Convert contacts to glasses (and back).

No. Glasses sit about 12 mm in front of the eye and contacts sit on it, so the same eye needs a slightly different power on each plane. Below about ±4.00 the two round to the same 0.25 step. A contact order also carries a base curve and a diameter that a glasses paper does not list.

Divide the glasses power by one minus the vertex distance in meters times that power: F_contact = F_glass ÷ (1 − 0.012 × F_glass). Round the result to the nearest 0.25. The calculator above runs it in both directions, and the chart lists every step from −1.00 to −12.00.

Vertex distance is the gap between the back of a lens and the front of the eye, about 12 mm for a typical pair of glasses. A contact lens sits on the cornea, so its vertex distance is zero. That gap changes how much power actually reaches the eye, which is why the two numbers separate as they grow.

For a nearsighted eye the contact number is the weaker of the two. Moving a minus lens closer to the eye makes it more effective, so less power is needed. A −6.00 glasses power converts to about −5.50 in contacts, while a −2.00 stays −2.00 once it is rounded to the nearest 0.25.

Not as it stands. The power needs converting above about ±4.00, and a contact order also carries a base curve and a diameter that no glasses paper lists. Those two come from a fitting at the shop. The converted number tells you what to expect, not what to type into the order.

Cylinder uses the same formula, but it is usually left as printed below about −3.00 because the shift is smaller than the 0.25 rounding step. Above that, sphere and cylinder are converted separately and the axis stays as written. Toric contacts come in their own set of available steps.