This back vertex distance (BVD) calculator adjusts a lens power when the vertex distance —
the distance between the back surface of a lens and the front of the eye — changes.
Opticians and optometrists most often use it to work out the power of a contact lens from a
spectacle prescription.
Enter any valid spectacle Rx (sphere, cylinder and axis) along with the current and new
vertex distances, and the compensated Rx appears straight away. Set the new distance to 0 mm
to convert a spectacle prescription to a contact lens prescription.
How it's calculated
Each principal meridian is compensated separately using the standard vertex formula, where F is the original power in dioptres and d is the change in vertex distance in metres:
F′ = F / (1 − d × F)
The sphere and cylinder are then rebuilt from the two compensated meridians; the axis is
unchanged. Vertex compensation matters most for higher powers — typically above ±4.00 D.
Options
- − Cyl / + Cyl switches between minus- and plus-cylinder notation.
- 0.25 D rounds each compensated meridian to the nearest quarter dioptre, as
lenses are usually supplied.
This tool is a calculation aid and doesn't replace professional clinical judgement.
This site uses cookies for advertising. See our privacy policy.