Non-SI unit
Name Symbol Quantity Expressed in terms of SI base units Expressed in terms of other SI units
dioptre D optical refractive power,
reciprocal focal length
m−1 1 / m


The dioptre, symbol D, is a non-SI unit of measurement of the optical power of a lens or curved mirror.

The dioptre is not accepted for use with the SI.
One dioptre is equal to the reciprocal of the focal length measured in metres.

1 \ \text{D} = 1 \ \text{m}^{-1}

Although the dioptre is based on the metre, it is not included in the SI. The unit for optical power should therefore be specified explicitly as the reciprocal metre, m−1.