Check your eyesight online.
A real letter chart, sized to your screen and your distance. Free, two minutes, nothing uploaded. Not a substitute for an exam, and it says so.
Step 1 · Two things to have ready
A bank card, ID or driver's licence — any of them is 85.6 mm wide, which is how the chart learns the size of your screen. And room to sit back from the screen: one metre is the minimum, two or three is better, and the tool will tell you which lines it can honestly draw at the distance you pick.
Nothing is uploaded. The measurement happens in the browser and stops there.
How a browser can size a chart at all
A wall chart works because two numbers are fixed: the letter is a known height and the wall is a known distance. A screen has neither by default — the same CSS pixel is a different physical size on every display, and the browser has no idea where your head is. So the test measures both before it draws anything.
The screen is scaled against an ISO/IEC 7810 ID-1 card, which is the 85.6 mm rectangle in everyone's wallet: match a rectangle to it and the page knows its own pixels per millimetre. The distance is either one you step back to and set, or one the camera reads from the millimetres between your pupils — the same calibration the reading-distance tool runs. With both in hand, a 20/20 optotype is 2 · d · tan(2.5′) tall, and every other line is that scaled by its own fraction.
The lines are Sloan letters — C D H K N O R S V Z, the ten the standard uses — drawn on the 5 × 5 grid the standard defines rather than typeset in a font, because a font's cap height is not its em size and the physical height is the entire point. Each row is a fresh random draw, so a second run cannot be passed from memory.
Where a screen runs out
The fine lines are the limit. At one metre a 20/20 optotype is about 1.45 mm tall, which on an ordinary 96-pixel-per-inch display is five and a half pixels — a stroke about one pixel wide, thin enough that what you would be reading is the pixel grid rather than the letter. The test withholds any line whose stroke falls under a pixel and a half and tells you what it can draw at the distance you chose instead. Stepping back fixes it: the further away you are, the physically bigger the same visual angle becomes.
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Frequently asked
Can you test your eyesight online?
You can test one part of it. A browser can size a letter chart correctly if it knows two things: how many pixels your screen puts in a millimetre, and how far away you are sitting. Give it both and the sharpness measurement is real. Everything else an eye exam covers is out of reach.
How accurate is an online eye test?
About one chart line either way, once the screen is scaled against a bank card and the distance is measured rather than guessed. That is roughly the spread you get reading the same person twice on a wall chart. Uncalibrated tests that skip both steps are not measuring anything.
What does 20/20 mean?
A letter you read at 20 feet that a typical eye also reads at 20 feet. The fraction compares your distance to the reference distance, so 20/40 means the letter had to come in to 20 feet for you and a typical eye had it at 40. It is an average, not a ceiling.
Is 20/40 vision bad?
It is the line most US states use for an unrestricted driver's licence, so distance detail arrives at about half the usual range and close-up work is untouched. On the glasses scale it sits near a quarter to three quarters of a diopter, which is the mild end.
Can an online test tell me my prescription?
No. A chart line and a lens number are different measurements, and the same line comes from different numbers in different eyes. This tool shows the approximate band a line usually sits in and stops there. The number itself comes from an optician, or from measuring your own blur distance.
When do I need a real eye exam?
Any pain, a sudden change, flashes, new floaters, or one eye going worse on its own: the same day, with a professional. Otherwise on whatever schedule your optometrist sets. This tool never diagnoses anything and does not replace that appointment.
EndMyopia treats myopia as a refractive state, not a medical condition. This is a free educational tool — not medical advice, diagnosis, or a substitute for a licensed optometrist. This is not an eye exam. Measurements depend on how carefully the screen is scaled and the distance is set; expect about one chart line of spread either way. For lens orders, eye health concerns, or any task requiring full 20/20 vision, see a licensed professional.
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