Quantitative applications of mobile multibeam sonar in aquatic ecology and fisheries require accurate and efficient in-tank calibration methodologies. Calibration factors for a Simrad SM20 multibeam sonar are experimentally extracted thereby enabling sonar estimation of target strength and volume backscattering strength. Measured and modeled sonar characteristics are systematically compared and show good overall correlation. Due to the limited angular span of the sonar head array, well quantified sonar operation is restricted to an equatorial angular sector of only 80° (vs. rated 120°) in 'imaging' mode. In 'echo-sounder' mode, the 'high' power transmit setting appears to introduce artifacts. A routine in-tank measurement procedure is described which for given multibeam sonar minimizes the time required for quality multibeam calibration