Question: are all bicycle handlebars the same diameter?
Answer: The ISO standard for the stem clamping area of a handlebar is 25.4 mm (1 in), which is used on mountain bikes and many Japanese-made road handlebars. ... A new standard is an "oversize" 31.8 mm (1.252 in) or 31.7 mm (1.248 in) clamp for both MTB and road bars.