Diopter Calculator

A diopter is a unit of optical power equal to the reciprocal of the focal length measured in meters. A lens with a 0.5 m focal length has a power of 2 diopters, and a 0.25 m lens has 4 diopters.

Use P = 1 / f with the focal length f in meters. A 0.25 m lens is 4 diopters and a 2 m lens is 0.5 diopters. If your focal length is in centimeters, divide by 100 to get meters first.

Invert the power: f = 1 / P, in meters. A 2 D lens has a 0.5 m focal length, which the tool also shows as 50 cm on the third result card.

For thin lenses held close together, the total power is the simple sum of the individual powers. Enter each one in the Combine lenses box and the calculator adds them, so 2 D and 3 D placed together make 5 D.

Yes. Negative values represent diverging lenses and subtract from the total, so a list like 2, 3, -1 sums to 4 diopters. The result is undefined only if the powers cancel to exactly zero.

A negative power is a diverging lens with a negative focal length, the kind used to correct nearsightedness. A positive power is a converging lens that brings light to a focus.

Power is the reciprocal of focal length, so a focal length of zero has no defined power, and a power of zero would mean an infinite focal length. The combine mode also rejects a set of powers that sums to zero for the same reason.

The diopter figures line up with the sphere value on a prescription, but full prescriptions also include cylinder and axis for astigmatism, which this tool does not model. Treat it as a physics and optics helper rather than a substitute for an eye exam.

No. The reciprocal and the sum both run in your browser, so the numbers you type are never uploaded.