Focus on the retina
A reference illustration of distance light focused on the retina.
20/20 Vision Calculator
Understand your acuity notation. Explore optotypes. Make sense of your prescription.
Converts an entered eye-chart result. This page does not measure your vision or replace an eye examination.
Acuity & Measurement Inputs
ft
ft
D is the distance assigned to the smallest chart line read, not the distance you held a screen or book.
Decimal acuity = d / D
logMAR = −log₁₀(d / D)Both distances must use the same unit.
logMAR = −log₁₀(d / D)Both distances must use the same unit.
20/20 is a reference, not perfect vision.
Acuity describes central detail. It does not assess your entire visual field, eye health or ability to drive.
Visual Acuity Summary
Entered result · ODNORMALIZED SNELLEN NOTATION
20/206/6·Decimal 1.00
Standard reference acuity · not an eye-health diagnosis
Decimal acuity1.00logMAR0.000MAR1 arcmin
Explore the tumbling E
Illustration onlyRelative symbol sizes illustrate chart design. Screen size and viewing distance are not calibrated; this is not a home vision test.
One measurement. Four ways to write it.
| Snellen · feet | Snellen · metres | Decimal | logMAR |
|---|---|---|---|
| 20/20 | 6/6 | 1.00 | 0.000 |
| 20/40 | 6/12 | 0.50 | 0.301 |
| 20/100 | 6/30 | 0.20 | 0.699 |
| 20/200 | 6/60 | 0.10 | 1.000 |
6-metre and 20-foot notation express the same ratio; 6 m is not an exact physical conversion of 20 ft.
Lens power is not visual acuity.
Sphere · SPH−2.00 D
Cylinder · CYL−1.00 D
Spherical equivalent · SPH + CYL / 2−2.50 DThis summarizes lens power. It does not predict a reliable 20/20 or 20/40 result.
A prescription and an eye-chart resultanswer different questions.
SPH, CYL, axis and age cannot replace an acuity measurement.
Use the prescription tab to understand the notation.
Why focus and visual acuity are different
Explore the opticsSimplified distance-focus diagrams · illustrative, not to scale · accommodation relaxed
Myopia · focus in front
Distance light comes to a focus before reaching the retina.
Hyperopia · focus behind
Without accommodation, the optical focus falls behind the retina.
Presbyopia is a different focusing problem. With age, the lens becomes less flexible, making near focusing harder. Age alone does not determine a Snellen score.
Explore image focus, without guessing your vision
Interactive illustrationMove the slider to change the image effect. This has no conversion to Snellen acuity, lens power or an individual visual experience.
SharperMore defocus
Blur is not the whole story. Contrast, glare, pupil size, lighting and eye health also affect vision. Two people with the same prescription can have different acuity results.
Interpret the notation with context
20/20 · a reference
Standard central-detail acuity. A result alone does not establish that your eyes are healthy.
20/40 · not a driving clearance
Licence requirements vary by jurisdiction and may include corrected vision, visual fields and other criteria.
20/200 · context matters
Some US definitions use this threshold in the better eye with best correction. Visual-field criteria also apply.
Formula & Worked Example
Example: entered Snellen 20/40
1
Compute the acuity ratio:
20 / 40 = 0.50
2
Normalize the metric notation:
6 / 0.50 = 12 → 6/12
3
Convert to logMAR:
−log₁₀(0.50) ≈ 0.301
Decimal acuity is a notation ratio, not a percentage of total sight.
Frequently Asked Questions
Can my glasses prescription predict 20/20 vision?
No. Sphere, cylinder and age do not determine a reliable Snellen acuity. Acuity must be measured; this page keeps the lens-power explanation separate from acuity conversions.
Does 20/20 mean perfect eye health?
No. 20/20 is a reference for central detail at distance. It does not assess all aspects of vision, visual fields or eye health.
Can I use this screen as a vision test?
No. These optotypes are uncalibrated illustrations. Screen size, distance, lighting and testing procedure are not controlled.
Does 0.5 mean I have 50% of my vision?
No. Decimal acuity is a ratio used to write an acuity measurement. It is not a percentage of your total vision.
Does 20/200 automatically mean legal blindness?
No. Some US definitions use 20/200 or worse in the better eye with best correction, or a restricted visual field. Eligibility depends on a professional assessment and local criteria.
Why does lower logMAR mean better acuity?
logMAR is the base-10 logarithm of the minimum angle of resolution. A smaller angle means finer detail can be resolved, so lower logMAR denotes better measured acuity.
Sudden vision loss or a new curtain/shadow in your sight needs urgent medical assessment. Do not use this converter to delay care.
Sources & References
Visual acuity & chart notationEyeWiki · clinical measurement context.
Understanding low visionNIH · patient information.
The Spherical EquivalentLens-power formula and limitations.Independent educational references · No institutional endorsement or completed medical review claimed.
Further reading: NEI · Types of Refractive Errors · Presbyopia
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Understand your acuity notation. Explore optotypes. Make sense of your prescription.
