53
53 is a prime number and the atomic number of iodine, essential to thyroid function.
- Odd
- Prime
Where 53 fits
Why 53 is special
53's biology tie (iodine) is more notable than its unremarkable mathematical profile as an ordinary two-digit prime.
Same number, different meanings
How 53 is understood across different contexts — never implying one meaning because of another.
- Mathematics
53's biology tie (iodine) is more notable than its unremarkable mathematical profile as an ordinary two-digit prime.
- Everyday life
The thyroid cannot make its hormones without element 53, and in regions whose soil and water hold little of it, deficiency causes goitre and — in pregnancy — irreversible harm to a child's neurological development. Adding a trace of iodine to table salt is one of the cheapest public-health measures ever devised: Switzerland and the United States began in the 1920s, and universal salt iodisation has since been adopted in most of the world. It is the reason a supermarket salt packet says 'iodised' at all.
- Technology & computing
The port the Domain Name System uses is 53 (worldwide, IANA registry). It runs over UDP for ordinary lookups and TCP for large responses, which is unusual — most services pick one. Blocking it breaks essentially everything, since almost nothing on the internet is reached by address.
- Science
53 is the atomic number of iodine. The thyroid gland needs it to make hormones, and adding traces to table salt eliminated goitre and a major cause of preventable developmental harm across whole countries.
Factors & digit properties
- Divisors (2)
- 1, 53
- Prime factorisation
- 53
- Digit sum
- 8
- Digital root
- 8
Representations
- Decimal
- 53
- Scientific notation
- 5.300 × 10^1
Roman numerals
L I I I
Binary
1 1 0 1 0 1
1 × 32 + 1 × 16 + 0 × 8 + 1 × 4 + 0 × 2 + 1 × 1 = 53
Octal
6 5
6 × 8 + 5 × 1 = 53
Hexadecimal
3 5
3 × 16 + 5 × 1 = 53
Read as powers of two: 32 + 16 + 4 + 1 = 53.
In historical numeral systems
Egyptian
5 hobbles, 3 strokes
5 × 10 + 3 × 1 = 53. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
5 corner wedges + 3 vertical wedges
53 fits in a single sexagesimal place: 5 corner wedges + 3 vertical wedges.
Greek (Ionic)
nu, then gamma, marked as a numeral
nu-gamma + keraia
nu + gamma written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
nun, then gimel
nun-gimel
nun (50) + gimel (3) = 53, written in descending order.
Chinese numerals
wǔ shí sān
wǔ shí sān
五 before 十 multiplies it: 5 tens. 三 after it adds 3. The written form mirrors the arithmetic — "wǔ shí sān". The financial (anti-fraud) form is 伍拾參.
Maya
2 dots (upper, twenties place) over 2 bars + 3 dots (lower, units place)
2 × 20 + 13 = 53. 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
- Deficient number
- Sophie Germain prime
Related numbers & connections
53 and 52 are consecutive integers.
View 52 →Port 53 carries DNS and is the port most restrictive networks leave open, which makes the assignment operationally significant.
DNS is defined in RFC 1034 and carried on port 53, the one port most restrictive networks leave open.
View 1034 →Nearby in a sequence
- is adjacent to52
why?
Strength 0.50, confidence "high".
Numbers Around Us
- appears inNumbers assigned as identifiers
why?
Strength 0.50, confidence "high".
- appears inNumbers that make the internet work
why?
Strength 0.50, confidence "high".
Everyday life
The thyroid cannot make its hormones without element 53, and in regions whose soil and water hold little of it, deficiency causes goitre and — in pregnancy — irreversible harm to a child's neurological development. Adding a trace of iodine to table salt is one of the cheapest public-health measures ever devised: Switzerland and the United States began in the 1920s, and universal salt iodisation has since been adopted in most of the world. It is the reason a supermarket salt packet says 'iodised' at all.
DocumentedEveryday lifeGeography: Switzerland and the United States first; now most countriesPeriod: 1920s – present
Technology & computing
The port the Domain Name System uses is 53 (worldwide, IANA registry). It runs over UDP for ordinary lookups and TCP for large responses, which is unusual — most services pick one. Blocking it breaks essentially everything, since almost nothing on the internet is reached by address.
DocumentedTechnology & computingGeography: worldwide, IANA registryPort 53 carries DNS, the lookup that turns a hostname into an IP address before any connection is made. It is the only port most restrictive networks leave open, which is why it is both critical infrastructure and a favoured channel for tunnelling traffic past captive portals.
DocumentedTechnology & computing
Science
53 is the atomic number of iodine. The thyroid gland needs it to make hormones, and adding traces to table salt eliminated goitre and a major cause of preventable developmental harm across whole countries.
DocumentedScience
Mathematics
53 is prime — the 16th prime number.
DocumentedMathematics
Common questions
Why is salt iodised?
Because the thyroid cannot make its hormones without iodine, and in regions where soil and water hold little of it, deficiency causes goitre and — in pregnancy — irreversible harm to a child's brain development. Adding a trace to table salt reaches almost everyone at almost no cost. Switzerland and the United States began in the 1920s; most of the world has followed.
Is 53 a prime number?
Yes, 53 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 53?
The factors of 53 are 1, 53.
What is 53 in Roman numerals?
53 in Roman numerals is LIII.
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