1: I, 5: V, 10: X, 50: L, 100: C, 500: D, 1000: M
If a digit is 3 or less, use up to 3 I, X, C, or M. For example, 3 is III. If a digit is 4, use I, X, or C followed by V, L, or D. For example, 4 is IV. If a digit is 8 or less, use V, L, or D followed by up to 3 I, X, or C. For example, 8 is VIII. If a digit is 9, use I, X, or C followed by X, C, or M. For example, 9 is IX.