Here is info from some real research:

Approximately 10 percent to 20 percent of the THC is transferred into the body when smoking a joint.

About 40 percent to 50 percent of the THC is transferred into the body when using a pipe.

Oral bioavailability of THC, whether given in the pure form or as THC in marijuana, also is low and extremely variable, ranging between 5 percent and 20 percent. The amount of drug absorbed is about one-third of that which would be absorbed by smoking.