is a number rather than a word.
Hebrew numerals
tav
tav
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
- Powerful number
- Harshad (Niven) number
Related numbers & connections
400 is the square of 20 (20² = 400).
View 20 →400 appears in Numbers in living things, the topic covering numbers that carry a documented biology or natural-history fact.
A pattern that backtracks exponentially is uncontrolled resource consumption written by accident.
View 1333 →Mathematically related
- is the square of20
why?
Strength 0.50, confidence "high".
Numbers Around Us
- appears inLiving things
why?
Strength 0.50, confidence "high".
- appears inHTTP status codes
why?
Strength 0.50, confidence "high".
- appears inStandard athletics track lap distance
why?
Strength 0.50, confidence "high".
Science
The tensile strength of structural steel is about 400 million pascals (worldwide, common grades). 400 megapascals. Concrete is strong in compression and weak in tension, which is why the two are used together.
DocumentedScienceGeography: worldwide, common gradesThere are about 400 million tonnes of new plastic manufactured each year. Figure as of 2023. Roughly half is made for packaging and other single-use items, which is why production and waste figures track each other so closely.
DocumentedScienceGeography: worldwideThere are about 400 thousand described species of beetle. About a quarter of all described animal species. The pattern prompted the remark that a creator would have had an inordinate fondness for beetles.
DocumentedScienceGeography: worldwide
Technology & computing
CWE-400 is uncontrolled resource consumption, the parent of the more specific CWE-770. Regular expression backtracking and unbounded recursion are common instances.
DocumentedTechnology & computingGeography: International — MITRE CWEPeriod: 2006–The short-wavelength end of the visible spectrum is about 400 nanometres (typical human vision; the limits vary between people). Usually given as 380 to 750 nanometres, but sensitivity fades gradually rather than stopping, and the lens yellows with age — so the short end is a property of the observer as much as of the light.
DocumentedTechnology & computingGeography: typical human vision; the limits vary between people400 is the HTTP status for a malformed request — syntax the server cannot parse at all, as distinct from 422's valid syntax with impossible meaning. It is the catch-all of the 4xx family, which is why a bare 400 with no explanation is one of the least useful responses an API can give.
DocumentedTechnology & computing
Sport
A standard outdoor athletics track measures 400 metres in lane 1, making it the reference distance from which other track events are derived — 800 m is two laps and the 4 × 400 m relay one lap per runner.
DocumentedSportGeography: athletics (international)
Mathematics
400 is a perfect square: 20² = 400.
DocumentedMathematics
Common questions
Is 400 a prime number?
No, 400 is not prime.
What are the factors of 400?
The factors of 400 are 1, 2, 4, 5, 8, 10, 16, 20, 25, 40, 50, 80, 100, 200, 400.
What is 400 in Roman numerals?
400 in Roman numerals is CD. CD is subtractive: a hundred before five hundred — five hundred minus a hundred. Written additively it would be CCCC, 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: 8 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026