A contact lens power chart converts your glasses prescription into the power your contact lenses actually need, and it only starts to matter once your prescription climbs above about four diopters. The reason is the vertex distance: your glasses sit roughly 12 to 14 millimetres from your eye, while a contact lens sits right on it, and that gap slightly changes the effective power. A contact lens power chart is the quick way to see how far those two numbers drift apart.
Why the Two Powers Are Not the Same
Think of the difference as how far the lens is from the surface it is correcting. A spectacle lens corrects from a distance, so its focusing ability must be tuned for that gap. A contact lens corrects at zero distance, so it needs a slightly different power to achieve the same result.
For a low prescription the difference is negligible — around a quarter of a diopter or less — and most people simply carry the same number over. Above roughly four diopters, however, the shift becomes large enough that copying your glasses order straight onto a lens leaves you a little over or under corrected. This is the exact point where a chart or a conversion tool earns its keep.
How to Read the Chart Direction
There is one rule that makes the whole chart obvious: the lens shrinks the number closer to zero, in both directions. If you are short-sighted with a minus power, the contact lens power is less negative than the glasses power. If you are long-sighted with a plus power, the lens power is less positive.
That direction holds because a lens sitting on the eye is effectively slightly stronger than the same optical design sitting further away. So to match the glasses, the contact power has to be reduced in magnitude — not increased. This is why you will often see a chart column for “retail minus” and one for the smaller “contact minus” value next to it.
Example Values on a Contact Lens Power Chart
Concrete numbers make the pattern clearer, and these are typical of how the values move step by step. They are examples, not a prescription — your own eyes need a proper conversion.
| Glasses Power | Approx. Contact Power | Direction |
|---|---|---|
| -2.00 | -2.00 | No change |
| -4.00 | -3.75 | Slightly less minus |
| -6.00 | -5.50 | Less minus |
| -8.00 | -7.25 | Less minus |
| -10.00 | -8.75 | Less minus |
| -12.00 | -10.00 | Less minus |
| +4.00 | +3.75 | Slightly less plus |
| +8.00 | +7.25 | Less plus |
Notice the numbers move toward zero in every row. And note the caveat: these are illustrative figures that assume a standard assembly distance and a spherical correction. With astigmatism or a very high prescription, the actual values depend on your own measurements.
Does a High Prescription Need More Correction?
Yes, and the effect compounds. The higher your glasses power, the bigger the gap between your glasses value and your contact value, because the vertex correction grows with the strength of the prescription.
That is why a chart that reads the same for everyone is not quite accurate for the people who need it most. Someone at -2.00 sees no change, while someone at -12.00 can be a full two diopters apart. If your prescription sits at the stronger end, this is not a number you want to eyeball from a table.
How to Convert Your Own Prescription Accurately
Run your numbers through the vertex lens calculator on vertexlenscalc.com — it applies the vertex distance formula directly to your sphere, cylinder, and axis rather than relying on a generic table.
The calculator uses the same principle the chart is based on, but it accounts for your specific prescription and the fitting distance, which is what a one-size table cannot do. Optical practices use this conversion for a reason, since this is general guidance rather than medical advice, and if your prescription changes it is worth getting an eye exam and a proper fit rather than matching a chart on your own.
