Clamp-on ultrasonic flow metering can provide a non-invasive and portable means for flow measurement. However, it indicates flow rates with low measurement accuracy at low flow velocity in pipe flows. Typical accuracy of the clamp-on ultrasonic flow metering amounts as low as ${pm}1%$ if the flow velocity in a pipe is greater than 0.5 m/s. The accuracy can be increased greater than ${pm}2%$ if the flow velocity is lowered smaller than 0.5 m/s. Inner pipe diameter can be also an influential factor in flow metering when the exact value of the inner diameter is not known. The inner pipe diameter cannot be found if the pipe is too large to measure or if there are erosions or adhesions on the inner pipe surface due to small particles in the flow. These shortcomings of the clamp-on ultrasonic flow metering can be overcome by combining two transit times along a Z-shaped and a V-shaped ultrasonic path. This technique is termed combined V/Z clamp-on ultrasonic flow metering. With the water flow standard system in KRISS, this combined technique exhibited intermediate performance between the two flow metering techniques along the Z-shaped and the V-shaped ultrasonic paths. Notably, the combined technique showed better performance (expanded uncertainty less than 0.76%, k = 2) than the two flow metering techniques (1.61% and 1.17%, k = 2) in the flow range of (100~400) $m^3/h$ with pipe diameter of 250 mm.