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).

The problem in measuring your IPD is that IPD depends on eye convergence; as your eyes rotate inwards to foveate on close-by objects, IPD decreases because the eyeballs rotate around their center points. Meaning, if you hold a ruler up to your eyes and try to focus on it, you’re going to measure your near-converged IPD, not the infinity-converged IPD (which is what you want).

But there is a very simple and elegant solution; it only requires a mirror and a ruler. Check out Figure 1 below. If you look at your reflection in a mirror, and, say, focus on your left eye, then your left eye is going to look straight out. If you then focus on your right eye, now your right eye is going to look straight out, meaning the left- and right-eye viewing directions in steps 1 and 2 are parallel, as if you had foveated on an object at infinity.

So here’s the procedure: Look at your own face in a mirror, and hold up a ruler such that it’s right underneath both your pupils, and hold the ruler parallel to the mirror (that’s important!). Now focus both eyes on your left eye’s reflection, and move the ruler such that the “0” marker, as seen in the mirror, is directly underneath the center of your left pupil. Then hold the ruler very still, and focus both eyes on your right eye’s reflection. Whatever tick mark shows up underneath the center of your right pupil is your precise infinity-converged inter-pupillary distance (and also the distance between your eyeballs’ centers). Voila!

In theory you can hold the ruler anywhere between your eyes and the mirror, but the closer it is to your eyes, the easier it is to read the tick marks when focused on your eyes’ reflections.

Update: After this post went online, Twitter user Sebastian Sylvan pointed out that I had forgotten an important detail — eye dominance. Let’s say, for the sake of discussion, that Alice is left-eye dominant, meaning that if there is discrepancy between what she sees with her left and right eyes, the impression in the left eye will override the one in the right eye. Now apply that to Figure 1, and you get Figure 2:

Figure 2: Eye dominance problem when measuring IPD. In this example, the viewer is left-eye dominant, and marks down a wrong position for point B due to the parallax induced by the ruler being some distance in front of the eyes. Solution: close the “off” eye when marking down points A and B.

In the first step, Alice correctly moves the ruler so that the zero tick mark coincides with point A, but in the second step, notice how she marks down the wrong point B, because she places it where her dominant left eye sees her right-eye pupil behind the ruler, instead of where her right eye sees it. The ruler being held some distance in front of her eyes causes a parallax shift, and leads her to underestimate her IPD. The solution is quite simple: when focusing with the left and right eyes in steps 1 and 2, respectively, just close the “off” eye, i.e., the right eye in step 1 and the left eye in step 2, to avoid dominance issues. Just be careful not to move the ruler while opening one eye and closing the other. I recommend holding the ruler against your nose to keep it securely in place.