Eye Prescription (Diopters) to 20/20 Visual Acuity Converter
Convert your eyeglass or contact lens Spherical (SPH) and Cylindrical (CYL) diopter prescription into estimated uncorrected Snellen 20/X visual acuity and focal point distance.
How to Use the Eye Prescription (Diopters) to 20/20 Visual Acuity Converter
Optometrists and ophthalmologists record eyeglass prescriptions in Diopters (D)—the reciprocal of focal length in meters—whereas everyday vision is described using the Snellen fraction (such as 20/20 or 20/100). While diopters measure the optical power of the corrective lens required to focus light directly onto your retina, Snellen acuity measures the smallest letter size you can resolve at 20 feet.
Formula and Calculation Methodology
- SPH: Spherical lens power in diopters (- for nearsightedness, + for farsightedness)
- CYL: Cylindrical astigmatism correction power in diopters
- SE: Spherical Equivalent diopter power used to estimate Snellen visual acuity
By first computing your Spherical Equivalent (SE = Sphere + 0.5 × Cylinder), you can accurately estimate both your uncorrected Snellen chart line and your physical Far Point—the maximum distance from your eyes at which objects remain crystal clear without glasses. For example, a -2.00 D myopic eye has a far point of exactly 100 ÷ 2.00 = 50 centimeters (20 inches), corresponding to roughly 20/125 to 20/150 uncorrected vision.
Diopter Prescription (SPH) to Snellen 20/X Chart & Clear Focus Distance
| Diopters (SPH) | Estimated Snellen Acuity | Metric Acuity (6/X) | Clear Far Point Distance |
|---|---|---|---|
| 0.00 D | 20 / 20 | 6 / 6 | Infinity (Clear at all distances) |
| -0.50 D | 20 / 30 | 6 / 9 | 200 cm (6.5 feet) |
| -1.00 D | 20 / 50 – 20 / 60 | 6 / 15 | 100 cm (39 inches) |
| -1.50 D | 20 / 80 – 20 / 100 | 6 / 24 | 67 cm (26 inches) |
| -2.00 D | 20 / 125 – 20 / 150 | 6 / 38 | 50 cm (20 inches) |
| -2.50 D | 20 / 200 | 6 / 60 | 40 cm (16 inches) |
| -3.00 D | 20 / 250 – 20 / 300 | 6 / 75 | 33 cm (13 inches) |
| -4.00 D | 20 / 400 | 6 / 120 | 25 cm (10 inches) |
Frequently Asked Questions
Why isn’t there a single exact formula between diopters and 20/20 vision?
Diopters measure physical lens refraction, whereas 20/X Snellen acuity also depends on pupil diameter (pinhole effect in bright light), retinal cone density, and neural image processing.
What diopter prescription equals 20/200 on the eye chart?
An uncorrected refractive error of approximately -2.50 D to -2.75 D typically corresponds to 20/200 uncorrected vision (the big ‘E’ at the top of the Snellen chart).
Does 20/200 uncorrected vision mean I am legally blind?
No! Legal blindness is defined as vision that cannot be corrected to better than 20/200 even WITH glasses or contact lenses. If glasses correct your -3.00 D eyes to 20/20, you are not legally blind.