Ruler for automatically measuring diameter at breast height

The utility model relates to a ruler for automatically measuring diameter at breast height. The ruler comprises a locking seat and a scale ruler connected with the locking seat. The ruler is characterized in that the locking seat is provided with a guide passage through which the scale ruler can pass; the center of the guide passage is provided with an L-shaped elastic locking tongue; the root portion of the locking tongue is connected with the locking seat; a working surface of the locking tongue, which is opposite to the scale ruler, is provided with at least one toothed protrusion; a surface of the scale ruler, which contacts with the locking tongue, is provided with a toothed bar matched with the locking tongue; and the outer surface of the scale ruler is provided with graduations indicating diameter.