Health & Fitness

Eye Prescription to 20/20 Scale Calculator

Convert your glasses prescription (sphere and cylinder) into an approximate 20/20 Snellen scale equivalent to understand what your uncorrected vision looks like.

⚕️ Note: This is an approximate educational conversion. Only a qualified optometrist or ophthalmologist can measure your actual visual acuity. This calculator does not account for all individual factors affecting vision.
eye-rx-calculator

Enter your right eye (OD) prescription values. You can repeat for left eye (OS) separately.

Approximate visual acuity (uncorrected)
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Eye—
Spherical equivalent (SE)—
Approx. uncorrected acuity—
Refractive category—
What this means—

Understanding your glasses prescription

Your glasses prescription is written in a standardised format using diopters — units of refractive power. The numbers might look confusing at first, but each value tells your optician exactly how to shape the lenses to correct your vision.

The sphere (SPH) is the main correction. Negative sphere values correct myopia (nearsightedness) — the eye is too long or the cornea too curved, causing light to focus in front of the retina. Positive values correct hyperopia (farsightedness) — light focuses behind the retina. The larger the absolute value, the stronger the prescription.

The cylinder (CYL) corrects astigmatism — an uneven curvature of the cornea or lens that causes images to appear distorted or double at any distance. The axis tells the lab at which angle to apply the cylinder correction.

The spherical equivalent (SE) is a useful single number that combines sphere and cylinder: SE = Sphere + (Cylinder ÷ 2). This allows for a meaningful comparison of prescription strength across people with and without astigmatism.

The 20/20 scale describes visual acuity — the sharpness of vision at a standard test distance. Knowing what your prescription means in 20/xx terms helps you understand what you can see without glasses and whether you meet common vision requirements (driving, piloting, etc.).

Reference: American Academy of Ophthalmology — understanding prescription numbers.

Conversion formula and scale

Spherical Equivalent (SE): SE = Sphere + (Cylinder ÷ 2) Approximate Snellen equivalent (uncorrected): SE = 0.00 → 20/20 (no correction needed) SE = -0.25 to -0.50 → 20/25 – 20/30 SE = -0.75 to -1.00 → 20/40 – 20/60 SE = -1.25 to -2.00 → 20/80 – 20/120 SE = -2.25 to -3.00 → 20/150 – 20/200 SE = -3.25 to -5.00 → 20/250 – 20/400 SE = -5.25 to -8.00 → 20/500 – 20/800 SE below -8.00 → 20/1000+ (legally blind without correction)

Frequently asked questions

  • 20/20 vision means you can see at 20 feet what a person with normal vision sees at 20 feet. The top number is the test distance (always 20 feet in the US), and the bottom number is the distance at which a person with normal vision could read the same line. 20/40 means you need to be at 20 feet to see what someone with normal vision sees from 40 feet — half as sharp. 20/200 is the legal blindness threshold in the US.
  • A glasses prescription typically includes: Sphere (SPH) — the main correction power in diopters; negative numbers mean nearsighted (myopia), positive means farsighted (hyperopia). Cylinder (CYL) — the astigmatism correction; can be zero if no astigmatism. Axis — the angle of astigmatism correction, from 1 to 180 degrees. Addition (ADD) — for reading glasses or bifocals, usually +1.00 to +3.50. Prism — less common, corrects alignment issues.
  • Legal blindness in the US is defined as best-corrected visual acuity of 20/200 or worse in the better eye, or a visual field of 20 degrees or less. The key phrase is "best corrected" — meaning the measurement is taken WITH the best possible glasses or contacts. Someone with a very high prescription who can still achieve 20/40 with corrective lenses is not legally blind. The prescription number alone does not determine legal blindness.
  • A sphere of -5.00 diopters indicates moderate to severe myopia (nearsightedness). Without correction, someone with -5.00 can typically see clearly only up to about 20cm (8 inches) in front of them. Their uncorrected visual acuity is roughly 20/400. With proper correction (glasses or contacts), they can typically achieve 20/20 or near 20/20. A -1.00 corresponds to roughly 20/80 uncorrected; -3.00 to roughly 20/200.
  • Yes — astigmatism (cylinder) contributes to overall blur. For conversion purposes, the spherical equivalent (SE) is often used, calculated as SE = Sphere + (Cylinder ÷ 2). This gives a single number representing the total refractive error. For example, a prescription of -2.00 / -1.00 has a spherical equivalent of -2.50. The calculator uses spherical equivalent to estimate visual acuity.
  • Yes. Some people have 20/15 or even 20/10 vision — they can see at 20 feet what a person with normal vision can only see at 15 or 10 feet. This is above-average sharpness. LASIK surgery sometimes achieves 20/15 results. Eagle-like vision at 20/10 is rare but possible in young adults with exceptionally healthy corneas and lenses.
  • Axis alone does not affect how much blur you experience — it only determines the direction of the astigmatism correction. However, an incorrect axis in your glasses will cause blurred and distorted vision just as much as an incorrect sphere. The axis number (1–180 degrees) tells the lab at what angle to grind the cylinder correction into the lens. Axis only matters in combination with cylinder.
  • Contact lens prescriptions differ from glasses prescriptions in several ways. Because contacts sit directly on the eye rather than 12mm in front of it, the power is slightly different (the vertex distance correction). Contact lens prescriptions also include base curve (BC) and diameter (DIA) measurements that determine the physical fit of the lens. You legally need a separate, valid contact lens prescription from your eye care provider — a glasses prescription is not the same document.