Measure the body from the front of the body, but keep in mind that it's just a guideline and all bars are not measured the same. All brands measure the width at the ends, but some measure center to center and others measure outside to ouside, which makes them about 2cm narrower than the same size measured c-c. Also, there are some bars that place the brake hoods 1-2cm narrower than the width measured at the ends and it's really the brake hood c-c measurement that's most important. I owned some FSA compact bars that were 40cm at the ends, but the brake hood spacing was only 38cm.