Prism & PD Calculator

OD and OS, live as you type

Enter the prescription and the prism you want, and see how far each lens needs to move.

Illustration of an eyeglass lens shifting away from the pupil to create prism — Illustration, not a measurement diagram.

Which way

One direction at a timeThis works out horizontal or vertical prism on its own. With an oblique cylinder axis the movement also adds prism in the other direction; you will see the amount below the result when it reaches 0.05 Δ.

Result

How to use it

Pick horizontal or vertical prism at the top.
Enter the sphere, minus cylinder and axis from the prescription. Leave cylinder and axis empty for a sphere-only lens.
Enter the prism amount and its base direction.
Add each eye's monocular PD in millimetres. The result is the PD to order for each lens.

Good to know

Prescriptions are read in minus-cylinder form. The middle steps (the power in the working meridian and the signed movement) are calculated behind the scenes and kept hidden.

Movement follows Prentice's rule: prism equals the lens power in that meridian times the distance moved, and the requested prism checks out in an independent 2D optics test. One direction at a time: with an oblique cylinder axis the same movement also adds a little prism in the other direction, and the requested prism does not cancel it.

Works fully offline. Nothing is ordered from this page, and nothing you enter is saved or sent anywhere.