Calc Drills cover the optical math on the ABO exam. Pick a drill and work through 10 problems. The numbers are typed, not picked from a list, and each answer is checked against the same formulas the site’s free optical calculators use. Every problem gets fresh numbers, so you can repeat a drill without memorizing the answers. After each check you see the right answer and the working, step by step.

There are 4 drills:

• Transpose It. Convert prescriptions between plus- and minus-cylinder form. You enter the new sphere, new cylinder and new axis.
• Prentice Drill. Induced prism from decentration — magnitude and base. You enter the prism (±0.25 Δ) and base direction (BI, BO, BU or BD).
• Vertex Drill. Compensate lens power for a change in vertex distance. You enter the compensated power (±0.12 D).
• Decentration Drill. Per-lens decentration and minimum blank size from frame specs. You enter the decentration per lens (±0.1 mm) and minimum blank size (±0.5 mm).

A ± figure is how far a typed answer can be off and still count. Other answers must match exactly, though an axis of 0 counts the same as 180. Your best score out of 10 on each drill is saved in this browser. Calc Drills come with every paid plan, starting with Basic.