# How to Measure Your IPD

Update: There have been complaints that the post below is an overly complicated and confusing explanation of the IPD measurement process. Maybe that’s so. Therefore, here’s the TL;DR version of how the process works. If you want to know why it works, read on below.

1. Stand in front of a mirror and hold a ruler up to your nose, such that the measuring edge runs directly underneath both your pupils.
2. Close your right eye and look directly at your left eye. Move the ruler such that the “0” mark appears directly underneath the center of your left pupil. Try to keep the ruler still for the next step.
3. Close your left eye and look directly at your right eye. The mark directly underneath the center of your right pupil is your inter-pupillary distance.

Here follows the long version:

I’ve recently talked about the importance of calibrating 3D displays, especially head-mounted displays, which have very tight tolerances. An important part of calibration is entering each user’s personal inter-pupillary distance. Even when using the eyeball center as projection focus point (as I describe in the second post linked above), the distance between the eyeballs’ centers is the same as the inter-pupillary distance.

So how do you actually go about determining your IPD? You could go to an optometrist, of course, but it turns out it’s very easy to do it accurately at home. As it so happened, I did go to an optometrist recently (for my annual check-up), and I asked him to measure my IPD as well while he was at it. I was expecting him to pull out some high-end gizmo, but instead he pulled up a ruler. So that got me thinking.

Figure 1: How to precisely measure infinity-converged inter-pupillary distance using only a mirror and a ruler. Focus on the left eye in step one and mark point A; focus on the right eye in step two and mark point B; the distance between points A and B is precisely the infinity-converged inter-pupillary distance (and also the eyeball center distance).