To provide a cardiothoratic ratio scale capable of easily and accurately measuring the cardiothoratic ratio without requiring a calculation when measuring the cardiothoratic ratio on an X-ray photograph of a chest part.

Scales from 22 cm to 33 cm different in the length by 1 cm are made, gradations by dividing the total length into equal 100 parts with the left ends as 0 and the right ends as 100 are marked, the whole scales are superposed in length order, the left ends are aligned, and are loosely joined in a part close to the left ends by a machine screw nut.