Even though all real objects are three dimensional, imaging and metrology performed by using electron-beam tools such as scanning electron microscopes is inherently two dimensional. Any information about the third dimension must therefore be obtained by inference, or by time consuming special methods such as stereo-photogrammetry. If, however, the structures of interest are thin enough to be electron transparent then quantitative three dimensional metrology can be performed directly by using off-axis transmission electron holography. Here we demonstrate the application to a SCALPEL lithography mask which consists of chromium lines on a silicon support film. The off-axis holography was performed in a field emission transmission electron microscope, a Hitachi HF2000 operated at 200keV. The sample is positioned so that half the beam passes through the specimen while the rest travels only through the vacuum. An electrostatic biprism then recombines these two components to form the hologram which is recorded onto a CCD camera.