I do have callipers just for this purpose but when I tried to use them, I couldn't tell if they got all the way around the pipe or not. So I thought that I may as well use the formula for the circumference of a circle to confirm my measurement.
I used a flexible tape measure used for sewing to measure around the pipe, which gave me the circumference. In my case it was 243mm.
Now the formula for the circumference is 2 × pi × r. With that and a little bit of algebra it's easy enough to work out the radius...
Now that of course gives the radius and I was after the diameter, so just multiply by two (or I could have just not multiplied pi by 2 above).
So the diameter of my pipe according to the calculation is 77.35mm. This is not a standard copper pipe size, which means my circumference measurement was wrong (confirmed later it was off by 5mm). It's important to get this measurement right, however it's not a show stopper if you just need an approximate value and can just match to the closest standard size (you're not likely to have non-standard sized pipes).
Incidentally this is what my callipers said...76mm, which was spot on after all.