404
404 is the HTTP status code for 'Not Found', one of the most recognisable numbers on the web.
- Even
- Composite
Where 404 fits
Why 404 is special
404 is the number people meet when something is missing, and the digits are not arbitrary: HTTP groups its status codes by first digit, so 4xx announces a client-side problem before the rest identifies which one. It is worth knowing what it rules out — a 404 means the server was reached and had nothing there, which is a working site with a broken address, not an outage and not a security problem.
Same number, different meanings
How 404 is understood across different contexts — never implying one meaning because of another.
- Mathematics
404 is the number people meet when something is missing, and the digits are not arbitrary: HTTP groups its status codes by first digit, so 4xx announces a client-side problem before the rest identifies which one. It is worth knowing what it rules out — a 404 means the server was reached and had nothing there, which is a working site with a broken address, not an outage and not a security problem.
- History
404 is not an arbitrary label: HTTP response codes are grouped by their first digit — 1xx informational, 2xx success, 3xx redirection, 4xx client error, 5xx server error — so 404 announces a client-side problem before its second and third digits identify which one. The scheme was set out in RFC 1945 for HTTP/1.0 in 1996 and has been carried through every revision since.
- Technology & computing
A 'soft 404' is a page that tells the reader it does not exist while returning status 200 to the machine, usually because a site serves a friendly error page without setting the code. Search engines treat this as a defect: the crawler is told the page is fine, so it stays in the index and competes with real content. The failure is a mismatch between what the page says and what the protocol says, and it is invisible to anyone reading the site in a browser.
- Everyday life
A 404 means the server was reached but had nothing at that address — so the site is working and the specific page is not. The usual causes are a mistyped URL, a page that has been moved or deleted, or a stale link on someone else's site. It is not a sign of a security problem, and it is distinct from a 403 (the page exists but you may not see it) and a 500 (the server itself failed).
- Culture
Beyond its technical meaning, '404' is used in English internet slang for something or someone missing, unavailable or oblivious — as in 'his attention span is 404'. The usage borrows the error page's familiarity and says nothing about the number itself.
Not just a mathematical number
404is 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).
404 is part of
Factors & digit properties
- Divisors (6)
- 1, 2, 4, 101, 202, 404
- Prime factorisation
- 2^2 × 101
- Digit sum
- 8
- Digital root
- 8
Representations
- Decimal
- 404
- Scientific notation
- 4.040 × 10^2
Roman numerals
C D I V
CDIV uses the subtractive pair IV: one before five — five minus one.
Binary
1 1 0 0 1 0 1 0 0
1 × 256 + 1 × 128 + 0 × 64 + 0 × 32 + 1 × 16 + 0 × 8 + 1 × 4 + 0 × 2 + 0 × 1 = 404
Octal
6 2 4
6 × 64 + 2 × 8 + 4 × 1 = 404
Hexadecimal
1 9 4
1 × 256 + 9 × 16 + 4 × 1 = 404
Read as powers of two: 256 + 128 + 16 + 4 = 404.
In historical numeral systems
Egyptian
4 coils of rope, 4 strokes
4 × 100 + 4 × 1 = 404. 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 + 4 vertical wedges (units place)
6 × 60 + 44 = 404. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
upsilon, then delta, marked as a numeral
upsilon-delta + keraia
upsilon + delta written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
tav, then dalet
tav-dalet
tav (400) + dalet (4) = 404, written in descending order.
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
- Palindromic number
- Deficient number
- Happy number
Related numbers & connections
HTTP 404 is a status code registry entry. It is probably the most widely recognised assigned number in the world, and it means nothing outside the registry.
The two ends of the same conversation. Every HTTP response carries a status, 200 meaning the request succeeded and 404 meaning the server has nothing at that address. Users only ever see the failure, which is why 404 is famous and 200 is not.
View 200 →Numbers Around Us
- appears inNumbers assigned as identifiers
why?
Strength 0.50, confidence "high".
- appears alongside451
why?
Strength 0.50, confidence "high".
- appears alongside407
why?
Strength 0.50, confidence "high".
- appears alongside418
why?
Strength 0.50, confidence "high".
- appears inHTTP status codes
why?
Strength 0.50, confidence "high".
Other connections
- contrasts with200
why?
Strength 0.50, confidence "high".
- contrasts with503
why?
Strength 0.50, confidence "high".
- is associated with2616
why?
Strength 0.50, confidence "high".
- contrasts with500
why?
Strength 0.50, confidence "high".
- contrasts with410
why?
Strength 0.50, confidence "high".
- is associated with100
why?
Strength 0.50, confidence "high".
- contrasts with403
why?
Strength 0.50, confidence "high".
History
404 is not an arbitrary label: HTTP response codes are grouped by their first digit — 1xx informational, 2xx success, 3xx redirection, 4xx client error, 5xx server error — so 404 announces a client-side problem before its second and third digits identify which one. The scheme was set out in RFC 1945 for HTTP/1.0 in 1996 and has been carried through every revision since.
DocumentedHistoryPeriod: 1996 – presentA widely repeated story holds that 404 was named after room 404 at CERN, where a web server sat and staff would report that a file could not be found. It is not true: the code's first digit denotes a client error and the remaining digits were allocated sequentially in the earliest HTTP drafts, alongside 400, 401, 403 and the rest. Tim Berners-Lee's own accounts and the specification history give no such room. The story survives because it explains a number that otherwise looks arbitrary and is not.
DisputedHistory
Technology & computing
A 'soft 404' is a page that tells the reader it does not exist while returning status 200 to the machine, usually because a site serves a friendly error page without setting the code. Search engines treat this as a defect: the crawler is told the page is fine, so it stays in the index and competes with real content. The failure is a mismatch between what the page says and what the protocol says, and it is invisible to anyone reading the site in a browser.
DocumentedTechnology & computingHTTP status code 404 ('Not Found') is formally defined in IETF specifications (originally RFC 2068, now RFC 9110) to indicate that a requested web resource could not be located on the server.
DocumentedTechnology & computing
Everyday life
A 404 means the server was reached but had nothing at that address — so the site is working and the specific page is not. The usual causes are a mistyped URL, a page that has been moved or deleted, or a stale link on someone else's site. It is not a sign of a security problem, and it is distinct from a 403 (the page exists but you may not see it) and a 500 (the server itself failed).
DocumentedEveryday life
Culture
Beyond its technical meaning, '404' is used in English internet slang for something or someone missing, unavailable or oblivious — as in 'his attention span is 404'. The usage borrows the error page's familiarity and says nothing about the number itself.
Widely documentedCulture
Common questions
Was 404 named after a room at CERN?
No. The story is widely repeated and there is no evidence for it. HTTP status codes are grouped by first digit — 4xx means the client's request was at fault — and the remaining digits were allocated sequentially in the earliest drafts, alongside 400, 401 and 403. The room story persists because it explains a number that looks arbitrary and is not.
What is the difference between a 404, a 403 and a 500?
404 means the server was reached and has nothing at that address. 403 means the page exists but you may not see it. 500 means the server itself failed while trying. A 404 is not a security problem and usually means a mistyped address, a moved page, or a stale link on someone else's site.
Is 404 a prime number?
No, 404 is not prime.
What are the factors of 404?
The factors of 404 are 1, 2, 4, 101, 202, 404.
What is 404 in Roman numerals?
404 in Roman numerals is CDIV. IV is subtractive: one before five — five minus one. Written additively it would be CCCCIIII, 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: 6 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026