Main text The objective of the Key Comparison CCM.FF-K1.2015 for water flow measurement was to support and prove the Calibration and Measurement Capabilities (CMC) of the participating NMIs of Japan (AIST), Mexico (CENAM), Chinese Taipei (ITRI), Korea (KRISS), P.R. China (NIM), Germany (PTB), USA (NIST), Sweden (RISE), UK (NEL), Turkey (TUBITAK UME) and Netherlands (VSL). The comparison was organized as a round robin, started in December 2015 at PTB and finished in April 2018, also at PTB. As pilot laboratory, the national metrology institute of Germany (PTB) organised the comparison. A combined setup of a turbine meter and Coriolis meter was used as a transfer standard (TS), which was provided by the pilot laboratory. The nominal calibration conditions of the KC were defined within the flow range between 30 m3/h and 200 m3/h, 20 °C fluid temperature and 3 bar line pressure. A special focus of the comparison was to estimate the uncertainties of the transfer standard (u TS). Both transfer meters were subjected to extensive characterization measurements at pilot laboratory, with the following investigated parameters: fluid temperature, line pressure, repeatability, flow stability, meter sensitivity to varying inflow conditions and hysteresis effects. For turbine meter, all labs passed the E N criteria of ≤ 1.20. The calibrations of the turbine meter were strongly affected by the presence of the large values for u TS with > 0.12 % k =1) which were mainly caused by the meter sensitivity to disturbed inflow conditions. This effect led to inconclusive calibration results for all laboratories. The evaluation criteria u comp/u base exceeded the critical value of 2.00. Finally, the turbine meter was not suitable for a confirmation of all submitted CMC values. For Coriolis meter, all labs passed the E N criteria of ≤ 1.20. In contrast to turbine meter, the evaluation criteria u comp/u base exceeded the critical value of 2.00 for one laboratory, only. The maximum uncertainty u TS of Coriolis meter was estimated with 0.022 % (k =1). In summary, the comparison was successfully finished for a confirmation of the submitted CMC values, related to mass calibrations. For volume related CMCs this comparison was not suitable. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).