TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT maximum storage sizes

From the documentation (MySQL 8) :

Type | Maximum length
-----------+-------------------------------------
  TINYTEXT |           255 (2 8−1) bytes
      TEXT |        65,535 (216−1) bytes = 64 KiB
MEDIUMTEXT |    16,777,215 (224−1) bytes = 16 MiB
  LONGTEXT | 4,294,967,295 (232−1) bytes =  4 GiB

Note that the number of characters that can be stored in your column will depend on the character encoding.

Expansion of the same answer

This SO post outlines in detail the overheads and storage mechanisms. As noted from point (1), A VARCHAR should always be used instead of TINYTEXT. However, when using VARCHAR, the max rowsize should not exceeed 65535 bytes. As outlined here http://dev.mysql.com/doc/refman/5.0/en/charset-unicode-utf8.html, max 3 bytes for utf-8.

THIS IS A ROUGH ESTIMATION TABLE FOR QUICK DECISIONS!

So the worst case assumptions (3 bytes per utf-8 char) to best case (1 byte per utf-8 char) Assuming the english language has an average of 4.5 letters per word x is the number of bytes allocated

x-x

      Type | A= worst case (x/3) | B = best case (x) | words estimate (A/4.5) - (B/4.5)
-----------+---------------------------------------------------------------------------
  TINYTEXT |              85     | 255               | 18 - 56
      TEXT |          21,845     | 65,535            | 4,854.44 - 14,563.33  
MEDIUMTEXT |       5,592,415     | 16,777,215        | 1,242,758.8 - 3,728,270
  LONGTEXT |   1,431,655,765     | 4,294,967,295     | 318,145,725.5 - 954,437,176.6

Please refer to Chris V's answer as well : https://stackoverflow.com/a/35785869/1881812

Rising to @Ankan-Zerob's challenge, this is my estimate of the maximum length which can be stored in each text type measured in words:

      Type |         Bytes | English words | Multi-byte words
-----------+---------------+---------------+-----------------
  TINYTEXT |           255 |           ±44 |              ±23
      TEXT |        65,535 |       ±11,000 |           ±5,900
MEDIUMTEXT |    16,777,215 |    ±2,800,000 |       ±1,500,000
  LONGTEXT | 4,294,967,295 |  ±740,000,000 |     ±380,000,000

In English, 4.8 letters per word is probably a good average (eg norvig.com/mayzner.html), though word lengths will vary according to domain (e.g. spoken language vs. academic papers), so there's no point being too precise. English is mostly single-byte ASCII characters, with very occasional multi-byte characters, so close to one-byte-per-letter. An extra character has to be allowed for inter-word spaces, so I've rounded down from 5.8 bytes per word. Languages with lots of accents such as say Polish would store slightly fewer words, as would e.g. German with longer words.

Languages requiring multi-byte characters such as Greek, Arabic, Hebrew, Hindi, Thai, etc, etc typically require two bytes per character in UTF-8. Guessing wildly at 5 letters per word, I've rounded down from 11 bytes per word.

CJK scripts (Hanzi, Kanji, Hiragana, Katakana, etc) I know nothing of; I believe characters mostly require 3 bytes in UTF-8, and (with massive simplification) they might be considered to use around 2 characters per word, so they would be somewhere between the other two. (CJK scripts are likely to require less storage using UTF-16, depending).

This is of course ignoring storage overheads etc.

This is nice but doesn't answer the question:

"A VARCHAR should always be used instead of TINYTEXT." Tinytext is useful if you have wide rows - since the data is stored off the record. There is a performance overhead, but it does have a use.