Step 1: Determine your current fit numbers
What we want is the X/Y coordinate to the top/center of your aerobar pads, measured from the bottom bracket. Most bike fitters can provide this number to you when performing a bike fit (although some provide it to the top/back of the pads rather than top/center). If you already know these numbers, you can skip this step. Otherwise, follow the steps below to determine these numbers for your current bike setup:
1-Place your bicycle up against the corner of a room, with the back tire abutting the wall.
2-Next, measure the vertical distance from the floor to the center of the bottom bracket. We'll call this measurement Floor Offset (FO).
3-Measure the horizontal distance from the wall to the center of the bottom bracket, we'll call this measurement Wall Offset (WO).
4-Measure the vertical distance from the floor to the top of your aerobar pads. This measurement will be Pads-To-Floor (PTF).
5-Measure the horizontal distance from the wall to the center of your aerobar pads. This will be called Pads-To-Wall (PTW).
6-Subtract Floor Offset from Pads-To-Floor (PTF minus FO). This is your Pad Stack.
7-Subtract Wall Offset from Pads-To-Wall (PTW minus WO). This is your Pad Reach.