The magnetotail magnetic flux (MTF) is an important global variable to describe the magnetospheric state and dynamics. Existing methods of MTF estimation on the basis of the polar cap area, inferred from observations of global auroras and field-aligned currents, do not allow benchmarking due to the absence of a gauge for comparison; besides, they rarely allow a systematic nearly real time MTF monitoring. We describe three modifications (F0, F1, and F2) of the method to calculate the MTF, based on simultaneous spacecraft observations in the magnetotail and in the solar wind, suitable for real-time MTF monitoring. The MTF dependence on the solar wind parameters and the observed tail lobe magnetic field is derived from the pressure balance conditions. An essential part of this study is the calibration of our approximate method against global 3-D MHD simulations and the empirical T14 magnetospheric field model. The calibration procedure provides all variables required to evaluate F0, F1, and F2 quantities and, at the same time, computes the reference MTF value through any tail cross section. It allowed us to extend the method to be used in the near tail, investigate its errors, and define the applicability domain. The method was applied to Cluster and THEMIS measurements and compared with methods of polar cap area calculation based on IMAGE and AMPERE observations. We also discuss possible applications and some recent results based on the proposed method. ; This work was supported by Russian Science Foundation grant 14-17-00072. The CLUSTER-based tail magnetic flux computation for years 2001–2009 was supported by FP7 ECLAT project, they are now available via CSA. We are grateful to all teams for the opportunity to use their observational data. We thank NASA CDAWeb (http://cdaweb.gsfc.nasa.gov/istp_public/) for IMF, solar wind, and magnetic index data; the AMPERE project (http://ampere.jhuapl.edu) for AMPERE data; the THEMIS website (http://themis.ssl.berkeley.edu/) for THEMIS spacecraft data, Cluster Science Archive (http://www.cosmos.esa.int/web/csa) for Cluster data; the IMAGE-FUV data were supplied by the NASA Space Science Data Centre(NSSDC). The global MHD simulations were made possible due to NASA Community Coordinated Modeling Center effort (http://ccmc.gsfc.nasa.gov/). S.E.M. was supported by the Science and Technology Facilities Council (STFC), UK, grant ST/K001000/1. The work at the Birkeland Centre for Space Centre, University of Bergen, Norway, was supported by the Research Council of Norway/CoE under contract 223252/F50. We also thank Stepan Dubyagin for help in calculating THEMIS total pressure, Peter Boakes for data on magnetotail plasma domains, and Marianna Kholeva for help with the manuscript preparation. ; Peer-reviewed ; Publisher Version