21 in Roman numerals, how to write?

21 = XXI
20 = ? ... 22 = ?
Explanations below.

Roman numerals used to make the conversion:


I = 1; X = 10;

Reading rules

1. Break the number (decompose it) into place value subgroups:

21 = 20 + 1;

2. Convert each subgroup:

20 = 10 + 10 = X + X = XX;
1 = I;

3. Wrap up the Roman numeral:

21 = 20 + 1 = XX + I = XXI;

Final answer:


21 = XXI

In Roman numerals, how to write:
20 = ? ... 22 = ?

In Roman numerals online converter

Latest conversions of numbers to Roman numerals

21 = XXI Nov 17 10:49 UTC (GMT)
221,162 = (C)(C)(X)(X)MCLXII Nov 17 10:49 UTC (GMT)
425,469 = (C)(D)(X)(X)(V)CDLXIX Nov 17 10:49 UTC (GMT)
25,372 = (X)(X)(V)CCCLXXII Nov 17 10:49 UTC (GMT)
20,816 = (X)(X)DCCCXVI Nov 17 10:49 UTC (GMT)
105,000 = (C)(V) Nov 17 10:48 UTC (GMT)
31,975 = (X)(X)(X)MCMLXXV Nov 17 10:48 UTC (GMT)
923,006 = (C)(M)(X)(X)MMMVI Nov 17 10:48 UTC (GMT)
21,990 = (X)(X)MCMXC Nov 17 10:48 UTC (GMT)
2,024 = MMXXIV Nov 17 10:48 UTC (GMT)
1,582,018 = (M)(D)(L)(X)(X)(X)MMXVIII Nov 17 10:48 UTC (GMT)
1,992 = MCMXCII Nov 17 10:48 UTC (GMT)
19,942 = (X)M(X)CMXLII Nov 17 10:48 UTC (GMT)
converted numbers, see more...

The set of basic symbols of the Roman system of writing numerals

The major set of symbols on which the rest of the Roman numberals were built:

  • I = 1 (one); V = 5 (five);

  • X = 10 (ten); L = 50 (fifty);

  • C = 100 (one hundred);

  • D = 500 (five hundred);

  • M = 1,000 (one thousand);

    • For larger numbers:
    • (*) V = 5,000 or |V| = 5,000 (five thousand); see below why we prefer this notation: (V) = 5,000.

    • (*) X = 10,000 or |X| = 10,000 (ten thousand); see below why we prefer this notation: (X) = 10,000.

    • (*) L = 50,000 or |L| = 50,000 (fifty thousand); see below why we prefer this notation: (L) = 50,000.

    • (*) C = 100,000 or |C| = 100,000 (one hundred thousand); see below why we prefer this notation: (C) = 100,000.

    • (*) D = 500,000 or |D| = 500,000 (five hundred thousand); see below why we prefer this notation: (D) = 500,000.

    • (*) M = 1,000,000 or |M| = 1,000,000 (one million); see below why we prefer this notation: (M) = 1,000,000.

(*) These numbers were written with an overline (a bar above) or between two vertical lines. Instead, we prefer to write these larger numerals between brackets, ie: "(" and ")", because:

  • 1) when compared to the overline - it is easier for the computer users to add brackets around a letter than to add the overline to it and
  • 2) when compared to the vertical lines - it avoids any possible confusion between the vertical line "|" and the Roman numeral "I" (1).

(*) An overline (a bar over the symbol), two vertical lines or two brackets around the symbol indicate "1,000 times". See below...

Logic of the numerals written between brackets, ie: (L) = 50,000; the rule is that the initial numeral, in our case, L, was multiplied by 1,000: L = 50 => (L) = 50 × 1,000 = 50,000. Simple.

(*) At the beginning Romans did not use numbers larger than 3,999; as a result they had no symbols in their system for these larger numbers, they were added on later and for them various different notations were used, not necessarily the ones we've just seen above.

Thus, initially, the largest number that could be written using Roman numerals was:

  • MMMCMXCIX = 3,999.

Roman numerals reading rules, summary: