73
73 is a prime number — the 21st prime — and the atomic number of tantalum.
- Odd
- Prime
Where 73 fits
Why 73 is special
73's mirror number, 37, is the 12th prime — and 21 reversed is 12, a genuine, verifiable pattern that made 73 an internet-famous 'best number' after being popularised by the sitcom The Big Bang Theory.
Same number, different meanings
How 73 is understood across different contexts — never implying one meaning because of another.
- Mathematics
73's mirror number, 37, is the 12th prime — and 21 reversed is 12, a genuine, verifiable pattern that made 73 an internet-famous 'best number' after being popularised by the sitcom The Big Bang Theory.
- Culture
A 2010 episode of The Big Bang Theory has a character argue that seventy-three is the best number, citing that it is the twenty-first prime and that its digits reversed give the twelfth. The claim is arithmetically correct and the conclusion is a joke.
- Media & popular culture
73 is widely called 'the best number' because of a 2010 episode of the American sitcom The Big Bang Theory, in which the character Sheldon Cooper recites its properties — the 21st prime, its mirror 37 being the 12th, and 7 x 3 = 21 — and the writers later used 73 as a running joke. The properties are real and were known long before the programme; what the programme did was make them famous, and it is the reason most people who look 73 up are looking it up.
- Science
73 is the atomic number of tantalum. Tantalum capacitors store a lot of charge in very little space, which is why they appear in phones and other compact electronics.
73 is part of
Factors & digit properties
- Divisors (2)
- 1, 73
- Prime factorisation
- 73
- Digit sum
- 10
- Digital root
- 1
Representations
- Decimal
- 73
- Scientific notation
- 7.300 × 10^1
Roman numerals
L X X I I I
Binary
1 0 0 1 0 0 1
1 × 64 + 0 × 32 + 0 × 16 + 1 × 8 + 0 × 4 + 0 × 2 + 1 × 1 = 73
Octal
1 1 1
1 × 64 + 1 × 8 + 1 × 1 = 73
Hexadecimal
4 9
4 × 16 + 9 × 1 = 73
Read as powers of two: 64 + 8 + 1 = 73.
In historical numeral systems
Egyptian
7 hobbles, 3 strokes
7 × 10 + 3 × 1 = 73. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
1 vertical wedge (sixties place), 1 corner wedge + 3 vertical wedges (units place)
1 × 60 + 13 = 73. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
omicron, then gamma, marked as a numeral
omicron-gamma + keraia
omicron + gamma written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
ayin, then gimel
ayin-gimel
ayin (70) + gimel (3) = 73, written in descending order.
Chinese numerals
qī shí sān
qī shí sān
七 before 十 multiplies it: 7 tens. 三 after it adds 3. The written form mirrors the arithmetic — "qī shí sān". The financial (anti-fraud) form is 柒拾參.
Maya
3 dots (upper, twenties place) over 2 bars + 3 dots (lower, units place)
3 × 20 + 13 = 73. 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
- Lucky number (mathematical sieve)
- Twin prime
Related numbers & connections
71 and 73 are twin primes — two primes exactly two apart.
View 71 →The televised claim turns on seventy-three being the twenty-first prime and its reversal the twelfth.
View 37 →Culture
A 2010 episode of The Big Bang Theory has a character argue that seventy-three is the best number, citing that it is the twenty-first prime and that its digits reversed give the twelfth. The claim is arithmetically correct and the conclusion is a joke.
DocumentedCultureGeography: United StatesPeriod: 2010–
Media & popular culture
73 is widely called 'the best number' because of a 2010 episode of the American sitcom The Big Bang Theory, in which the character Sheldon Cooper recites its properties — the 21st prime, its mirror 37 being the 12th, and 7 x 3 = 21 — and the writers later used 73 as a running joke. The properties are real and were known long before the programme; what the programme did was make them famous, and it is the reason most people who look 73 up are looking it up.
DocumentedMedia & popular cultureCulture: American televisionPeriod: 2010 – present
Science
73 is the atomic number of tantalum. Tantalum capacitors store a lot of charge in very little space, which is why they appear in phones and other compact electronics.
DocumentedScience
Mathematics
73 is prime and the 21st prime number; its mirror, 37, is prime and the 12th prime number — and 21 reversed is 12, a genuine, verifiable pattern popularised (including by the sitcom The Big Bang Theory) as 'the best number'.
DocumentedMathematics
Common questions
Why is 73 called the best number?
From a 2010 episode of The Big Bang Theory, in which Sheldon Cooper points out that 73 is the 21st prime, that its mirror 37 is the 12th, and that 7 × 3 = 21. Every part of that is true and none of it was new — what the programme did was make it famous, which is why most people looking 73 up are looking it up because of a sitcom.
Is 73 a prime number?
Yes, 73 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 73?
The factors of 73 are 1, 73.
What is 73 in Roman numerals?
73 in Roman numerals is LXXIII.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 4 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026