How I measured my Pupillary Distance

Surfing the net, I ran into a site selling relatively cheap prescription eyeglasses (free plug: Zenni Optical, frames as cheap as $8.95, I got the 3148 Rimless Flexible (Memory) Titanium for about $50 with all options). I was able to locate my 2 year old prescription, which I know is still good because I see perfectly with the scratched pair of glasses I am replacing. I have astigmatism, which doesn’t change very often, and my prescription is a rather minimal -0.50 SPH in both eyes.

I started to input my prescription into the site, and it seemed that I had everything required to order prescription eyeglasses over the Internet! But just as I was getting ready to hit “order,” I saw this: “PD-Pupillary Distance — MUST SELECT.” I said to myself, okay, it must be on my prescription somewhere. I looked and looked and I could not find it anywhere. So I clicked the question mark, which brought up the following (emphasis mine):

The PD, or Pupillary Distance, is the distance between your two eyes, center to center, the center of one pupil to the center of the other, in Millimeters (1” = 25.4mm). Unfortunately most optometrists leave this quite necessary measurement off from your prescription, but you can measure this yourself.

You can measure this yourself in a mirror (not very accurate, and not generally recommended) or have a friend measure it for you by holding a ruler just under the pupils of your eyes and measuring the distance center to center. You should be looking straight forward fixing your focus at distant object, and not at the person measuring you.

Average is about 62mm, and most, 97% or so, fall within 54 to 74 range. If you are coming up with a number outside of this range consider that you might be making a measurement mistake. Do NOT just guess, or assume you’re just an average 62, and DO NOT measure your old glasses to try to come with this number.

Fair enough. I located a ruler and started measuring my PD looking in the bathroom mirror. I am pretty thorough as I think that any task worth doing is worth doing right, and after a few trials I ended up with something like 65mm.

But.. my scientific mind wondered if there was a more precise way to go about doing this. After all, these are my eyes we’re talking about. There had to be a better way! My Canon 400D digital SLR came to mind, which I set on my Manfrotto tripod, taking about 5 shots and ending up with this:

Pupillary Distance
I used photo-editing software to measure the width of each of my pupils in pixels, split that in half and drew a line straight down to the metric end of the ruler. I then took the reading at each point, subtracted 125mm from 187mm, et viola!, I’m dead-average with a PD of 62mm! I was off by 3mm, or about 5% — I’m very glad I took the time to do this right!

In case you’re wondering why the metric end of the scale is upside-down: the glare from the flash was making it unreadable the other way around, so I flipped it to make the US end of it (inches) unreadable instead. This is perfectly acceptable and does not affect the accuracy of this method. You should be able to do this with any digicam, just make sure the ruler is aligned straight with your eyes!

Note: this method should only be used to measure your distance vision PD, not your near-vision PD (i.e. reading glasses). I am not an optometrist, and all information presented above is purely for informational purposes — use at your own risk, if in doubt go to a professional!

9 Responses to “How I measured my Pupillary Distance”

  1. Tom Says:

    Wow – I was considering doing the Zenni order myself (same frames too – how do you like them???) but did not have my PD. Your method makes good sense. Thanks!

  2. Administrator Says:

    Hi Tom! I just got the glasses (shipping was pretty fast!) and they are great — much better than what I expected. Even the optional clip-on sunshades aren’t that bad. I got 2 different pairs and I’m very satisfied with them so far.

  3. Tom Says:

    Good to hear! I’m going to place my order next week after my checkup (and a fresh prescription in hand). Thanks!

  4. steve Says:

    Great solution to a frustrating problem. Thanks.

  5. Mike Says:

    Thanks for the information. I did this and my PD measued at 66mm. Just to confirm for myself I asked my optician for a “Travel Prescription”, which she gave freely. Their PD of my eyes was exactly the same – 66mm. I am trying Zenni also. Hope it goes well. I have a progressive & astigmatic prescription. I’ll let you know.

  6. Tara Says:

    That’s pretty cool, however, you could have just called the place you had the exam done at and asked them to look at your chart :) If that’s not an option, you can walk into any eyeglass place and ask them to do it and they would for free.

  7. Bes Says:

    That’s a really good solution. I popped in the place that took my prescription and they said they would charge me £25 (~$40) just for the measurement of the PD.
    Thanks

  8. Diskdoctor Says:

    You need to be careful using this method as it will generally give you a reading that is too small. How small depends how close your camera is to your face (further away is better) and how close the ruler is to being in the same plane as your pupils (you want it as close as possible).

    In your picture it looks like the ruler is about 2cm from the pupils. If we assume the camera was 1m away from the pupils and centered then the ratio of the actual pupillary distance to the distance measured using this method is 1020/1000 (it’s just basic geometry). You measured a 62mm PD using the method described. Assuming the numbers I estimated are real then your actual pupillary distance would be 62*1020/1000 = 63.2mm. This is why when they describe how to take this measurement the person doing the measuring needs to open one eye at a time rather than looking with one eye from the middle.

  9. Gisell Says:

    Ha! Awesome! I ran into this exact problem, googled ‘how do i measure my pupillary distance?!’ and found this! First time Google got me exactly what I needed.

    Thanks!

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