A 700c with a 38mm tyre will measure up at 27 inches or perhaps a little under. More like ~ 680-690mm rather than the 700mm that the size would suggest.
For mountain bikes the following is true when using a normal mountain bike tyre measured from edge to edge.
Also included the actual outer rim diameter too.
"26 inch" = 26 inches - outer rim diameter = 559mm
"650b" = 27.5 inches - outer rim diamater = 584mm
"29er" = 29 inches - outer rim diameter = 622mm
29er rims are the same diameter as those used for 700c wheels, it's just the tyre sizes that make for a different diameter.
The volume and side-wall height of the tyre make up part of the wheel diameter so there is quite alot of variation.