Abstract: An apparatus for determining the thickness of a slab structure in which a cylindrical bore has been made. The device comprising a cylindrical tube having opposed longitudinal oriented openings in the wall at the distal end of the cylinder. The cylinder having an indicia scale of length running longitudinally and visibly on the outer surface of the cylinder with the zero point starting at the inner end of the openings. The device having a shaft within the tube having a length greater than the length of the tube, a handle on the proximal end, and a pair of butterfly arms on the distal end of the shaft. The butterfly arms being biased in the outward direction so that when the arms are opposed to the openings in the tube they will extend outward through the openings to engage the outer surface of the slab with the tube extending through the bore in the slab so that the indicia can be read on the inside of the slab to determine the thickness of the slab.