Educational starting-power tool
Estimate contact lens starting powers from your glasses prescription, or compare toric and spherical options using your current contact lens prescription.
If you already wear toric contact lenses, you can also estimate the spherical equivalent and explore available lens options.
Enter one or both eyes. Use the sphere, cylinder and axis exactly as written on your current glasses prescription.
Example prescriptionSPH −4.00 · CYL −1.00 · Axis 180
Glasses sit in front of the eyes; contact lenses sit at the corneal plane. At stronger powers, this distance changes the effective lens power. For glasses prescriptions, the calculator therefore applies vertex conversion after first estimating spherical equivalent where cylinder is present.
Vertex distance is the space between a spectacle lens and the eye. This tool assumes 12 mm. Your actual fitting distance can differ, so the result remains a starting estimate.
Spherical equivalent combines sphere with half the cylinder into one estimated spherical power. Because a spherical lens does not correct cylinder, clarity may reduce as astigmatism increases. Toric lenses correct sphere and cylinder but must still be assessed for rotation and fit.
Many coloured lenses are spherical only. Whether uncorrected cylinder provides acceptable visual clarity varies by wearer. Product matching uses exact orderable sphere values when available, or clearly labelled published-range data when exact values are unavailable. It does not assess fit or guarantee equivalent vision.
Yes, especially for new wearers or a change of lens design. A fitting assesses base curve, diameter, movement, comfort, material and corneal health—factors a spectacle-power calculation cannot assess.
A glasses prescription lists spectacle powers. A contact lens prescription can also specify the lens design, material, base curve and diameter. This converter cannot determine those fitting details.
You can mathematically estimate a starting contact lens power, but an optometrist must confirm the contact lens prescription and fit.
Contact lenses sit on the eye while glasses sit several millimetres away. Vertex distance can change the effective power, particularly at stronger prescriptions.
It uses a configurable 12 mm back vertex distance.
Sometimes, particularly at lower cylinder values, but omitting cylinder can reduce clarity. The calculator explains the level without guaranteeing vision.
Spherical equivalent is sphere plus half the cylinder. It estimates one spherical power when cylinder correction is omitted.
Toric lenses preserve cylinder correction and are the shopping pathway shown when astigmatism is significant. An optometrist can assess lens rotation, fit and vision to determine the final prescription.
Use it only as an educational starting estimate. It does not confirm fit or expected visual clarity for a coloured lens, and it will not automatically populate an order.
Yes. A fitting assesses eye health, comfort, movement, base curve, diameter, material and the performance of the actual lens on your eye.
You can calculate its spherical equivalent by adding half the cylinder to the sphere. This removes the cylinder correction and does not guarantee the same vision through a spherical lens.
No. A current contact lens prescription is already at the corneal plane, so this calculator does not apply vertex-distance conversion in contact lens mode.
It is the sphere plus half the cylinder, rounded here to the nearest 0.25 dioptre. It is an educational estimate, not a replacement prescription.
Not necessarily. A spherical lens omits the cylinder correction, so clarity may reduce, particularly as astigmatism increases.
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