Roman Numerals -- The Basics

A smaller number in front of a larger number means subtraction, all else means addition. For example, IV means 4, VI means 6.

You would not put more than one smaller number in front of a larger number to subtract. For example, the number 3 is III, not IIV.

You must separate ones, tens, hundreds, and thousands as separate items. That means that 99 is XCIX, 90 + 9, but never should be written as IC. Similarly, 999 cannot be IM and 1999 cannot be MIM.
I The numeral one. II is two, III is three. You seldom see IIII as 4, since IV can also mean 4, plus its shorter to write.
V The numeral 5. IV is 4, VI is 6, VII is 7, VIII is 8.
X The numeral 10. IX is 9, XI is 11, etc.
L The numeral 50. XL would be 40.
C The numeral 100. Think of Century having a hundred years. C is short for the Latin word Centum, but that's not very easy to remember.
D The numeral 500.
M The numeral 1000.
Sometimes you will see a numeral with a line over it. That means to multiply it by 1000. A numeral V with a line over it means 5000.


Roman Numerals -- Advanced

There are four basic principles for reading and writing Roman numerals:
  • 1. A letter repeats its value that many times (XXX = 30, CC = 200, etc.).
    A letter can only be repeated three times.


  • 2. If one or more letters are placed after another letter of greater value, add that amount.
    VI = 6 (5 + 1 = 6)
    LXX = 70 (50 + 10 + 10 = 70)
    MCC = 1200 (1000 + 100 + 100 = 1200)

  • 3. If a letter is placed before another letter of greater value, subtract that amount.
    IV = 4 (5 – 1 = 4)
    XC = 90 (100 – 10 = 90)
    CM = 900 (1000 – 100 = 900)
    Several rules apply for subtracting amounts from Roman numerals:
    • a. Only subtract powers of ten (I, X, or C, but not V or L)
      For 95, do NOT write VC (100 – 5).
      DO write XCV (XC + V or 90 + 5)
    • b. Only subtract one number from another.
      For 13, do NOT write IIXV (15 – 1 - 1).
      DO write XIII (X + I + I + I or 10 + 3)
    • c. Do not subtract a number from one that is more than 10 times greater (that is, you
          can subtract 1 from 10 [IX] but not 1 from 20—there is no such number as IXX.)
      For 99, do NOT write IC (C – I or 100 - 1).
      DO write XCIX (XC + IX or 90 + 9)

  • 4. A bar placed on top of a letter or string of letters increases the numeral's value by 1,000 times.
    XV = 15, (X-bar)(V-bar)= 15,000


Roman Numerals -- Examples

One I Eleven XI Thirty XXX
Two II Twelve XII Forty XL
Three III Thirteen XIII Fifty L
Four IV Fourteen XIV Sixty LX
Five V Fifteen XV Seventy LXX
Six VI Sixteen XVI Eighty LXXX
Seven VII Seventeen XVII Ninety XC
Eight VIII Eighteen XVIII One hundred C
Nine IX Nineteen XIX Five hundred D
Ten X Twenty XX One thousand M