The rules of roman numerals
The rules of roman numerals
There are six basic rules when it comes to roman numerals. Stick to these rules and you'll be writing perfect numerals in no time.
If in doubt, use our numeral converter. If you make a mistake, our numeral converter will tell you where you made the mistake and provide you with the correct solution too! If you want to quickly convert a number to numerals, use our number converter.
Rule 1: The numerals
All roman numerals are written using a combination of base numerals and subtractive numerals.
There are only seven base numerals which can be used to create any numeral sequence or number representation:
Number Numeral Words 1 I one 5 V five 10 X ten 50 L fifty 100 C one hundred 500 D five hundred 1000 M one thousand
Utilising the base numerals, there are also six subtractive numerals which act as shortcuts to represent numerals which would otherwise require four or five individual numerals:
Number Numeral Words 4 IV four 9 IX nine 40 XL forty 90 XC ninety 400 CD four hundred 900 CM nine hundred
Combining the base numerals and the subtractive numerals, this gives a total of 13 individual numerals which can be used in combination in any sequence.
Number Numeral Words 1 I one 4 IV four 5 V five 9 IX nine 10 X ten 40 XL forty 50 L fifty 90 XC ninety 100 C one hundred 400 CD four hundred 500 D five hundred 900 CM nine hundred 1000 M one thousand
Rule 2: The calculation
The value of a roman numeral sequence is calculated by simply adding together the numerals, from left to right.
Be careful though! Remember to look out for (and utilise when appropriate) subtractive numerals.
For example, the number 1988 converted to roman numerals is MCMLXXXVIII which is calculated from left-to-right as:
Rule 4: Sequencing
Roman numerals are written from left to right, and from highest to lowest (in terms of individual numeral value).
A higher value numeral cannot appear after a lower value numeral (unless in the context of an individual subtractive numeral).
Rule 5: Repetition
An individual numeral cannot appear more than three times consecutively in any sequence.
If you find yourself with four same-value numerals consecutively, it can always be simplified by using a subtractive numeral in its place.
A special case for this simplification rule is four M's. This exceeds the 3999 limit and would be invalid unless vinculum notation or bracket notation were adopted. Using bracket notation, four M's becomes (IV).
Rule 6: Single use numerals
There are individual numerals which can only appear once in any sequence.
All subtractive numerals can only appear once in any sequence.
From the set of base numerals: V, L, and D should only appear once.
The only caveat to this is when using another notation such as bracket notation. A single-use numeral can appear once within the brackets and once outside the brackets. For example, 5005 in bracket notation is (V)V.
Number Numeral Words 4 IV four 5 V five 9 IX nine 40 XL forty 50 L fifty 90 XC ninety 400 CD four hundred 500 D five hundred 900 CM nine hundred