210
A4 paper is 210 mm wide.
- Even
- Composite
Where 210 fits
Why 210 is special
The width follows from the standard's own rule rather than being chosen: an A0 sheet is one square metre with sides in the ratio of the square root of two, and halving it four times gives 210 by 297 mm. That is why A4 fits an envelope exactly when folded in three.
Same number, different meanings
How 210 is understood across different contexts — never implying one meaning because of another.
- Mathematics
The width follows from the standard's own rule rather than being chosen: an A0 sheet is one square metre with sides in the ratio of the square root of two, and halving it four times gives 210 by 297 mm. That is why A4 fits an envelope exactly when folded in three.
- Everyday life
A4 is 210 mm wide because A0 is one square metre with sides in the square-root-of-two ratio, halved four times. Nothing about 210 was chosen directly, which is why it is not a round number — and why A4 folded in three fits a standard envelope exactly.
- Science
Polonium-210 emits alpha particles that are harmless outside the body and lethal within it. Its use in the 2006 killing of Alexander Litvinenko made it one of the few isotopes widely known by number.
210 is part of
Factors & digit properties
- Divisors (16)
- 1, 2, 3, 5, 6, 7, 10, 14, 15, 21, 30, 35, 42, 70, 105, 210
- Prime factorisation
- 2 × 3 × 5 × 7
- Digit sum
- 3
- Digital root
- 3
Representations
- Decimal
- 210
- Binary
- 11010010
- Octal
- 322
- Hexadecimal
- d2
- Roman numeral
- CCX
- Scientific notation
- 2.100 × 10^2
Other mathematical patterns
- Triangular number
- Abundant number
- Pentagonal number
- Harshad (Niven) number
Related numbers & connections
A5 is 148 by 210 mm and A4 is 210 by 297: the width of each size is the height of the next one down. That only works because every sheet shares the square-root-of-two ratio.
View 148 →ISO 216 is the standard that fixes A4 at 210 by 297 mm, by defining A0 as one square metre with sides in the square-root-of-two ratio.
View 216 →Everyday life
A4 is 210 mm wide because A0 is one square metre with sides in the square-root-of-two ratio, halved four times. Nothing about 210 was chosen directly, which is why it is not a round number — and why A4 folded in three fits a standard envelope exactly.
DocumentedEveryday life
Science
Polonium-210 emits alpha particles that are harmless outside the body and lethal within it. Its use in the 2006 killing of Alexander Litvinenko made it one of the few isotopes widely known by number.
DocumentedScienceGeography: GlobalPeriod: 1898–
Common questions
Is 210 a prime number?
No, 210 is not prime.
What are the factors of 210?
The factors of 210 are 1, 2, 3, 5, 6, 7, 10, 14, 15, 21, 30, 35, 42, 70, 105, 210.
What is 210 in Roman numerals?
210 in Roman numerals is CCX.
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: 3 August 2026