How to read and write Roman numerals
HESI A2 topic: Roman numerals
- I=1, V=5, X=10, L=50, C=100, D=500, M=1,000.
- Larger to smaller, you add: XVII = 17.
- Subtract only in six pairs: IV, IX, XL, XC, CD, CM.
- Only I, X, C and M may repeat, up to three times.
- To write a number, convert one place value at a time.
Key facts
- I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1,000.
- When symbols go from larger to smaller, add them: LXVI = 50 + 10 + 5 + 1 = 66.
- Only six subtractive pairs are standard: IV (4), IX (9), XL (40), XC (90), CD (400), CM (900).
Show all 7 key factsShow fewer
- I, X, C and M can repeat up to three times in a row; V, L and D are not repeated.
- Roman numerals have no zero and no place value, so you read them symbol by symbol, not digit by digit.
- To write a number, split it by place value and write each part: 48 = 40 + 8 = XL + VIII = XLVIII.
- In anatomy, the 12 cranial nerves are numbered with Roman numerals, CN I to CN XII.
Quick check1 question · 2 more at the end
What is the value of the Roman numeral XLVI?
Correct answer: D.
X comes before the larger L, so XL is a subtractive pair: 50 − 10 = 40. The rest goes from larger to smaller, so add: V + I = 6. The total is 40 + 6 = 46.
A This adds every symbol (10 + 50 + 5 + 1) instead of treating XL as 50 − 10 = 40.
B This reads VI as IV (4); VI has the smaller symbol second, so it is 5 + 1 = 6.
C This drops the X and adds L + V + I = 56, ignoring the subtractive pair XL.
D Correct: XL = 40 and VI = 6, so XLVI = 46.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Roman numerals use seven symbols: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1,000. Symbols written from largest to smallest are added (XVII = 17). A smaller symbol before a larger one is subtracted, but only in six pairs: IV, IX, XL, XC, CD and CM.
1 What do the seven symbols mean?Every Roman numeral is built from just seven letters, and each one has a fixed value. 1 min
Every Roman numeral is built from just seven letters, and each one has a fixed value. One college's description of HESI A2 Math score reports lists Roman numerals under general math facts, so these seven are worth knowing cold.
| Symbol | Value | Pattern |
|---|---|---|
| I | 1 | a "one" |
| V | 5 | a "five" |
| X | 10 | a "one" |
| L | 50 | a "five" |
| C | 100 | a "one" |
| D | 500 | a "five" |
| M | 1,000 | a "one" |
The "ones" (I, X, C, M) are powers of 10, and the "fives" (V, L, D) sit halfway between them. The system has no zero and no place value.
2 When do you add and when do you subtract?You add when symbols go from larger to smaller, and you subtract only when a "one" sits directly in front of one of the next two bigger symbols. 1 min
You add when symbols go from larger to smaller, and you subtract only when a "one" sits directly in front of one of the next two bigger symbols.
- Add: CLXII = 100 + 50 + 10 + 1 + 1 = 162.
- Repeat: I, X, C and M may appear up to three times in a row (XXX = 30). V, L and D never repeat.
- Subtract: only six pairs are standard. I goes before V or X (IV = 4, IX = 9). X goes before L or C (XL = 40, XC = 90). C goes before D or M (CD = 400, CM = 900).
Because of the three-in-a-row limit, 4 is IV rather than IIII, and 400 is CD rather than CCCC.
3 How do I turn a Roman numeral into a number?Read from left to right, treat each subtractive pair as one chunk, then add the chunks. 1 min
Read from left to right, treat each subtractive pair as one chunk, then add the chunks.
Worked example 1. What is CDXLVII?
- Spot the pairs: CD (smaller before larger) and XL (smaller before larger).
- Chunks: CD = 400, XL = 40, V = 5, I = 1, I = 1.
- Add: 400 + 40 + 5 + 1 + 1 = 447.
Classic wrong turn: adding every symbol (C + D + X + L + ...) and getting 667. Whenever a smaller symbol comes first, that pair is a subtraction.
4 How do I write a number as a Roman numeral?Break the number into thousands, hundreds, tens and ones, write each part, and join them in order. 1 min
Break the number into thousands, hundreds, tens and ones, write each part, and join them in order.
Worked example 2. Write 1,968.
- Split: 1,000 + 900 + 60 + 8.
- Write each part: M, CM, LX, VIII.
- Join: MCMLXVIII.
Worked example 3. Write 349.
- Split: 300 + 40 + 9.
- Write: CCC, XL, IX.
- Join: CCCXLIX.
Classic wrong turn: writing 349 as CCCIL, which subtracts I from L. I can only come before V or X, so each place value must be written on its own.
To check your practice answers, use the free Roman numeral converter. It splits each answer by place value.
5 Where will I see Roman numerals in health care?In anatomy, the 12 cranial nerves are numbered with Roman numerals, from CN I through CN XII. 1 min
In anatomy, the 12 cranial nerves are numbered with Roman numerals, from CN I through CN XII. For example, CN I is the olfactory nerve, CN X is the vagus nerve and CN XII is the hypoglossal nerve.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: 99 is XCIX, not IC; I can be subtracted only from V and X.
Watch out: 4 is IV and 40 is XL in the standard form; do not repeat a symbol four times (IIII, XXXX).
Watch out: V, L and D are never subtracted: 45 is XLV, not VL.
Watch out: CD is 400 and DC is 600; the order decides whether you subtract or add.
Watch out: Read a subtractive pair as one unit: in XIV, the I belongs to the V, so it is 10 + 4 = 14.
How to remember it
The symbols in order from biggest to smallest are M, D, C, L, X, V, I.
Memory hook: "My Dear Cat Loves Xtra Vitamins Intensely" (M, D, C, L, X, V, I).
Self-check:
- What is MMXIV?
- Write 94 and 444 in Roman numerals.
- Explain why VL is not a correct way to write 45.
Show answers
- 2014.
- Chunks: MM = 2,000, X = 10, IV = 4 (I before V is a subtractive pair).
- Add: 2,000 + 10 + 4 = 2,014.
- XCIV and CDXLIV.
- 94 = 90 + 4 = XC + IV = XCIV.
- 444 = 400 + 40 + 4 = CD + XL + IV = CDXLIV.
- Check: no symbol repeats four times, and every pair is one of the six standard pairs.
- V is never subtracted.
- Only I, X and C can go before a bigger symbol, so VL is not a standard pair.
- Write 45 one place at a time: 40 + 5 = XL + V = XLV.