diff --git a/docs/reference/bluetooth/advertise-url.md b/docs/reference/bluetooth/advertise-url.md
index 76a981e2..852f4090 100644
--- a/docs/reference/bluetooth/advertise-url.md
+++ b/docs/reference/bluetooth/advertise-url.md
@@ -23,7 +23,7 @@ bluetooth.advertiseUrl("https://pxt.microbit.org/", 7, true);
 
 ### Parameters
 
-* ``url`` - a [string](/reference/types/string) containing the URL to broadcast, at most 18 characters long
+* ``url`` - a [string](/reference/types/string) containing the URL to broadcast, at most 17 characters long, excluding the protocol (eg: ``https://``) which gets encoded as 1 byte.
 * ``power`` - a [number](/reference/types/number) representing the power level between 0 (short) and 7 (maximum range).
 
 ### Example: Broadcast a secret code
@@ -38,4 +38,4 @@ bluetooth.advertiseUrl("https://pxt.io?secret=42", 7, true);
 
 ```package
 bluetooth
-```
\ No newline at end of file
+```