71
71 is a prime number, the atomic number of lutetium, and close to the commonly cited ~71% figure for the proportion of Earth's surface covered by water.
- Odd
- Prime
Where 71 fits
Why 71 is special
71's near-coincidental match with Earth's water coverage is a genuine, widely cited approximate figure in Earth science, not a mathematical property of the number itself.
Same number, different meanings
How 71 is understood across different contexts — never implying one meaning because of another.
- Mathematics
71's near-coincidental match with Earth's water coverage is a genuine, widely cited approximate figure in Earth science, not a mathematical property of the number itself.
- Geography
Approximately 71% of Earth's surface is covered by water, mostly oceans — one of the most widely cited approximate figures in Earth science.
- Science
71 is the atomic number of lutetium. Lutetium-based scintillator crystals convert gamma rays into visible light inside PET scanners, making the technique practical.
71 is part of
Factors & digit properties
- Divisors (2)
- 1, 71
- Prime factorisation
- 71
- Digit sum
- 8
- Digital root
- 8
Representations
- Decimal
- 71
- Scientific notation
- 7.100 × 10^1
Roman numerals
L X X I
Binary
1 0 0 0 1 1 1
1 × 64 + 0 × 32 + 0 × 16 + 0 × 8 + 1 × 4 + 1 × 2 + 1 × 1 = 71
Octal
1 0 7
1 × 64 + 0 × 8 + 7 × 1 = 71
Hexadecimal
4 7
4 × 16 + 7 × 1 = 71
Read as powers of two: 64 + 4 + 2 + 1 = 71.
In historical numeral systems
Egyptian
7 hobbles, 1 stroke
7 × 10 + 1 × 1 = 71. 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 + 1 vertical wedge (units place)
1 × 60 + 11 = 71. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
omicron, then alpha, marked as a numeral
omicron-alpha + keraia
omicron + alpha 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 alef
ayin-alef
ayin (70) + alef (1) = 71, written in descending order.
Chinese numerals
qī shí yī
qī shí yī
七 before 十 multiplies it: 7 tens. 一 after it adds 1. The written form mirrors the arithmetic — "qī shí yī". The financial (anti-fraud) form is 柒拾壹.
Maya
3 dots (upper, twenties place) over 2 bars + 1 dot (lower, units place)
3 × 20 + 11 = 71. 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
- Twin prime
Related numbers & connections
71 and 73 are twin primes — two primes exactly two apart.
View 73 →Roughly seventy-one percent of Earth's surface is covered by water, which is the figure usually quoted for ocean coverage.
Shares a distinctive class
- is associated with73
why?
Strength 0.50, confidence "high".
Numbers Around Us
- appears inEarth's water coverage
why?
Strength 0.50, confidence "high".
Geography
Approximately 71% of Earth's surface is covered by water, mostly oceans — one of the most widely cited approximate figures in Earth science.
Widely documentedGeography
Science
71 is the atomic number of lutetium. Lutetium-based scintillator crystals convert gamma rays into visible light inside PET scanners, making the technique practical.
DocumentedScience
Common questions
Is 71 a prime number?
Yes, 71 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 71?
The factors of 71 are 1, 71.
What is 71 in Roman numerals?
71 in Roman numerals is LXXI.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 2 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026