Maximum length for MySQL type text

TINYTEXT: 256 bytes
TEXT: 65,535 bytes
MEDIUMTEXT: 16,777,215 bytes
LONGTEXT: 4,294,967,295 bytes

See for maximum numbers: http://dev.mysql.com/doc/refman/5.0/en/storage-requirements.html

TINYBLOB, TINYTEXT       L + 1 bytes, where L < 2^8    (255 Bytes)
BLOB, TEXT               L + 2 bytes, where L < 2^16   (64 Kilobytes)
MEDIUMBLOB, MEDIUMTEXT   L + 3 bytes, where L < 2^24   (16 Megabytes)
LONGBLOB, LONGTEXT       L + 4 bytes, where L < 2^32   (4 Gigabytes)

L is the number of bytes in your text field. So the maximum number of chars for text is 2¹⁶-1 (using single-byte characters). Means 65 535 chars(using single-byte characters).

UTF-8/MultiByte encoding: using MultiByte encoding each character might consume more than 1 byte of space. For UTF-8 space consumption is between 1 to 4 bytes per char.

TINYTEXT 256 bytes
TEXT 65,535 bytes ~64kb
MEDIUMTEXT 16,777,215 bytes ~16MB
LONGTEXT 4,294,967,295 bytes ~4GB

TINYTEXT is a string data type that can store up to to 255 characters.

TEXT is a string data type that can store up to 65,535 characters. TEXT is commonly used for brief articles.

LONGTEXT is a string data type with a maximum length of 4,294,967,295 characters. Use LONGTEXT if you need to store large text, such as a chapter of a novel.

Basically, it's like:

"Exact Max. Length Allowed" = "Approx. Length" in bytes - 1

Note: If using multibyte characters (like Arabic, where each Arabic character takes 2 bytes), the column "Exact Max. Length Allowed" for TINYTEXT can hold be up to 127 Arabic characters (Note: space, dash, underscore, and other such characters, are 1-byte characters).