Contact lens power calculator: why contacts differ from glasses
This contact lens power calculator converts a glasses prescription to the power needed at the surface of the eye, using the vertex distance formula opticians use. Glasses sit about 12 mm in front of the cornea; a contact lens sits on it. Moving a lens closer to the eye changes its effective power, so a −6.00 glasses prescription corresponds to about −5.60 at the cornea, and a +6.00 to about +6.47.
Enter sphere (SPH), cylinder (CYL) and axis for each eye, check the vertex distance, and read the exact power, the nearest available step, and for astigmatism the toric lens equivalent and the spherical equivalent. You can also convert the other way, from contact lenses to glasses.
It is for understanding your prescription and checking the arithmetic — useful for students and people curious why their contact lens box shows a different number. A lens still has to be fitted by an optician or optometrist.
How to convert glasses to contact lens power
- Choose Glasses → contacts (or Contacts → glasses).
- Copy SPH, CYL and AXIS for the right eye (OD) and left eye (OS) from your prescription. Leave CYL empty if there is none; plus or minus cylinder both work.
- Keep the vertex distance at 12 mm unless your optician measured a different one.
- Read the exact power, the nearest available step and, for astigmatism, the toric and spherical-equivalent options.
- Press Copy result to take the numbers to your appointment.
Contact Lens Power Calculator features
Exact vertex formula
Fc = F ÷ (1 − d × F), with d in metres, applied to each principal meridian.
Astigmatism handled properly
Sphere and cylinder are converted meridian by meridian; plus-cylinder prescriptions are transposed to the minus-cylinder form contact lenses use.
Both directions
Glasses to contacts, or contacts back to glasses with F = Fc ÷ (1 + d × Fc).
Available steps
Rounds to 0.25 D up to ±6.00 and 0.50 D beyond, the way most soft lens ranges are made, and shows the nearest common toric cylinder.
Visual vertex distance
A small diagram moves the glasses lens as you change the distance.
The vertex distance formula and a worked example
Effective power at the cornea: Fc = F ÷ (1 − d·F), where F is the glasses power in dioptres and d is the vertex distance in metres (12 mm = 0.012 m). The American Academy of Ophthalmology’s EyeWiki gives the same relation for moving a correction to the corneal plane (checked 24 October 2026). Minus prescriptions become weaker at the eye and plus prescriptions stronger.
Astigmatism example: −6.00 / −1.50 × 180. The meridian at 180° has −6.00, which becomes −6 ÷ 1.072 = −5.60. The meridian at 90° has −7.50, which becomes −7.5 ÷ 1.09 = −6.88. The new cylinder is −6.88 − (−5.60) = −1.28, so the corneal-plane prescription is −5.60 / −1.28 × 180 — about −5.50 / −1.25 × 180 in available steps. The axis does not change.
| Glasses | At the cornea (12 mm) | Nearest step |
|---|---|---|
| −12.00 | −10.49 | −10.50 |
| −10.00 | −8.93 | −9.00 |
| −8.00 | −7.30 | −7.50 |
| −6.00 | −5.60 | −5.50 |
| −5.00 | −4.72 | −4.75 |
| −4.00 | −3.82 | −3.75 |
| +4.00 | +4.20 | +4.25 |
| +5.00 | +5.32 | +5.25 |
| +6.00 | +6.47 | +6.50 |
| +8.00 | +8.85 | +9.00 |
| +10.00 | +11.36 | +11.50 |
When the conversion matters, and what it cannot tell you
- Below about ±4.00 D the change is smaller than a quarter dioptre, so contact lenses usually have the same power as glasses.
- Above that, myopes need less minus and hyperopes more plus in contact lenses. At −10.00 the difference is over a dioptre.
- Toric soft lenses are usually made in a few cylinders (often −0.75, −1.25, −1.75 and −2.25) and in 10° axis steps; small cylinders are often handled with a spherical lens at the spherical equivalent. Your fitter decides, after seeing how the lens sits and rotates.
- Base curve, diameter, material, wearing schedule and eye health checks come from a fitting, not from a formula. In the US the FTC Contact Lens Rule treats a contact lens prescription as separate from a glasses prescription, and in the UK contact lenses are fitted under the Opticians Act.
- Multifocal, rigid gas-permeable, orthokeratology and keratoconus lenses follow different fitting rules.
Contact Lens Power Calculator FAQ
How do I convert my glasses prescription to contact lenses?
Apply Fc = F ÷ (1 − 0.012 × F) to each meridian, then round to the available steps. The contact lens power calculator above does this for both eyes; an optician must still fit and check the lenses.
Why is my contact lens power different from my glasses?
Glasses sit about 12 mm in front of your eye, contact lenses on it. That distance changes the effective power, so strong minus prescriptions need less minus in contacts and strong plus prescriptions need more plus.
Do I need vertex conversion for −3.00?
Usually not. −3.00 at 12 mm becomes −2.90 — closer to −3.00 than to −2.75 — so the power stays the same. The effect becomes significant from about ±4.00 D.
What about astigmatism (cylinder and axis)?
Convert the two principal meridians separately; the axis stays the same and the cylinder usually gets slightly smaller in minus prescriptions. Toric lenses come in set cylinders and axes, so your fitter picks the closest that sits well.
Can I order contact lenses with this result?
No. You need a contact lens prescription from a fitting, which includes the base curve, diameter and brand. This tool explains the power part for information only and is not medical advice.