Originally Posted by T-Mar

Typically, the diameter is stamped on the seat post, either a at the top or near the bottom. You will have to remove the post to check the latter.

Alternatively, you can wrap a 1cm x 10cm strip of paper tightly around a clean, unmarred section of the seat post. Using a sharp pencil, mark where it overlaps itself. Remove the paper and measure from the end of the paper strip to the mark. This will provide the circumference which you divide by pi to determine the diameter. Repeat to ensure accuracy. Using this method you should be capable of measuring the circumference accurately to within 0.5mm which will provide a diameter accurate to within 0.16mm, While this method is not as good as a set of calipers, it is far more accurate than trying to measure the diameter directly, using a ruler. By measuring the circumference, as opposed to the diameter, you effectively increase the resolution by a factor of pi.