A spherometer is an instrument used for the precise measurement of the radius of curvature of a curved surface. Originally, these instruments were primarily used by opticians to measure the curvature of the surface of a lens.

A spherometer usually consists of:

On new spherometers, the vertical scale is marked off in units of 0.5 mm. One complete turn of the dial also corresponds to 0.5 mm and each small graduation on the dial represents 0.005 mm. The graduations on old spherometers are 0.001 mm.

To measure the radius of a sphere—e.g. the curvature of a lens—the spherometer is leveled and read, then placed on the sphere, adjusted until the four points exert equal pressure, and read again. The difference gives the thickness of that portion of the sphere cut off by a plane passing through the three feet. A contact-lever, delicate level or electric contact may be attached to the spherometer in order to indicate the moment at which the four points exert equal pressure.

The spherometer directly measures a sagitta, h. If the mean length between two outer legs is a, the spherical radius R is given by the formula

Using a spherometer with a circle cup of diameter D, the spherical radius R is instead given by the formula

Since the spherometer is essentially a type of micrometer, it can be employed for purposes other than measuring the curvature of a spherical surface. For example, it can be used to measure the thickness of a thin plate.

To do so, the instrument is placed on a perfectly level plane surface and the screw turned until the point just touches; the exact instant when it does so is defined by a sudden diminution of resistance followed by a considerable increase. The dial and scale are read, the screw is raised, the thin plate slipped under it, and the process is repeated. The difference between the two readings gives the required thickness.