diff --git a/README.md b/README.md
index b678ac5..25131d8 100644
--- a/README.md
+++ b/README.md
@@ -25,6 +25,12 @@ from sms_counter import SMSCounter
 >>> {'length': 29, 'messages': 1, 'remaining': 41, 'per_message': 70, 'encoding': 'UTF16'}
 ```
 
+The meaning of the `length`, `remaining` and `per_message` values returned by `SMSCounter.count()` depend on the encoding. 
+
+For GSM_7BIT_EX encoding, `length`, `remaining` and `per_message` count the number of 7-bit characters in the message, __including__ the escape character that must precede any characters in the "extended" character set. For example, the `length` of the message '€' is 2, because it takes 2 7bit characters to encode '€' in GSM_7BIT_EX.
+
+For UTF16 and GSM_7BIT encoding, `length`, `remaining` and `per_message` count the number of characters (since all characters have an equal bit width).
+
 #### Mentions
 
 * Original idea : [danxexe/sms-counter](https://github.com/danxexe/sms-counter)