Your contact lens prescription looks different from your glasses because the two sit at different distances from your eye. A contact lens rests right on the cornea, while glasses sit about a centimetre away, and that gap changes how much power each lens needs to focus your vision. For low prescriptions, the two usually read the same. For stronger ones, the contact lens is usually a little weaker for nearsightedness and a little stronger for farsightedness. Neither is more powerful, they are simply measured for different positions.
Plenty of Canadians notice this the first time they compare the two side by side. The numbers do not line up, and it can look like a mistake. It is not. Here is what causes the gap and how to read each prescription correctly.
What Causes the Gap Between the Two Prescriptions?
It comes down to where the lens sits. A pair of glasses is held roughly 12 millimetres ahead of your eye by the frame. A contact lens floats directly on the surface of the eye. Light has to travel that extra bit of distance through glasses before it reaches you, so the lens power has to be set for that position.
Move the correction closer, as a contact lens does, and the power required shifts. Nearsighted prescriptions need slightly less minus power on a contact, and farsighted prescriptions need slightly more plus power. Optometrists adjust for this using vertex distance, which is why your two prescriptions rarely carry the exact same figure.
Does This Apply to Every Prescription?
No, and this is the part most people miss. If your prescription is mild, the adjustment is so tiny it rounds away to nothing, and your glasses and contacts can show an identical number.
The shift starts to matter once you go beyond about 4.00 dioptres. Below that, no real conversion is needed. Above it, the difference grows with every step up in strength. A wearer at -1.00 will see the same number on both. A wearer at -10.00 will see a noticeably lower number on their contacts, around -8.93, because the stronger the correction, the more that 12 millimetre gap changes things.
Glasses Power Next to Contact Lens Power
This table shows how sample glasses powers shift once they are converted for a contact lens sitting on the eye.
| Glasses Power | Contact Lens Power | Noticeable Difference? |
|---|---|---|
| -1.00 | -1.00 | No |
| -3.00 | -2.87 | Barely |
| -5.00 | -4.75 | Slight |
| -8.00 | -7.27 | Yes |
| -10.00 | -8.93 | Yes |
| +5.00 | +5.37 | Slight |
| +8.00 | +8.93 | Yes |
The figures above assume a 12 millimetre vertex distance and are examples only. Your real contact lens power is confirmed by your optometrist at a fitting.
Which Details Appear Only on a Contact Lens Prescription?
Beyond the power, a contact lens prescription carries fit information that a glasses prescription has no reason to include. A lens has to physically sit on your eye, so its shape and size are part of the prescription.
- Base curve, which matches the curve of the lens to the curve of your cornea.
- Diameter, which sets how far the lens extends across your eye.
- Lens name, since every lens fits and feels differently and cannot be swapped freely.
- Power, already corrected for the position on your eye.
A glasses prescription skips all of these and instead notes your pupillary distance so the lenses can be centred in the frames. That single difference is why the two cannot be traded for one another.
Can You Use One Prescription To Buy the Other?
No. A glasses prescription will not let you order contacts, and a contact lens prescription will not let you order glasses. The measurements and the corrected powers are built for different products.
There is also a health step involved. A contact lens has to be fitted, because a lens that sits poorly on the eye can cause irritation or worse over time. In Canada, that fitting is its own appointment, separate from the basic eye exam that produces your glasses prescription. Once your optometrist signs off, the prescription is yours to fill anywhere, including online, until it expires, which is usually after one to two years.
Reading Your Two Prescriptions Without Guesswork
Lay them side by side and begin with the sphere figure, the headline power number. If your prescription is light, the two spheres will look almost the same. If it is strong, the contact figure will sit a bit lower for minus powers and a bit higher for plus powers, exactly as the table above shows.
After that, hunt for base curve, diameter, and a lens name. Those three only live on a contact lens prescription. No base curve or diameter means you are looking at a glasses prescription. Last, glance at the expiry date, since an out-of-date prescription cannot be used to reorder.
Frequently Asked Questions
Is a bigger number on my contacts a sign of an eye problem?
No. The number only describes how much correction your vision needs. A larger figure means more correction, not a less healthy eye.
Can I work out my own contact lens power from my glasses?
You should not. A formula exists, but a legal contact lens prescription only comes from a fitting that checks the lens against your eye and your eye health.
Why are my contacts a weaker number than my glasses?
Because for nearsightedness a lens sitting on the eye needs a touch less power than one held in front of it. The weaker figure is correct, not an error.
At what point do the two prescriptions stop matching?
Around 4.00 dioptres. Below that they tend to match, and above it they drift apart more as the prescription strengthens.
Can both prescriptions ever show the same number?
Yes. With a mild prescription the difference is too small to register, so the glasses and contact figures can be identical.
When your two prescriptions disagree, that is the system doing exactly what it should. Once your fitting is done and your prescription is current, Vision Pros lets you order your contact lenses online from a broad range of lenses and manufacturers.
References
Wikipedia. Vertex Distance. Wikipedia.org. 2025.
Canadian Association of Optometrists. Contact Lens Fitting and Care. CAO.ca. 2023.
American Academy of Ophthalmology. Contact Lens Prescriptions. AAO.org. 2023.
Health Canada. Medical Devices and Contact Lenses. Canada.ca. 2023.
