Wondering how your glasses numbers translate into “20/20”? This eye prescription to 20/20 tool gives you a plain-English, educational estimate of how sharp your uncorrected distance vision may be, then explains the 20/20 scale, how to read your prescription, and what figures like 20/40 or 20/200 actually mean. Enter your spherical (SPH) value below to see an approximate Snellen result — and remember that only a measured eye exam can confirm your true visual acuity.
Eye Prescription → 20/20 Scale Estimator
Estimate approximate uncorrected distance vision on the 20/20 (Snellen) scale from a glasses prescription. Screening & education only — it does not measure acuity.
Enter prescription
Estimated 20/20-scale result
| Reading | Meaning |
|---|---|
| 20/15 | Sharper than average — reads at 20 ft what others read at 15 ft |
| 20/20 | Normal reference acuity |
| 20/25–20/30 | Slightly below normal; often unremarkable |
| 20/40 | Mild reduction; a common minimum for unrestricted driving (varies by U.S. state) |
| 20/70–20/100 | Moderate reduction in distance clarity |
| 20/200 | Best-corrected 20/200 or worse in the better eye is one U.S. criterion for legal blindness |
| 20/400 | Severe reduction in distance vision |
Visual acuity measures sharpness only — not colour vision, depth perception, peripheral vision or eye health.
| Spherical equivalent (myopia) | Approx. unaided acuity | Category |
|---|---|---|
| 0 to −0.50 D | ~20/20 – 20/40 | Normal–mild |
| −0.50 to −1.00 D | ~20/30 – 20/70 | Mild |
| −1.00 to −1.50 D | ~20/50 – 20/100 | Mild–moderate |
| −1.50 to −2.00 D | ~20/70 – 20/150 | Moderate |
| −2.00 to −3.00 D | ~20/100 – 20/200 | Moderate |
| −3.00 to −4.00 D | ~20/200 – 20/400 | Significant |
| −4.00 to −6.00 D | ~20/300 – worse than 20/400 | High |
| Stronger than −6.00 D | Markedly reduced (often counting-fingers range) | Very high |
Applies to nearsightedness. Farsightedness (plus power) does not track unaided acuity because focusing (accommodation) can hide the blur — so this tool will not estimate a number for plus prescriptions. Astigmatism and pupil size add further variation.
| Term | Meaning |
|---|---|
| OD / OS / OU | Right eye / left eye / both eyes |
| SPH (Sphere) | Main lens power in D; minus = nearsighted, plus = farsighted |
| CYL (Cylinder) | Power correcting astigmatism |
| Axis | Orientation of the cylinder, 1–180° |
| ADD | Added reading power (bifocals/progressives) |
| PD | Pupillary distance (mm) |
Spherical equivalent used here = SPH + (CYL ÷ 2).
Last reviewed: July 2026. Published by clinicaltoolslibrary.com as an educational reference.
- American Academy of Ophthalmology — What Does 20/20 Vision Mean?
- American Optometric Association — Visual Acuity
- Cleveland Clinic — 20/20 Vision
- National Eye Institute — Refractive Errors
- National Eye Institute — Nearsightedness (Myopia)
- Uncorrected Refractive Error & Distance Visual Acuity in Children (peer-reviewed) — supports why unaided acuity poorly reflects hyperopia
Full disclaimer & how to report an error
This calculator provides an approximate, educational estimate of uncorrected distance visual acuity from a spherical-equivalent refraction. It is not a medical device, not a substitute for a professional eye examination, and must not be used to diagnose disease, judge fitness to drive, determine legal blindness, or decide on lenses or treatment. Estimates apply mainly to simple nearsightedness; results vary widely between individuals with pupil size, age, astigmatism and ocular health. Always rely on a measured refraction and the guidance of a qualified optometrist or ophthalmologist, and confirm every figure against your own prescription or report.
Found an error in this tool? Please report it to clinicaltoolslibrary.com. For a suspected problem with a medical device or product in the U.S., you can report to the FDA MedWatch program. Clinical concerns about your vision should go to your eye-care professional.
Educational reference · not medical advice · results depend on the values you enter · Last reviewed July 2026.
How the eye prescription to 20/20 calculator works
Your glasses prescription is written in diopters (D), a measure of the lens power needed to focus light on your retina. A “20/20” reading, by contrast, comes from a Snellen chart and describes the clarity of what you can read at 20 feet. These are two different measurements, so there is no exact formula that converts one into the other. What this calculator does is estimate your likely uncorrected distance acuity from the spherical equivalent of your prescription, using general clinical relationships for nearsightedness (myopia).
The spherical equivalent combines your sphere (SPH) with half of your cylinder (CYL) into a single number, which is why the estimate works best for straightforward myopia and is least reliable for significant astigmatism or farsightedness, where the eye’s focusing (accommodation) can mask blur. Treat every result as a ballpark for learning — not a clinical acuity score.
