We measured the surface roughness of three unpatterned Si wafers by four different instruments: atomic force microscope, wafer inspection station (based on light scattering), Nomarski optical microscope, and interferometric profiler. The root‐mean‐square surface roughness (Rrms) values vary between 1.2 Å as measured by an atomic force microscope over a area to 19.4 Å as measured by an interferometric profiler over a 1.32 mm path length. To explain the observation that samples showing high roughness values when measured with one technique but low when measured with another technique, we discuss the lateral (within the surface plane) characteristics of the different methods and show that the experimental results are not contradictory. We suggest that a parameter describing the lateral properties of a surface roughness measurement technique should be included when reporting surface roughness data.