521
521 is a non-standard Cloudflare status code meaning the origin server refused the connection.
- Odd
- Prime
Where 521 fits
Why 521 is special
521 is the most diagnostic of Cloudflare's invented codes: the origin is not accepting connections at all, usually because the service is down or a firewall is blocking the proxy's addresses. Unlike a generic 502 it tells you which end failed and how.
Not just a mathematical number
521is 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).
521 is part of
Factors & digit properties
- Divisors (2)
- 1, 521
- Prime factorisation
- 521
- Digit sum
- 8
- Digital root
- 8
Representations
- Decimal
- 521
- Scientific notation
- 5.210 × 10^2
Roman numerals
D X X I
Binary
1 0 0 0 0 0 1 0 0 1
1 × 512 + 0 × 256 + 0 × 128 + 0 × 64 + 0 × 32 + 0 × 16 + 1 × 8 + 0 × 4 + 0 × 2 + 1 × 1 = 521
Octal
1 0 1 1
1 × 512 + 0 × 64 + 1 × 8 + 1 × 1 = 521
Hexadecimal
2 0 9
2 × 256 + 0 × 16 + 9 × 1 = 521
Read as powers of two: 512 + 8 + 1 = 521.
In historical numeral systems
Egyptian
5 coils of rope, 2 hobbles, 1 stroke
5 × 100 + 2 × 10 + 1 × 1 = 521. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
8 vertical wedges (sixties place), 4 corner wedges + 1 vertical wedge (units place)
8 × 60 + 41 = 521. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
phi, then kappa, then alpha, marked as a numeral
phi-kappa-alpha + keraia
phi + kappa + alpha 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 qof, then kaf, then alef
tav-qof-kaf-alef
tav (400) + qof (100) + kaf (20) + alef (1) = 521, 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
- Deficient number
- Lucas number
- Twin prime
Related numbers & connections
521 is Cloudflare's code for an origin server that actively refused the connection.
Numbers Around Us
- appears inHTTP status codes
why?
Strength 0.50, confidence "high".
- appears alongside520
why?
Strength 0.50, confidence "high".
Technology & computing
521 is Cloudflare's code for an origin server that actively refused the connection, which usually means the web server is down or a firewall is rejecting Cloudflare's addresses. Like the rest of the 52x range it appears in no RFC.
DocumentedTechnology & computing
Common questions
Is 521 a prime number?
Yes, 521 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 521?
The factors of 521 are 1, 521.
What is 521 in Roman numerals?
521 in Roman numerals is DXXI.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 1 reviewer-confirmed record, sourced and reviewed before publication.
Last reviewed: 15 July 2026