它的规则如下：

1.所有的索引字段，如果没有设置not null，则需要加一个字节。
2.定长字段，int占四个字节、date占三个字节、char(n)占n个字符。
3.对于变成字段varchar(n)，则有n个字符+两个字节。
4.不同的字符集，一个字符占用的字节数不同。latin1编码的，一个字符占用一个字节，gbk编码的，一个字符占用两个字节，utf8编码的，一个字符占用三个字节。


举例说明:

mysql> desc person;
+-------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id| varchar(100) | NO| PRI | ||
| name | varchar(100) | YES | MUL | NULL||
| age| int(10) | YES | | NULL||
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)


查看索引情况:
mysql> show index from person;
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| person | 0 | PRIMARY |1 | id | A |2 | NULL | NULL| | BTREE | ||
| person | 1 | idx_name |1 | name| A |2 | NULL | NULL| YES | BTREE | ||
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)


mysql> explain select name from person;
+----+-------------+--------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra|
+----+-------------+--------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | person | index | NULL | idx_name | 103 | NULL |2 | Using index |
+----+-------------+--------+-------+---------------+----------+---------+------+------+-------------+
1 row in set (0.00 sec)


根据以上计算规则:
所有的索引字段，如果没有设置not null，则需要加一个字节
对于变成字段varchar(n)，则有n个字符+两个字节
因此:key_len=100+2+1=103