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.
- Technology & computing
The width of a sheet of A4 paper is 210 millimetres (worldwide, ISO 216). The 297 by 210 ratio is the square root of two, so halving a sheet gives the same shape again. That is why enlarging A4 to A3 is exactly 141 per cent.
- 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
- Scientific notation
- 2.100 × 10^2
Roman numerals
C C X
Binary
1 1 0 1 0 0 1 0
1 × 128 + 1 × 64 + 0 × 32 + 1 × 16 + 0 × 8 + 0 × 4 + 1 × 2 + 0 × 1 = 210
Octal
3 2 2
3 × 64 + 2 × 8 + 2 × 1 = 210
Hexadecimal
d 2
d × 16 + 2 × 1 = 210
Read as powers of two: 128 + 64 + 16 + 2 = 210.
In historical numeral systems
Egyptian
2 coils of rope, 1 hobble
2 × 100 + 1 × 10 = 210. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
3 vertical wedges (sixties place), 3 corner wedges (units place)
3 × 60 + 30 = 210. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
sigma, then iota, marked as a numeral
sigma-iota + keraia
sigma + iota written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
resh, then yod
resh-yod
resh (200) + yod (10) = 210, written in descending order.
Maya
2 bars (upper, twenties place) over 2 bars (lower, units place)
10 × 20 + 10 = 210. 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
- 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
Technology & computing
The width of a sheet of A4 paper is 210 millimetres (worldwide, ISO 216). The 297 by 210 ratio is the square root of two, so halving a sheet gives the same shape again. That is why enlarging A4 to A3 is exactly 141 per cent.
DocumentedTechnology & computingGeography: worldwide, ISO 216
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: 3 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026