The maths is generally not so simple as to suit an online calculator. That said, it's not overwhelming complex either. "Building the Timber Frame House" by Tedd Benson has a fairly straightforward treatment of it at the back of the book. Failing that, I can dig out the various articles I used to build the software that I verified my beams with.*

*Before you ask, the software is far from ready for public consumption for a great many reasons, and offering it for use would open me up to legal nightmares if I've got the maths wrong anywhere!