81
81 is a composite number, odd, a calendar year.
81 (eighty-one) is an odd 2-digit number. It is a composite number with 5 divisors, and factors as 3⁴. It is a perfect square (9²). Written other ways, in Roman numerals it is LXXXI and in binary, 1010001.
Interestingness
Historical context — 81 AD
Calendar year
A.D. 81 (LXXXI) was a common year starting on Monday of the Julian calendar.
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Historical context — 81 BC
Calendar year
Year 81 BC was a year of the pre-Julian Roman calendar.
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Year facts
- Year type
-
Common year
Standard 365-day year; not divisible by 4 (or divisible by 100 but not 400).
- Days in year
- 365
- ISO weeks
- 52
- Started on
-
Wednesday
January 1, 81
- Ended on
-
Wednesday
December 31, 81
- Friday the 13ths
-
1
One Friday the 13th this year.
- Decade
-
80s
80–89
- Century
-
1st century
1–100
- Millennium
-
1st millennium
1–1000
- Years ago
-
1,945
1945 years before 2026.
In other calendars
- Hebrew
-
3841 / 3842 AM
Rosh Hashanah falls in September/October.
- Chinese
-
Year of the zodiac:Metal zodiac:Snake
Sexagenary cycle position 18 of 60. Lunar new year falls in late January / mid-February.
- Buddhist Era
-
624 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
- Ethiopian
-
73 / 74 ET
Year boundary at Enkutatash (September 11/12).
- Indian National (Saka)
-
3 / 2 Saka
Indian national calendar; year starts in March.
Properties
- Parity
- Odd
- Digit count
- 2
- Digit sum
- 9
- Digit product
- 8
- Digital root
- 9
- Palindrome
- No
- Bit width
- 7 bits
- Reversed
- 18
- Flips to (rotate 180°)
- 18
- Recamán's sequence
- a(45) = 81
- Square (n²)
- 6,561
- Cube (n³)
- 531,441
- Square root (√n)
- 9
- Divisor count
- 5
- σ(n) — sum of divisors
- 121
- φ(n) — Euler's totient
- 54
- Sum of prime factors
- 12
Primality
Prime factorization: 3 4
Divisors & multiples
Sums & aliquot sequence
Representations
- In words
- eighty-one
- Ordinal
- 81st
- Roman numeral
- LXXXI
- Binary
- 1010001
- Octal
- 121
- Hexadecimal
- 0x51
- Base64
- UQ==
- One's complement
- 174 (8-bit)
- Scientific notation
- 8.1 × 10¹
- As a duration
- 81 s = 1 minute, 21 seconds
As an angle
Historical numeral systems
- Babylonian (base 60)
- 𒁹 𒌋𒌋𒁹
- Egyptian hieroglyphic
- 𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓏺
- Greek (Milesian)
- παʹ
- Mayan (base 20)
- 𝋤·𝋡
- Chinese
- 八十一
- Chinese (financial)
- 捌拾壹
Digit at this position in famous constants
- π — Pi (π)
- Digit 81 = 9
- e — Euler's number (e)
- Digit 81 = 9
- φ — Golden ratio (φ)
- Digit 81 = 4
- √2 — Pythagoras's (√2)
- Digit 81 = 3
- ln 2 — Natural log of 2
- Digit 81 = 1
- γ — Euler-Mascheroni (γ)
- Digit 81 = 7
Also seen as
As an ASCII codepoint, 81 is Q. Printable ASCII character Q.
As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.81.
- Address
- 0.0.0.81
- Class
- reserved
- IPv4-mapped IPv6
- ::ffff:0.0.0.81
Unspecified address (0.0.0.0/8) — "this network" placeholder.
Matches numbered highway designation:
- I-81 — Dandridge, TN to Wellesley Island, NY (Canadian border).
Heard as a frequency, 81 Hz is closest to:
- Concert pitch (A4 = 440 Hz): E2 (82.4 Hz, -30¢)
- Scientific pitch (C4 = 256 Hz): E2 (80.6 Hz, +8¢)
- Baroque pitch (A4 = 415 Hz): F2 (82.3 Hz, -29¢)
As a MIDI note number, 81 is A5 (880 Hz at concert pitch).
On the periodic table, atomic number 81 is Thallium (Tl) — period 6.
Related reading
- Babylonian numerals — The base-60 cuneiform system that gave us 60 minutes, 60 seconds, and 360°.