Basic Rules For Roman Numbers


In Roman Numeral the following letters are used to represent numbers:

$I = 1$
$V = 5$
$X = 10$
$L = 50$
$C = 100$
$D = 500$
$M = 1000$



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The following rules apply when writing roman numbers:

  • When a smaller digit appears before (to the left of) a larger digit, the smaller digit is subtracted from the larger digit. Example: $IV = 5 - 1 = 4$.
  • When a smaller digit appears after (to the right of) a larger digit the number is the smaller digit is added to the larger digit. Example: $XI = 10 + 1 = 11$.
  • Smaller digit to the right can be repeated at most $3$ times. More than that is not allowed.
    Example: To write $9$, we cannot use $VIIII$, but will have to use $IX$.
  • Roman number is formed by writing separate groups for digits in different places like units, tens, hundeds, etc..
    Example: To write $99$ do not use $IC$. It should be written as $90 + 9$, that is $XCIX$
  • When writing a smaller digit to the left of a larger digit, remember, that numbers starting with $5$ like $5 (V)$, $50 (L)$ or $500 (D)$ cannot precede a larger number. Hence $LC$ or $LD$ are not valid Roman numbers. To write $450$ you must use $CDL$ and not $LD$. Look at the following list of digits that can precede a larger digit.
    Only $I$ and can precede $X$
    Only $X$ can precede $C$ and $L$
    Only $C$ can precede $D$ and $M$