Convert between vision prescription formats including diopters, Snellen, and LogMAR. Plus glasses-to-contacts conversion with vertex distance correction.
The Vision Prescription Converter Calculator helps you convert between different formats used in eye care — diopters, Snellen fractions, and LogMAR (Logarithm of the Minimum Angle of Resolution). It also converts glasses prescriptions to contact lens prescriptions using vertex distance correction, which accounts for the difference in lens-to-eye distance.
Eye prescriptions contain sphere (SPH) for nearsightedness or farsightedness, cylinder (CYL) for astigmatism, and axis for cylinder orientation. Optometrists may write prescriptions using either plus cylinder or minus cylinder form. This calculator can transpose between these notations and apply vertex distance correction for prescriptions ±4.00 diopters or stronger.
Understanding your prescription helps you compare quotes from different providers, order glasses online, and verify that your contact lens prescription is correct relative to your spectacle prescription. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process.
Vision prescriptions are written in specialized notation that can vary between practitioners and countries. This converter bridges the gap between Snellen notation (20/20 system), metric Snellen (6/6), LogMAR (used in clinical research), and diopter values. It's also essential when switching between glasses and contacts, since prescriptions differ by the vertex distance — typically 12mm for glasses but 0mm for contacts.
Cylinder Transposition: New SPH = Old SPH + Old CYL; New CYL = −(Old CYL); New Axis = (Old Axis + 90) mod 180 Vertex Distance Correction: Corrected Power = Power / (1 − d × Power) where d = vertex distance in meters (typically 0.012m) Snellen to LogMAR: LogMAR = −log₁₀(Snellen fraction), e.g. 20/20 = −log(20/20) = 0.0 Approximate Diopter from Snellen: Diopter ≈ −(denominator − numerator) / denominator (rough estimate)
Result: Contact lens: SPH −3.35 / CYL −1.20 × 90°
With a glasses prescription of −3.50 / −1.25 × 90, vertex distance correction at 12mm gives: SPH = −3.50 / (1 − 0.012 × (−3.50)) = −3.50 / 1.042 = −3.36, rounded to −3.35. CYL is corrected similarly. The minus sign means the correction is less strong for contacts because they sit closer to the eye.
A diopter (D) is the unit of measurement for lens power. It equals the reciprocal of the focal length in meters: 1D = 1/1m focus. Negative diopters correct myopia (nearsightedness) by diverging light rays. Positive diopters correct hyperopia (farsightedness) by converging light rays. Most prescriptions fall between −20.00D and +20.00D, with the majority between −8.00D and +6.00D.
Vertex distance is the space between the back surface of a lens and the front of the cornea. For glasses, this is typically 12–14mm. For contact lenses, it's essentially zero. The vertex distance correction formula adjusts the lens power to account for this difference. The correction is more significant for higher prescriptions because the focal point shift becomes more pronounced at greater distances.
Astigmatism occurs when the cornea or lens has two different curvatures (like a football rather than a basketball). The cylinder value measures the difference between the steepest and flattest meridians. Regular astigmatism is correctable with spectacles or toric contact lenses. Irregular astigmatism (from keratoconus, scars, or surgery) may require rigid gas permeable lenses or specialty contacts.
SPH (sphere) corrects myopia (minus values) or hyperopia (plus values) and is measured in diopters. CYL (cylinder) corrects astigmatism — an irregularly shaped cornea. Axis is the angle (1–180°) that orients the cylinder correction. If your CYL is zero or blank, you have no astigmatism and axis is not needed.
They describe the same prescription in different notation. Minus cylinder is used by optometrists; plus cylinder is used by ophthalmologists. To convert: add the old CYL to the old SPH for the new SPH, negate the CYL, and rotate the axis by 90°. Example: −2.00 / −1.00 × 180 becomes −3.00 / +1.00 × 90.
Glasses sit about 12mm from your eye, while contacts sit directly on the cornea. This difference in distance (vertex distance) means the lens power needed is slightly different — typically less strong for contacts with minus prescriptions and more strong for plus prescriptions. The stronger the prescription, the larger the difference.
LogMAR (Logarithm of the Minimum Angle of Resolution) is a visual acuity scale where 0.0 equals 20/20 (Snellen), negative values are better than 20/20, and positive values are worse. It's preferred in clinical research because each 0.1 step represents the same optical change, unlike Snellen where the steps are unequal. LogMAR 0.3 = 20/40; LogMAR 1.0 = 20/200.
Not directly. Contact lenses require a separate fitting that determines base curve and diameter in addition to power adjustments. For prescriptions under ±4.00D the power may be essentially the same, but a contact lens exam is still required for proper fit. Using the wrong base curve can cause corneal damage.
20/20 means you see at 20 feet what a person with normal vision sees at 20 feet (using the imperial system). 6/6 is the metric equivalent using meters. 20/40 means you need to be at 20 feet to see what a normal eye sees at 40 feet. In many countries, 6/6 (meters) is the standard; in the US and Canada, 20/20 (feet) is used.