About the Glasses to Contacts Prescription Converter
This glasses-to-contacts converter adjusts a spectacle prescription for vertex distance — the gap of about 12 mm between the back of a glasses lens and the front of your eye. Because a contact lens sits directly on the cornea, the same focusing effect needs a slightly different power: weaker for short-sighted (minus) prescriptions and stronger for long-sighted (plus) ones. Enter the sphere, cylinder and axis from your glasses prescription and the vertex distance, and it returns the power at the eye, rounded to the 0.25 D steps lenses are made in.
The effect is small for low prescriptions but grows quickly: under about ±4.00 D the change is less than a quarter-dioptre step, so glasses and contact powers are usually the same, while a −8.00 glasses lens becomes about −7.25 at the eye. Astigmatic prescriptions are converted meridian by meridian, which also changes the cylinder, and the calculator shows a spherical equivalent for people trying spherical lenses.
Use it to understand why your contact lens box shows a different number from your glasses, or to check an optics homework problem. It does not replace a contact lens fitting: base curve, diameter, material and fit can only be set by an eye care professional, and in many countries contact lenses legally require a separate prescription.
How to use the glasses to contacts prescription converter
- 1Copy the sphere (SPH) from your glasses prescription, including its sign.
- 2Enter the cylinder (CYL) and axis if you have astigmatism, otherwise leave CYL at 0.
- 3Keep 12 mm vertex distance unless your prescription lists another value.
- 4Read the power at the eye, rounded to 0.25 D steps.
- 5Take the result to your contact lens fitting rather than ordering from it directly.
Formula and method
A lens of power F (dioptres) worn d metres in front of the cornea has an effective power Fc at the cornea given by Fc = F ÷ (1 − d × F). With d = 0.012 m, a −5.00 D lens gives −5 ÷ 1.06 = −4.72 D, while a +6.00 D lens gives +6 ÷ 0.928 = +6.47 D. Minus powers therefore get weaker at the eye and plus powers stronger.
For astigmatism the two principal meridians are converted separately: the sphere meridian S and the combined meridian S + C. The new cylinder is the difference between the converted meridians and the axis does not change. The exact values are then rounded to the nearest 0.25 D, and the spherical equivalent is sphere + cylinder ÷ 2 at the eye.
- F
- Glasses lens power in dioptres (each meridian)
- Fc
- Equivalent power at the cornea
- d
- Vertex distance in metres (12 mm = 0.012 m)
Worked examples
−5.00 −1.00 × 180 at 12 mm
The −5.00 meridian becomes −5 ÷ 1.06 = −4.717 D and the −6.00 meridian becomes −6 ÷ 1.072 = −5.597 D. The cylinder at the eye is −0.88 D. Rounded, that is −4.75 −1.00 × 180, with a spherical equivalent of about −5.25.
High myopia, −8.00 sphere only
−8 ÷ (1 + 0.012 × 8) = −8 ÷ 1.096 = −7.30 D, which rounds to −7.25 — three quarter-dioptre steps weaker than the glasses.
Hyperopic +6.00 −1.50 × 90
Plus lenses get stronger at the eye: +6 ÷ 0.928 = +6.466 D, and the +4.50 meridian becomes +4.5 ÷ 0.946 = +4.757 D. The cylinder grows to −1.71 D, rounding to +6.50 −1.75 × 90.
−10.00 −2.00 × 170 at 14 mm
With a 14 mm vertex distance, −10 ÷ 1.14 = −8.77 D and −12 ÷ 1.168 = −10.27 D, giving −8.75 −1.50 × 170 at the eye.
Frequently asked questions
Why is my contact lens prescription different from my glasses?+
Glasses sit about 12 mm in front of your eyes while contacts sit on the cornea. Moving the lens closer changes its effective power, so minus prescriptions need weaker contacts and plus prescriptions stronger ones. Fitting factors and toric lens availability can also change the numbers.
When does vertex distance matter?+
It is usually ignored below about ±4.00 D because the change is smaller than the 0.25 D step lenses come in. From around ±4.00 D upward the difference becomes one step or more and must be corrected.
Can I order contacts using my glasses prescription?+
You should not. A contact lens prescription also specifies base curve, diameter and brand, determined at a fitting. In the US, the FTC Contact Lens Rule requires a valid contact lens prescription, separate from a glasses prescription.
What is a normal vertex distance?+
Most glasses sit about 10–14 mm from the cornea, with 12 mm the common default. Optometrists often record the measured vertex distance on prescriptions above ±4.00 D so it can be used in exactly this conversion.
What is the spherical equivalent?+
The spherical equivalent is the sphere plus half the cylinder. It approximates an astigmatic prescription with a single spherical power and is sometimes used to choose spherical contact lenses when astigmatism is small.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.