Free tool · Calculator
From glasses power to contact lens power
Calculate why your contact lens power can differ from your glasses power — and why a fitting is still always needed.
⚠ Acute symptoms?Sudden vision loss, an eye injury, chemicals in the eye, severe pain, or new blind spots in your visual field? Don't wait for an online explanation — call your GP (huisarts), the out-of-hours GP service, or 112 immediately.
Converted to the cornea (contact lens plane)
Contact lens SPH
-4.75
exact: -4.72
Contact lens CYL
—
Above ±4.00 D the difference becomes noticeable: this is exactly why your contact lens power can differ from your glasses power.
This is arithmetic, not a fitting. A well-fitting contact lens also needs the right curvature (BC), diameter and material, and your eyes must be healthy enough for lenses. Never order lenses based on this number alone — have them fitted and checked by a contact lens specialist or optometrist.
Frequently asked questions
Why does my contact lens power differ from my glasses power?
Glasses sit about 12 mm in front of your eye, a contact lens directly on it. That changes the effective power — noticeably from about ±4 dioptres. Minus powers get slightly weaker as a lens, plus powers slightly stronger.
Can I order contact lenses based on this result?
No. Power is only one parameter: curvature (BC), diameter, material and the health of your eyes together determine whether a lens fits. Always have lenses fitted and checked.
Why does my lens box sometimes show a different cylinder?
Toric lenses come in limited cylinder steps and vertex correction slightly changes the cylinder value. The specialist picks the nearest fitting combination — which is why the box can differ from your glasses prescription.
Further reading in the knowledge base
Want real certainty?
A registered optometrist reviews your complete file — measurements, scans and your question — and signs a report. Within 48 hours.
See the review options