Roman numerals are expressed by letters of the alphabet:
I=1
V=5
X=10
L=50
C=100
D=500
M=1000

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

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