Artificial reefs are constructed for many reasons around the world, but they usually have to comply with some form of regulation which may require demonstration of accurate deployment through some form of post construction measurement. Reefs related to fisheries are often subjected to scientific census to assess whether production and/or residence is related to reef size and cost. Once deployed under water, there is the obvious problem of obtaining an accurate assessment of the size and shape of reefs. This study examined two methods for estimating reef size that were employed on a large-scale matrix of 30 separate artificial reefs constructed off the west coast of Scotland: (1) measurements made by standard SCUBA divers and (2) estimates based on raster graphic image analysis of images created from multibeam echosounder (MBES) xyz datasets. The degree of agreement ranged between the two methodologies with no obvious trends; it was assumed that the MBES data would be more accurate because of the lower numbers of assumptions. More simple-shaped blocks produced more shallow and scattered profiles than the blocks containing voids. More reef scatter (as measured by footprint area) was recorded at greater water depths.