No, and it is not a commercial technicality. A glasses prescription is missing three pieces of information nobody can guess, and even the numbers it does contain are the wrong ones above a certain strength.
What is missing
A spectacle prescription gives you power, plus cylinder and axis for astigmatism, plus an add for reading. That is enough to grind a lens sitting 12 millimetres in front of your eye.
A contact lens sits directly on your cornea. Three things become necessary:
- Base curve (BC) — the curvature of the lens’s back surface, typically 8.3 to 9.0 mm. It has to correspond to your corneal shape. Too flat and the lens slides about; too steep and it grips, restricting the tear exchange underneath — and a tight lens does not always feel tight.
- Diameter (DIA) — usually 13.8 to 14.5 mm. Determines how much of the eye is covered and how the edge sits.
- Brand and product — a contact lens prescription is product-specific. There is no generic equivalent: materials, thickness profiles and edge designs behave differently, and 8.6 in one brand is not 8.6 in another.
The powers are not the same either
This is the part that surprises people. From roughly ±4.00 dioptres upward, contact lens power differs from spectacle power, because the lens sits on the eye rather than at a distance from it. This is vertex distance compensation.
A −6.00 spectacle prescription usually becomes about −5.50 in a contact lens. Copy the number across and you are genuinely over-corrected, with eye strain and headaches to match.
With astigmatism the conversion is more delicate still, because a toric lens has to stabilise rotationally on a blinking eye — something spectacles never have to do.
Which is why the fitting exists
A contact lens fitting adds to the eye exam: corneal curvature measurement, tear film assessment, a trial lens, and an evaluation under the slit lamp after the lens has settled. Base curve, diameter and brand come out of that.
It is also why the fitting is billed separately — see what contacts cost.
What you can do
- If you have worn contacts before, your contact lens prescription exists — ask your optometrist for it. Federal law requires them to release it automatically, at no extra charge, with no purchase requirement. See your rights.
- If you never have, you need a fitting. There is no shortcut, and any site offering to “convert” a glasses prescription is operating outside the FTC Contact Lens Rule — see is it safe to buy online.
- If your contact lens prescription expired, there are options — see here.
It does not work the other way either
Your contact lens prescription will not order glasses. It has no pupillary distance, and the powers are vertex-compensated for a lens on the eye.
Two separate documents, two separate expiry dates. See how to read your prescription and when it expires.
Sources
- FTC Contact Lens Rule, 16 CFR Part 315 — §315.2 (definition), §315.3
- Clinical references on vertex distance compensation
General information. Your optometrist determines which lens fits your eye. Reviewed by Dr. Teresita Bollar, OD.
