Reading Roman numerals
To read a Roman numeral, you go letter by letter from left to right. There are two simple rules: when the values decrease you add, and when a smaller letter is before a bigger one you subtract.
The two-rule method
- Decreasing values — add the values.
- VIII: 5 + 1 + 1 + 1 = 8
- XV: 10 + 5 = 15
- LXX: 50 + 10 + 10 = 70
- A smaller before a bigger — subtract the small one from the big one, then continue.
- IV: 5 − 1 = 4
- IX: 10 − 1 = 9
- XL: 50 − 10 = 40
- XIV: 10 + (5 − 1) = 14
- XXIX: 10 + 10 + (10 − 1) = 29
The trick is to scan ahead one letter. If the next letter is bigger than the current one, you've found a subtractive pair — combine them before adding.
A step-by-step reading
Read LXXVIII.
Scan left to right:
- L = 50.
- X = 10. Next letter is X (same value, not bigger) → keep adding: 50 + 10 = 60.
- X = 10. Next letter is V (smaller) → keep adding: 60 + 10 = 70.
- V = 5. Next letter is I (smaller) → keep adding: 70 + 5 = 75.
- I = 1. End: 75 + 1 = 76.
- I = 1. End: 76 + 1 = 77.
Wait — I have one too many. Let me re-count the symbols. L X X V I I I — that's L + X + X + V + I + I + I. Compute: 50 + 10 + 10 + 5 + 1 + 1 + 1 = 78.
That's why slow, careful reading matters!
Reading with subtractive pairs
Read XCIV.
Scan left to right:
- X = 10. Next letter is C (bigger!) → this is a subtractive pair: XC = 100 − 10 = 90.
- I = 1. Next letter is V (bigger!) → IV = 5 − 1 = 4.
- Add the parts: 90 + 4 = 94.
The whole numeral breaks into two pieces: XC + IV = 90 + 4 = 94.
A reading table for common values
| Numeral | Value | Numeral | Value |
|---|---|---|---|
| I | 1 | XX | 20 |
| II | 2 | XXX | 30 |
| III | 3 | XL | 40 |
| IV | 4 | L | 50 |
| V | 5 | LX | 60 |
| VI | 6 | LXX | 70 |
| VII | 7 | LXXX | 80 |
| VIII | 8 | XC | 90 |
| IX | 9 | C | 100 |
| X | 10 |
Notice the pattern: tens go X, XX, XXX, then XL (40, subtractive), then L, LX, LXX, LXXX, then XC (90, subtractive), then C.
Common mistakes when reading
- Adding when you should subtract. IV is 4, not 6. The I comes before the V, not after.
- Reading right to left. Always left to right.
- Counting too many letters. III is 3, IIII is not standard (and would be 4 if it were).
- Confusing L with 1 or with I. L is fifty.