Bike Forums - Calibrating a computer

For my computer, I need to enter in wheel circumference. Now, I'm a math teacher -- I can compute just fine!!! But what I can't seem to get is an accurate measurement of my wheel's diameter. The old tape measurer just doesn't cut it.

I'm very much a newbie, so I don't fully understand how tire sizes are given. My tire says something like "27 by 1 1/8." Assuming the tire is inflated correctly, what would the diameter be? Oh yeah, I need to convert that to millimeters too. I can look up that conversion factor, unless anyone knows the answer off the top of their head.

Thanks!