Calibrate your screen against a credit card, then read a tumbling-E chart that asks only which way the E opens, to approximate decimal and Snellen acuity. It is not a substitute for an eye exam.
Asking only which way the E opens keeps the task identical for people who do not read Latin letters. The Snellen letter chart is public domain but still needs letter knowledge.
When a row would need fewer pixels than the screen can draw, the test stops and says so — going smaller would mean resolution, not vision, is what you cannot see.
Screen brightness, distance error and pixel limits are all mixed in. It is not a diagnosis — it tells you roughly where rows start to blur.
Anything to ISO 7810 ID-1 (85.60 × 53.98 mm) works. Credit and debit cards and most national ID and driving licences are all that size.
The radial dial below lets you check by eye. Lines in one direction looking much darker or blurrier can indicate astigmatism, but no score comes out of it.