...and can be measured at home using a ruler and a mirror, pretty simply. Close one eye, align the ruler at 0 under that pupil. Open the other eye, close the first, read off the distance to the 2nd pupil.
I measured by taking a picture of my face with a credit card for reference and measuring the pixels with the measuring tool in GIMP. (These were the actual instructions on the website I ordered my glasses from.)
Also worth mentioning I paid $40 ordering them online, in the article it says he paid $280!
https://www.glasses24.com/customer-support/shipping-and-deli...
I was amazed because until a friend told me about this site, I've been paying hundreds of euros just for the frames! At such a low cost, I could even order slight variations on the strength.
They have a lot of options for better lens material, have it darken in sunlight, anti glare coating etc, but I just got the cheapest one and it was fine.
This means the frame must be chosen and fitted to you before any of these are measured.
It becomes a 2D measurement.
There is a lot more to it (vertex distance, pantoscopic tilt etc) but the above is the absolute minimum.
Ed: i see @andai made a similar comment.
Might indeed be better to use a "long" object to indicate pixel length by division (credit card is 5432 px = 5 cm) rather than going by a ruler which might typically only be accurate to 0.5mm at best anyway.