Eye prescription calculator
Enter your spectacle prescription. Compensation only matters materially beyond about ±4.00 D.
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Typically 12 to 14 mm for glasses.
How do you convert glasses to contacts prescription?
Use the vertex distance formula: contact power equals spectacle power divided by one minus the vertex distance in metres times the spectacle power. Glasses sit 12 to 14 mm from the eye and contacts sit on it, so a strong minus prescription becomes less minus and a strong plus becomes more plus. Below about ±4.00 D the change rounds away.
Why the number changes at all
A lens bends light, and how much correction reaches your eye depends partly on how far from the eye the lens sits.
Glasses sit roughly 12 to 14 mm in front of the cornea. A contact lens sits directly on it. Moving the same lens closer changes its effective power, so the prescription that works at 12 mm is not the prescription that works at zero.
For a minus lens, moving it closer makes it effectively stronger, so the contact needs less minus power. For a plus lens the opposite happens and the contact needs more plus. The effect scales with the square of the power, which is why it is negligible at low prescriptions and substantial at high ones.
The vertex distance formula
Standard optometric compensation. d is in metres, so 12 mm is 0.012.
F_contact = F_spec ÷ (1 − d · F_spec)at d = 0.012 m (12 mm):−2.00 D → −1.95 D rounds to −2.00, no change−4.00 D → −3.82 D rounds to −3.75−8.00 D → −7.30 D rounds to −7.25+6.00 D → +6.47 D rounds to +6.50toric: compensate each meridian separately,then recover the cylinder as the difference
The rule of thumb in dispensing is that compensation matters beyond about ±4.00 D. Below that the shift is smaller than the 0.25 D steps lenses are manufactured in, so it rounds away to nothing.
For a prescription with cylinder, each principal meridian is compensated on its own. The tool converts the sphere and the sphere-plus-cylinder separately, then recovers the new cylinder as the difference between them — which is why a strong astigmatic prescription can see its cylinder change as well as its sphere.
Worked example: −8.00 / −1.50 × 180 at 12 mm
A moderately high myopic prescription with astigmatism, converted for contact lens wear.
- First meridian−8.00 D
- −8.00 ÷ (1 − 0.012 × −8.00)−7.30 D
- Second meridian: −8.00 + −1.50−9.50 D
- −9.50 ÷ (1 − 0.012 × −9.50)−8.52 D
- New cylinder = −8.52 − (−7.30)−1.22 D
Roughly −7.25 / −1.25 × 180 — the sphere drops by 0.75 D and the cylinder by 0.25 D.
Both numbers moved. A calculator that only compensates the sphere and carries the cylinder across unchanged gets this wrong, and the error grows with the prescription.
What a fitting covers that this does not
- Base curve, which must match the shape of your cornea
- Lens diameter and how it sits over the limbus
- Material and oxygen transmissibility for your wearing schedule
- Tear film and dry eye assessment
- Astigmatism orientation and whether a toric lens will rotate stably
- Presbyopia options, where multifocal design matters more than the numbers
This is why a contact lens prescription is a separate legal document from a spectacle prescription in Canada, and why it expires and requires follow-up.
Assumptions and limits
- Assumes a 12 mm vertex distance unless you change it
- Rounds to the 0.25 D steps contact lenses are made in
- Handles sphere and cylinder; does not handle prism or add power
- Ignores lens thickness and back vertex power effects, which matter at very high powers
- Optical conversion only — never a substitute for a fitting
Terms on this page
- Vertex distance
- The gap between the back of a spectacle lens and the front of the eye, usually 12 to 14 mm. Contacts sit on the eye, so theirs is zero.
- Dioptre
- The unit of lens power. Minus corrects short sight, plus corrects long sight.
- Cylinder and axis
- The astigmatic part of a prescription — how much correction is needed in one meridian and at what orientation.
- Base curve
- The curvature of a contact lens, matched to your cornea during a fitting. Not something a power conversion can tell you.
Common questions
Can I use my glasses prescription for contacts?
Not directly. Beyond about ±4.00 D the power needs vertex compensation, and a contact lens prescription also specifies base curve, diameter and material that a spectacle prescription does not contain. In Canada the two are legally separate documents.
What is vertex distance?
The distance from the back surface of a spectacle lens to the front of the eye, typically 12 to 14 mm. Contact lenses sit directly on the cornea, so their vertex distance is effectively zero — which is why the power has to change.
When does vertex compensation matter?
Beyond roughly ±4.00 D. Below that the calculated shift is smaller than the 0.25 D increments lenses are manufactured in, so the compensated power rounds back to the original.
Why does my cylinder change too?
Because each principal meridian is compensated separately, and the stronger meridian shifts more than the weaker one. The cylinder is the difference between them, so it changes as well. Tools that only compensate the sphere miss this.
Does this replace an eye exam?
No. It converts a number. It cannot assess corneal shape, tear film, eye health or how a lens will behave on your eye, all of which a fitting covers. Contact lenses are medical devices and require an optometrist.
Where these numbers come from
- Canadian Association of Optometrists National body; contact lens prescribing and eye health guidance in Canada.
- Vertex distance — optical compensation The standard compensation formula used here.
Last verified 2026-08-01 The method on this page is checked against the sources above at least once a year. Spotted something out of date? Tell us and we will fix it.
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