20/20 Vision Calculator: Convert Eye Prescription to Visual Acuity
Enter your eye prescription to find out what your uncorrected vision is on the 20/20 Snellen scale, plus the metric 6/6, decimal, and LogMAR equivalents. Both right and left eyes are calculated side by side. The calculator uses the standard weighted formula (sphere plus half the cylinder) and shows every step of the conversion.
Formula
Worked example
A right eye prescription of -2.00 sphere with -0.50 cylinder: effective diopters = -2.00 + 0.5 x (-0.50) = -2.25 D. From the table, -2.25 D falls in the 20/125 row (LogMAR 0.80, decimal 0.160, metric 6/38).
What does 20/20 vision actually mean?
The notation "20/20" comes from the Snellen eye chart, developed by Dutch ophthalmologist Hermann Snellen in 1862. The first number is the test distance in feet (20 feet, the standard chart distance), and the second is the distance at which a person with normal vision can read the same line. So 20/20 means you can read at 20 feet what a normally-sighted person reads at 20 feet. 20/40 means you can only read at 20 feet what a person with normal vision could read from 40 feet away. The metric equivalent is the 6/6 scale, where the test distance is 6 metres instead of 20 feet.
20/20 is NOT the best possible vision. Some people achieve 20/15 or even 20/10, meaning their visual detail is sharper than average. Legal requirements and clinical definitions use different thresholds: most US states set a 20/40 minimum for an unrestricted drivers license, and 20/200 in the better eye even with correction is the US legal definition of blindness.
How the prescription-to-acuity conversion works
Your eyeglass or contact lens prescription contains three key numbers for each eye:
- Sphere (SPH): the main focusing correction in diopters. Negative values correct nearsightedness (myopia); positive values correct farsightedness (hyperopia).
- Cylinder (CYL): the astigmatism correction in diopters. This is usually negative.
- Axis: the angle (1-180 degrees) of your astigmatism correction. Used when fitting lenses, but not in the acuity estimate.
The standard formula for estimating uncorrected acuity from a prescription uses the effective diopters: the sphere value plus half the cylinder. This weighted average approximates how much total blur you experience without glasses. A sphere of -2.00 D with a cylinder of -1.00 D gives an effective power of -2.50 D, which maps to roughly 20/160 on the Snellen chart.
The conversion from effective diopters to Snellen notation is a lookup table rather than a single formula, because each step on a Snellen chart corresponds to a 0.1-unit change in LogMAR (log10 of the inverse acuity), not a linear change in diopters.
The four visual acuity scales explained
Clinicians and researchers use four measurement scales interchangeably:
- Snellen (20/x): the standard US notation. The denominator is the distance in feet from which a normally-sighted person can read the same letter size. Common in the US, Canada, and many other countries.
- Metric Snellen (6/x): the same chart measured in metres. 6/6 = 20/20; 6/12 = 20/40. Standard in the UK, Australia, and much of Europe.
- Decimal acuity: simply 20 divided by the Snellen denominator. 20/20 = 1.0; 20/40 = 0.5; 20/200 = 0.1. Common in European and Japanese clinical records.
- LogMAR: the logarithm (base 10) of the Snellen denominator divided by 20. 20/20 = 0.00; 20/200 = 1.00. Used almost universally in clinical research because it has equal steps between chart lines (each 0.1 unit = one Snellen line), making it suitable for statistical analysis.
Limitations of prescription-based acuity estimates
The conversions this calculator produces are educated approximations, not clinical measurements. Several factors affect actual acuity that a prescription alone cannot capture:
- Astigmatism axis: the direction of the cylinder correction matters. The same cylinder power at different axes can produce different acuity outcomes.
- Contrast and lighting: Snellen acuity measured under bright clinical conditions is typically better than acuity in dim or real-world conditions.
- Neural adaptation: people with long-standing high myopia sometimes achieve slightly better functional acuity than the table predicts because the brain partially compensates.
- Chart design and testing method: the standard letter Snellen chart, the ETDRS chart, and optotype tests can give different readings for the same eye.
This calculator is for educational understanding of your prescription. For accurate legal or clinical vision documentation, see a licensed eye care professional.
Diopters to visual acuity conversion table
| Sphere (D) | 20/x Snellen | 6/x Metric | Decimal | LogMAR | Description |
|---|---|---|---|---|---|
| 0.00 | 20/20 | 6/6 | 1.000 | 0.00 | Normal / perfect |
| -0.25 | 20/25 | 6/8 | 0.800 | 0.10 | Near-normal |
| -0.50 to -0.75 | 20/32-40 | 6/10-12 | 0.500-0.625 | 0.20-0.30 | Mild reduction |
| -1.00 | 20/50 | 6/15 | 0.400 | 0.40 | Moderate reduction |
| -1.50 | 20/80 | 6/24 | 0.250 | 0.60 | Moderate reduction |
| -2.00 | 20/125 | 6/38 | 0.160 | 0.80 | Significant reduction |
| -3.00 | 20/200 | 6/60 | 0.100 | 1.00 | Significant reduction |
| -5.00 | 20/320 | 6/96 | 0.063 | 1.20 | Severe reduction |
| -7.00 | 20/400 | 6/120 | 0.050 | 1.30 | Severe reduction |
| -10.00 and above | 20/500 | 6/150 | 0.040 | 1.40 | Profound reduction |
Approximate uncorrected visual acuity by prescription sphere (with no cylinder). Individual variation applies.
Frequently asked questions
What is 20/20 vision in other notation systems?
20/20 is exactly equivalent to 6/6 on the metric scale, 1.000 in decimal acuity, and 0.00 in LogMAR. All four represent normal reference vision: reading the standard smallest line at the standard chart distance.
Is 20/20 the best vision possible?
No. 20/15 and 20/10 represent acuity sharper than the norm. Some people achieve these without correction, and LASIK or lens implant outcomes are sometimes reported in this range. 20/20 is simply the reference standard for normal adult vision.
How do I read my eye prescription?
A typical prescription looks like: OD (right eye) -2.00 / -0.50 x 180, OS (left eye) -1.75 / -0.75 x 170. The first number is the sphere, the second is the cylinder, and the number after "x" is the axis. Enter the sphere and cylinder values here; the axis is not needed for this acuity estimate.
What does LogMAR mean and why is it used?
LogMAR stands for Logarithm of the Minimum Angle of Resolution. Unlike the Snellen scale, LogMAR values are evenly spaced: each 0.1-unit step equals exactly one Snellen line of change. This regularity makes LogMAR suitable for statistical calculations and clinical research. A LogMAR of 0.00 is 20/20; 0.30 is roughly 20/40; 1.00 is 20/200.
What vision is needed to drive legally in the US?
Most US states require a minimum of 20/40 corrected acuity in at least one eye for a standard (unrestricted) drivers license. Requirements vary by state; some allow 20/70 with restrictions. This threshold applies to corrected vision, so wearing your glasses or contacts during the test is permitted.
Can I use this calculator instead of an eye exam?
No. This calculator gives an approximate educational estimate based on standard conversion tables. Accurate visual acuity requires a clinical refraction with a calibrated chart, professional equipment, and an eye care provider. This tool is useful for understanding what your prescription numbers mean, not for medical or legal documentation.
Why does the calculator use sphere plus half the cylinder?
The cylinder in your prescription corrects astigmatism in only one meridian of the eye. The other meridian is corrected by the sphere alone. Averaging these two meridians - the sphere and the sphere-plus-cylinder - gives the sphere equivalent, or equivalently sphere plus half the cylinder. This is the standard optometric shorthand for the overall blur level your eye experiences without correction.