Input panel
Enter your chart and far-point measurements
Test each eye separately. A far point is the farthest distance that stays clear without distance glasses.
Calculation method
Formula used
- Decimal acuity: test distance ÷ line denominator
- logMAR: log10(line denominator ÷ test distance)
- Simple-myopia screening estimate: −1 ÷ far point in meters
The third relation describes a simplified far-point optics model. It only applies to simple myopia screening. It is not a chart-to-prescription conversion and cannot calculate cylinder, axis, prism, or near addition.
Practical steps
How to use this calculator
- Use a distance chart at its stated viewing distance.
- Test one eye at a time without pressing the covered eyelid.
- Enter the test distance and smallest line you read correctly.
- Select whether the test was unaided or performed with correction.
- For a simple myopia screening estimate, measure and enter your unaided far point.
- Review the chart metrics, then book an examination for prescription decisions.
Understanding Eye Chart Results
An eye chart measures visual acuity. It compares the smallest readable line with a standard viewing distance. A result such as 20/40 means you read at twenty feet what a standard eye reads at forty feet. A lower fraction signals reduced sharpness. It does not identify the exact cause. Dry eye, cataract, retinal disease, focusing problems, and uncorrected refractive error may reduce acuity. Test each eye separately. Record correction status clearly each time.
Why a Chart Cannot Write a Prescription
Glasses prescriptions may include sphere, cylinder, axis, prism, and near addition. A distance chart does not measure every value. It cannot reliably separate myopia from astigmatism, hyperopia, or eye disease. It also cannot find the astigmatism axis. A refraction uses lenses and testing to compare choices. This calculator provides an educational screening estimate. It never creates a prescription for ordering glasses. Use it to prepare questions for an eye professional.
What the Calculator Converts
The tool changes a Snellen result into decimal acuity and logMAR. Decimal acuity equals the testing distance divided by the line denominator. LogMAR equals the base ten logarithm of the denominator divided by the testing distance. They compare chart formats. They do not directly equal lens power. It accepts a measured far point. The far point is the farthest distance that remains clearly focused without distance glasses. That measurement supports a simple myopia-only estimate.
Far Point and Theoretical Power
For a simple myopic eye, distance correction is approximately negative one divided by the far point in meters. A far point of 0.50 meters gives roughly minus 2.00 diopters. A far point of 2.00 meters gives roughly minus 0.50 diopters. This model assumes simple myopia and careful measurement. It ignores vertex distance, accommodation, astigmatism, unequal eyes, and eye health. It rounds the screening estimate to quarter-diopter steps. It cannot select glasses without refraction.
How to Test More Carefully
Use a chart at its stated distance. Cover one eye without pressing the eyelid. Record the smallest correct line. Repeat with the other eye. Do not squint or guess letters. Measure the far point during another attempt. Move a high-contrast target until it becomes unclear. Record the last clear distance. Children, people with diabetes, and anyone with changing vision should not rely on a home screening result. Consistent conditions make later comparisons more useful.
When Professional Care Matters
Arrange a comprehensive eye examination when vision is blurred, headaches occur during visual tasks, or one eye differs from the other. Seek urgent care for sudden vision loss, severe eye pain, flashes, new floaters, a curtain-like shadow, injury, or chemical exposure. A chart score cannot rule out serious conditions. Keep date, distance, line, eye, and correction records. Bring records to appointments. A clinician can perform refraction and health checks. Those steps determine whether glasses, contact lenses, treatment, or further testing are appropriate.
Common questions
Eye chart and glasses power FAQs
Can an eye chart provide my exact glasses prescription?
No. A chart measures clarity, not the full optical correction. A prescription needs refraction and may include sphere, cylinder, axis, prism, and near addition.
What does 20/40 vision mean?
It means you read at twenty feet what a reference eye can read at forty feet. It describes acuity under that test condition.
What is logMAR?
logMAR is a logarithmic acuity scale. A lower value generally represents better visual acuity. It helps compare results across chart formats.
What is a far point?
For someone with simple myopia, the far point is the farthest distance that appears clear without distance correction.
Why does the screening estimate use a minus sign?
Simple myopia is commonly corrected with minus-powered lenses. The sign here reflects the simplified far-point optics relation, not a prescription decision.
Can this calculator find astigmatism?
No. Astigmatism requires cylinder amount and axis. A standard distance chart and far-point measurement cannot reliably determine them.
Can it calculate reading glasses power?
No. Near addition depends on age, working distance, symptoms, focusing ability, and clinical findings. It needs a proper near-vision assessment.
Should I test while wearing my current glasses?
You may record a corrected chart result, but it cannot produce an unaided far-point screening estimate. Test unaided only when safe and appropriate.
What if I read 20/20 but still have headaches?
Book an eye examination. Good chart acuity does not rule out eye strain, focusing problems, binocular vision issues, dry eye, or other conditions.
When is urgent eye care needed?
Seek prompt care for sudden vision loss, severe pain, flashes, many new floaters, a curtain-like shadow, injury, or chemical exposure.
Can children use this calculator?
It may help record a screening result, but it should not guide lens choices. Children need professional assessment because focusing can mask some refractive errors.
Further reading