Online Eye Chart Test — Landolt C Self-Check

What it is

This is a screen-based eye chart for a quick look at your vision at home. It draws Landolt C rings, the same broken-ring symbol used in clinics, at sizes calculated for your screen and your distance, and you tell it which way each gap points. Because a pixel is a different size on every device, you first match an on-screen box to a real card or ruler. You can run it for the right eye, the left eye or both, and the results for each are listed side by side. It can hint that one eye is weaker, but it is not a diagnosis and does not replace a proper eye exam.

The result is given as decimal acuity, the scale used on Korean and many European charts, where 1.0 is the conventional reference level. It is not converted to Snellen fractions here.

How to use

1. Choose a calibration method. With the card method, hold any credit, debit or ID card against the screen and resize the box until it is exactly as wide as the card. On a narrow phone, use the ruler method and set the line to 50 mm.
2. Press “Save this calibration”.
3. Measure the distance from your eyes to the screen and enter it (1 to 6 m, default 3 m).
4. Pick the eye you are testing and cover the other one lightly with your palm.
5. Press “Start test” and answer each ring with the direction buttons or the arrow keys. Use “Not sure” when you cannot tell. From a few metres away, ask a helper to press the buttons or use a wireless keyboard.
6. Read your decimal acuity and check the size table below the result.

How it works

• Ring size: outer diameter h = distance × tan(5′ ÷ acuity), so at acuity 1.0 the ring spans five minutes of arc. At 3 m that is 4.36 mm for 1.0 and 43.6 mm for 0.1.
• Stroke and gap are one fifth of the diameter; the gap points up, down, left or right, drawn at random for every ring, so a pure guess is right one time in four.
• Levels: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.2, 1.5 and 2.0, the decimal steps found on Korean wall charts.
• Procedure (this tool’s own rule): five rings per level starting at 0.1; three or more correct passes the level. The test ends at the first level you do not pass, and your result is the last level passed, or “below 0.1”. If the 0.1 ring does not fit on the screen, or its gap is too small to draw, the test does not start and only explains why.
• A level whose gap would be under 1.5 CSS pixels cannot be drawn, so the test stops there. If even the 0.1 ring is larger than the test area, you are asked to move closer.
• Calibration: px per mm = on-screen length ÷ 85.6 mm (card) or ÷ 50 mm (ruler). Uncalibrated, the CSS standard of 96 px per inch (about 3.78 px per mm) is assumed.

Examples

Ring sizes at 3 m on a screen with 4 px per mm:

Decimal acuity	Ring diameter	Screen pixels
0.1	43.64 mm	174.5 px
0.5	8.73 mm	34.9 px
1.0	4.36 mm	17.5 px
2.0	2.18 mm	8.7 px

On the same screen at 1 m, the gap drops below 1.5 px from level 0.8 on, so the test can only go up to 0.7.

Good to know

Glare, brightness, lighting and small calibration or distance errors all change the outcome. If your vision drops suddenly, your eyes hurt or you see double, see an eye doctor whatever this page says. The colour vision test and the 20-20-20 eye break timer are useful companions.