Prescription Glasses Calculator
Use our free Prescription glasses Calculator to get personalized health results. Based on validated medical formulas and clinical guidelines.
Formula
Spherical Equivalent = Sphere + (Cylinder / 2) | Focal Length = 100 / |Sphere| cm
Where Spherical Equivalent combines sphere and cylinder into a single value for classification, and Focal Length indicates the distance at which light focuses through the lens. Negative sphere values indicate myopia, positive indicate hyperopia. Cylinder and axis correct astigmatism.
Worked Examples
Example 1: Moderate Myopia with Astigmatism
Problem: Right eye: -3.25 sphere, -1.00 cylinder at 90 degrees. Left eye: -2.75 sphere, -0.75 cylinder at 85 degrees. PD: 64mm. What lens recommendations apply?
Solution: Right Eye Spherical Equivalent: -3.25 + (-1.00 / 2) = -3.75 D\nLeft Eye Spherical Equivalent: -2.75 + (-0.75 / 2) = -3.125 D\nAnisometropia: |(-3.75) - (-3.125)| = 0.625 D (within normal range)\nVision Type: Both eyes are myopic (nearsighted)\nSeverity: Right = Moderate, Left = Moderate\nRecommended Lens Index: 1.59 Polycarbonate or 1.67 High Index\nFocal Length Right: 100 / 3.25 = 30.8 cm (far point without correction)
Result: Both eyes moderate myopia | SE: R -3.75, L -3.13 | Recommended: 1.67 High Index lenses
Example 2: Presbyopia with Bifocal Need
Problem: A 52-year-old with Right: +1.50 sphere, Left: +1.25 sphere, ADD +2.00. What are the near vision prescriptions?
Solution: Distance Prescription: R +1.50, L +1.25 (mild hyperopia)\nADD Power: +2.00 (moderate presbyopia)\nNear Vision Right: +1.50 + 2.00 = +3.50 D\nNear Vision Left: +1.25 + 2.00 = +3.25 D\nSpherical Equivalent Distance: R = +1.50, L = +1.25\nAnisometropia: |1.50 - 1.25| = 0.25 D (negligible)\nRecommended: Progressive or bifocal lenses\nLens Index: 1.50 Standard CR-39 (mild prescription)
Result: Mild hyperopia with presbyopia | Near Rx: R +3.50, L +3.25 | Progressive lenses recommended
Frequently Asked Questions
What does the sphere value on my prescription mean?
The sphere (SPH) value indicates the lens power needed to correct nearsightedness or farsightedness, measured in diopters. A negative sphere value means you are myopic (nearsighted) and have difficulty seeing distant objects clearly. A positive sphere value means you are hyperopic (farsighted) and may struggle with close-up vision. The higher the absolute number, the stronger the prescription and the more correction your eyes need. For example, a sphere of -3.00 indicates moderate myopia requiring a concave lens, while +2.00 indicates mild hyperopia requiring a convex lens. Most prescriptions fall between -8.00 and +6.00 diopters, though extreme cases can exceed these ranges.
What is cylinder and axis in an eyeglass prescription?
Cylinder (CYL) and axis values on your prescription correct astigmatism, a condition where your cornea or lens is shaped more like a football than a basketball. The cylinder value indicates the amount of lens power needed to correct the astigmatism, measured in diopters, and is always accompanied by an axis value. The axis is a number from 1 to 180 degrees that specifies the orientation of the astigmatism correction on the lens. Together, they tell the lab exactly where and how much curvature to add to your lens. For example, a cylinder of -1.25 at axis 90 means the astigmatism correction is applied along the vertical meridian. Without the correct axis, the cylinder correction would blur your vision instead of improving it.
What is the ADD power on a prescription for?
The ADD (addition) power is the additional magnifying power applied to the bottom portion of a multifocal lens for reading and close-up work. It is prescribed for presbyopia, the natural age-related loss of near focusing ability that typically begins around age 40 to 45. ADD values usually range from +0.75 to +3.00 diopters and increase gradually with age as the eye lens becomes less flexible. An ADD of +1.00 is typical for someone in their early 40s, while +2.50 or more is common for those over 60. The ADD value is always the same for both eyes. When added to the distance sphere, it creates the near vision prescription, and this is used in bifocals, trifocals, and progressive lenses to provide clear vision at multiple distances.
How do I choose the right lens index for my prescription?
Lens index refers to the refractive efficiency of the lens material, with higher index numbers producing thinner and lighter lenses. Standard CR-39 plastic (index 1.50) works well for mild prescriptions up to about plus or minus 2.00 diopters. Polycarbonate (index 1.59) is recommended for moderate prescriptions and is impact-resistant, making it ideal for children and sports. High index 1.67 lenses are best for prescriptions between plus or minus 4.00 and 6.00, reducing lens thickness by about 30 percent compared to standard plastic. Ultra high index 1.74 is reserved for the strongest prescriptions above plus or minus 6.00 and provides the thinnest possible lens. Higher index materials are more expensive but essential for strong prescriptions to avoid thick, heavy lenses that are uncomfortable and cosmetically unappealing.
Can my glasses prescription be used for contact lenses?
No, glasses and contact lens prescriptions are different and not directly interchangeable. The primary reason is vertex distance, the approximately 12 millimeter gap between your glasses lenses and your eyes. Contact lenses sit directly on the cornea, eliminating this distance, which changes the effective power of the lens. For prescriptions above plus or minus 4.00 diopters, the contact lens power must be recalculated using the vertex distance formula. For example, a glasses prescription of -6.00 might convert to approximately -5.50 in contacts. Additionally, contact lens prescriptions include parameters that glasses prescriptions do not, such as base curve and diameter, which must be determined through a separate contact lens fitting. Always get a specific contact lens prescription from your eye care provider rather than trying to convert your glasses prescription yourself.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.