Base64 Valid Character Set

I found this on a forum post, and just wanted to verify that this information is correct:

for base64 the valid charset is:
ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwx yz0123456789+/
the = is used as filler for the last bytes, as the length must be mulitple
of 3


The charset looks reasonable, but must the length be a multiple of 3? (Seems like a multiple of 4 would make more sense.)

Comments very much appreciated!