How many bytes should i allocate for base64 decode?

I would like to use this https://opensource.apple.com/source/QuickTimeStreamingServer/QuickTimeStreamingServer-452/CommonUtilitiesLib/base64.c.auto.html implementation of a base64 decoder but I don't know how much space should I allocate for it.

unsigned char *buff = calloc(strlen(input_data) * SOMETHING, sizeof(unsigned char));
int len = Base64decode(buff, input_data);