diff --git a/diff.c b/diff.c
index 0f839c11b7..ffe8a55234 100644
--- a/diff.c
+++ b/diff.c
@@ -838,29 +838,38 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 
 static int parse_num(const char **cp_p)
 {
-	int num, scale, ch, cnt;
+	unsigned long num, scale;
+	int ch, dot;
 	const char *cp = *cp_p;
 
-	cnt = num = 0;
+	num = 0;
 	scale = 1;
-	while ('0' <= (ch = *cp) && ch <= '9') {
-		if (cnt++ < 5) {
-			/* We simply ignore more than 5 digits precision. */
-			scale *= 10;
-			num = num * 10 + ch - '0';
+	dot = 0;
+	for(;;) {
+		ch = *cp;
+		if ( !dot && ch == '.' ) {
+			scale = 1;
+			dot = 1;
+		} else if ( ch == '%' ) {
+			scale = dot ? scale*100 : 100;
+			cp++;	/* % is always at the end */
+			break;
+		} else if ( ch >= '0' && ch <= '9' ) {
+			if ( scale < 100000 ) {
+				scale *= 10;
+				num = (num*10) + (ch-'0');
+			}
+		} else {
+			break;
 		}
 		cp++;
 	}
 	*cp_p = cp;
 
-	/* special case: -M100 would mean 1.0 not 0.1 */
-	if (num == 100 && scale == 1000)
-		return MAX_SCORE;
-
 	/* user says num divided by scale and we say internally that
 	 * is MAX_SCORE * num / scale.
 	 */
-	return (MAX_SCORE * num / scale);
+	return (num >= scale) ? MAX_SCORE : (MAX_SCORE * num / scale);
 }
 
 int diff_scoreopt_parse(const char *opt)