What this calculator does
Contact lens over refraction helps refine a trial lens after it is placed on the eye. The patient looks through the trial lens, then an extra refraction is measured over it. That extra measurement shows what power is still needed. This calculator combines both values and gives a practical ordered lens estimate.
Why vertex matters
Most small over refractions can be added directly. Higher powers need more care because the phoropter lens sits away from the cornea. The vertex option converts the over refraction toward the contact lens plane. This is most useful when the over refraction, current lens, or final result contains stronger sphere values.
Toric and rotation support
Toric fitting can be harder because cylinder axis matters. The calculator uses power vector math to combine cylinders across different axes. It also offers the LARS rule. Add degrees when the lens rotates left. Subtract degrees when it rotates right. The adjusted order axis can then match real lens behavior on the eye.
Formula used
Spherocylinder powers are converted into M, J0, and J45 vectors. The spherical equivalent is M equals sphere plus half cylinder. J0 equals negative half cylinder times cosine of two times the axis. J45 equals negative half cylinder times sine of two times the axis. The current lens vector is added to the adjusted over refraction vector. The result is converted back into sphere, cylinder, and axis. Vertex conversion uses corneal power equals spectacle power divided by one minus distance times spectacle power.
How to use this calculator
Enter the current trial lens values first. Add sphere, cylinder, and axis. Then enter the over refraction found during testing. Set the vertex distance when the over refraction is strong. Choose the rounding step used by your lens supplier. Select minus or plus cylinder output. If a toric lens rotates, enter its direction and degrees. Press calculate to review exact, rounded, spherical equivalent, and suggested order values. Download the report when you need a fitting record.
Professional note
The output is a fitting aid, not a prescription. Confirm comfort, movement, centration, acuity, and ocular health. Use clinical judgment before ordering lenses. Recheck unstable refractions, dry eye signs, and lens wetting during follow-up visits.