The Roman Numeral System is an old numeration system that we still used today, you can find some on buildings, clocks and books. I will outline the approach I used in dealing with converting Roman numerals to HinduArabic system and from HinduArabic numerals to Roman numerals.
Before we begin, it is important that you are familiar with place values (e.g., ones, tens, hundreds etc.) , as this would be our key tool. Dealing with Roman Numerals, we need to familiarize ourselves with some ground rules.
I. Get familiar with the basic Roman Numerals with fixed values.
HinduArabic Numerals 
1 
5 
10 
50 
100 
500 
1000 
Roman Numerals 
I 
V 
X 
L 
C 
D 
M 
II. We add the HinduArabic values for same or smaller values that is written on the right side of a Roman numeral.
Example: XXII = 10+10+1+1 = 22
III. We subtract the HinduArabic values for smaller values that is written on the left side of a Roman numeral.
Example: XL = 50 – 10 = 40
IV. Roman numeral symbols I, X, C and M can be written three times in a sequence.
Example: CCC = 100+100+100 = 300
V. Roman numeral symbols V, L, and D cannot be written repeatedly in a sequence.
It would be helpful if you get familiar with the table below:
HinduArabic Numerals 
4 
9 
40 
90 
400 
900 
Roman Numerals 
IV 
IX 
XL 
XC 
CD 
CM 
Identifying the Place Values plays a key role in expressing Roman numerals to HinduArabic and from HinduArabic to Roman Numerals.
Expressing Roman Numerals to HinduArabic Numerals. Our approach is laid out in the following steps
Take for example: CCXXIX
1. Identify the place values involved. Looking at our example the presence of the Roman numerals C indicate that we are dealing with a 3digit HinduArabic numeral to it’s hundreds place.
2. Group the Roman numerals according to their place values.
Hundreds 
Tens 
Ones 
CC 
XX 
IX 
3. Get the respective HinduArabic values in each place values. (Use the ground rules (II and/or III), we laid out earlier on this article, as necessary)
Hundreds 
Tens 
Ones 
CC 
XX 
IX 
100+100 
10+10 
101 
4. Get the sum of the resulting numbers in each place values.
Hundreds 
Tens 
Ones 
CC 
XX 
IX 
200 
20 
9 
229 
More examples:
Example: XCIV
Hundreds 
Tens 
Ones 
XC 
IV 

10010 
51 

90 
0 
4 
94 
Example: DXV
Hundreds 
Tens 
Ones 
D 
X 
V 
500 
10 
5 
515 
Expressing HinduArabic Numerals. Our approach is basically the same as mention above but this time it’s the other way around
Take for example: 612
1. Identify the place value involved. We can clearly see that this one contains a hundreds place value
2. Expand the HinduArabic numeral according to its place values.
Hundreds 
Tens 
Ones 
600 
10 
2 
3. Convert each HinduArabic place values to Roman symbols. Use the ground rules as necessary.
Hundreds 
Tens 
Ones 
600 
10 
2 
DC 
X 
II 
4. Combine the resulting roman numeral symbols in each place value, starting from left to right.
Hundreds 
Tens 
Ones 
600 
10 
2 
DC 
X 
II 
DCXII 
More examples:
Example: 974
Hundreds 
Tens 
Ones 
900 
70 
4 
CM 
LXX 
IV 
CMLXXIV 
Example: 450
Hundreds 
Tens 
Ones 
400 
50 
0 
CD 
L 

CDL 