Is it true that 26" rim actual diameter is 25.625"? How does tire size affect wheel diameter? Is there any special formula to calculate wheel diameter?
You could calculate the effective diameter by measuring rollout and dividing that number by pi. To measure the rollout start with your valve stem in the lowest possible position and make a mark on the floor next to the valve stem. Ride the bike one revolution until the valve stem is at the bottom of it's rotation again and make another mark on the floor. Measure the distance between the two marks to find the effective circumference.
I wouldn't trust any formula, since the actual outside diameter can be affected by so many things, and since the advertised tire width isn't always accurate. If you want the diameter, do like Russ said and measure it yourself. You might also be interested in what Sheldon Brown had to say on this subject.