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81 in words

81

eighty-one

81 is written eighty-one.

Ways to write it

As an ordinal: eighty-first
Roman numeral: LXXXI
Expanded form: 80 + 1

Place value

Place	Digit	Value
tens	8	80
ones	1	1

Number facts

Even or odd: Odd
Prime or composite: Composite
Factors: 1, 3, 9, 27 and 81 (5 factors)
First 10 multiples: 81, 162, 243, 324, 405, 486, 567, 648, 729, 810
Square: 81 × 81 = 6,561
Square root: 9
Sum of the digits: 8 + 1 = 9
Rounded to the nearest 10: 80
Number before: 80
Number after: 82
In binary: 1010001

Quick check

What number comes just after 81?
Show the answer
82
Is 81 even or odd?
Show the answer
Odd
What is 81 + 10?
Show the answer
91

Practise with a game

Count with Me

Count pictures up to 5, 10 or 20 and compare groups.

Place Value Quest

Ones, tens, hundreds and thousands.

Times Tables Challenge

Practise any times table from 2× to 12×.

Frequently asked questions

How do you write 81 in words?

81 is written "eighty-one".

Is 81 a prime number?

No. 81 is a composite number, because it has more than two factors.

What are the factors of 81?

The factors of 81 are 1, 3, 9, 27 and 81.

How do you write 81 in Roman numerals?

81 in Roman numerals is LXXXI.