Vertex Distance Power Converter
Move a prescription between glasses and contacts and see how the vertex gap changes the power — the correction that matters most in strong prescriptions.
Compensated contact-lens power
At this power, vertex compensation is clinically meaningful — the corrected value differs enough to affect your vision.
Why the same eye needs a different number on the eye
A spectacle lens focuses light a centimetre or so in front of your eye; a contact lens focuses it right at the cornea. That change in position changes where the light converges, and therefore the power needed to put the focus on your retina. The maths is a single formula, but the consequence is real: a strong spectacle prescription copied straight onto a contact lens would leave you over- or under-corrected.
This converter lets you see the shift in both directions. It's a teaching and sanity-check tool — handy for understanding your own numbers or a lab's — not a substitute for a contact-lens fitting, which also weighs the curvature and health of your eye.
Frequently Asked Questions
What is vertex distance?
Vertex distance is the gap between the back of a spectacle lens and the front of the eye (the cornea), typically 10–14 mm. Because a lens's effective power depends on how far it sits from the eye, moving a prescription onto the eye as a contact lens changes the power you actually need.
When does vertex compensation matter?
The stronger the lens, the bigger the effect. Below about ±4.00 dioptres the change is a small fraction of a dioptre and usually rounds away. From ±4.00 D upward it becomes clinically meaningful, and by ±8.00 D it can be a full dioptre or more — which is why high-power contact lenses are never simply the same number as the glasses.
How does the formula work?
It uses F_c = F_s / (1 − d·F_s), where F_s is the spectacle power, d is the vertex distance in metres, and F_c is the compensated power at the eye. For a minus (myopic) lens the contact power comes out less minus; for a plus (hyperopic) lens it comes out more plus. Converting the other way flips the sign of d.
Can I use this to order contact lenses myself?
No. This tool is educational — it shows how vertex distance shifts a number. Contact lenses are medical devices that also depend on base curve, diameter, tear film, and a proper fitting. Only an eye-care professional can finalise a contact-lens prescription.
Why show a value rounded to 0.25 D?
Spectacle and contact-lens powers are manufactured in quarter-dioptre steps, so the exact mathematical result is rounded to the nearest available step. The tool shows both so you can see the true figure and the one you'd actually order.
Educational estimate only — not a contact-lens prescription. A contact-lens fitting also depends on base curve, diameter, and eye health, and must be done by an eye-care professional.