What the calculator can and cannot tell you
It can give you a rough sense of how blurry distance objects might appear without correction, and help you interpret the numbers on your Rx. It cannot measure your true acuity, account for astigmatism precisely, assess eye health, or serve as a driving or legal-blindness assessment. The table below shows the broad, approximate ranges often seen with myopia; individual results vary widely.
| Spherical equivalent (myopia) | Approximate uncorrected acuity* | General description |
|---|---|---|
| -0.25 D to -0.50 D | ~20/25 to 20/40 | Mild blur at distance |
| -0.75 D to -1.25 D | ~20/40 to 20/70 | Noticeable distance blur |
| -1.50 D to -2.50 D | ~20/70 to 20/200 | Moderate myopia |
| -2.75 D to -4.00 D | ~20/200 to 20/400 | Significant myopia |
| Stronger than -4.00 D | Poorer than 20/400 | High myopia |
*Ranges are approximate, apply mainly to nearsightedness, and are for education only. Your measured acuity may differ substantially. Confirm with an eye-care professional.
Understanding the 20/20 vision scale (Snellen)
On the Snellen scale, the top number is your test distance (20 feet) and the bottom number is the distance at which a person with normal sight could read the same line. So 20/40 means you must be at 20 feet to read what a typical eye reads at 40 feet. A smaller bottom number is sharper than average; a larger one is blurrier.
| Snellen reading | What it means |
|---|---|
| 20/15 | Sharper than average — you read at 20 ft what others read at 15 ft |
| 20/20 | Normal reference acuity |
| 20/25 | Very slightly below normal; usually unremarkable |
| 20/40 | Mild reduction; a common minimum for an unrestricted driver’s licence (varies by state) |
| 20/70 to 20/100 | Moderate reduction in distance clarity |
| 20/200 | Best-corrected 20/200 or worse in the better eye is one U.S. criterion for legal blindness |
| 20/400 | Severe reduction in distance vision |
What does 20/20 vision mean?
20/20 vision is normal visual acuity: at 20 feet you see what an average person sees at 20 feet. It reflects sharpness only — not colour vision, depth perception, peripheral vision, or overall eye health — so a comprehensive exam is still important even with 20/20 sight.
Is 20/15 or 20/10 vision better than 20/20?
Yes. A smaller second number means better-than-average sharpness. With 20/15 you read at 20 feet what most people need to stand at 15 feet to read. 20/10 is sharper still, though relatively uncommon.
What do 20/40, 20/70 and 20/200 vision mean?
Each describes progressively reduced distance clarity. 20/40 is a mild reduction (and a frequent driving threshold), 20/70 is moderate, and 20/200 is a substantial reduction that, when it persists with best correction in the better eye, meets a common definition of legal blindness in the United States.
How to read your eyeglass prescription (SPH, CYL, axis, OD/OS)
Before you convert your eye prescription to 20/20 estimates, it helps to know what each column means. Prescriptions list the right and left eye separately, followed by lens powers and any astigmatism correction.
| Abbreviation | Meaning |
|---|---|
| OD | Oculus dexter — your right eye |
| OS | Oculus sinister — your left eye |
| OU | Oculus uterque — both eyes |
| SPH (Sphere) | Lens power in diopters; minus (-) for nearsighted, plus (+) for farsighted |
| CYL (Cylinder) | Power correcting astigmatism |
| Axis | Orientation of the cylinder, from 1 to 180 degrees |
| ADD | Added magnifying power for reading (bifocals/progressives) |
| PD | Pupillary distance between the eyes, in millimetres |
What does SPH mean on a glasses prescription?
SPH (sphere) is the main lens power measured in diopters. A minus value corrects nearsightedness; a plus value corrects farsightedness. The further the number is from zero, the stronger the correction.
What is the axis on an eye prescription?
The axis is a number from 1 to 180 that tells the lab which orientation to place your astigmatism (cylinder) correction. It only appears when a CYL value is present and has no effect on your acuity by itself.
What do OD and OS mean?
OD (oculus dexter) is your right eye and OS (oculus sinister) is your left eye. OU refers to both eyes together. If you also need to move between glasses and contact powers, see our contact lens vertex distance calculator and spherical equivalent calculator.
Frequently asked questions
Can you convert an eyeglass prescription to 20/20 vision?
Not exactly. Diopters and the 20/20 (Snellen) scale measure different things, so any conversion is an approximation of uncorrected acuity — most reliable for simple nearsightedness. Only a chart-based refraction measures your true acuity.
What does 20/40 vision mean?
It means you see at 20 feet what a person with normal vision sees at 40 feet — a mild reduction in sharpness. In many U.S. states, roughly 20/40 is the minimum acuity for an unrestricted driver’s licence.
Is 20/40 vision bad?
Not usually. 20/40 is only a mild reduction and is often correctable to 20/20 with glasses or contacts. Whether it needs correction depends on your daily tasks and your eye doctor’s assessment.
What is the best vision you can have?
Most healthy eyes reach 20/20 with correction, but some people achieve 20/15 or even 20/10. Acuity beyond that is limited by the eye’s optics and the density of retinal cells.
How do I convert my glasses prescription to contacts?
Powers around -4.00 D and stronger often need a “vertex” adjustment because contacts sit on the eye rather than in front of it. Use our contact lens vertex distance calculator, then have the result verified by your prescriber.
What does 20/200 vision mean?
You see at 20 feet what a typical eye sees at 200 feet. When this level persists in the better eye with best correction, it meets a common U.S. definition of legal blindness. Many cases of blurry 20/200 vision are simply uncorrected refractive error.
