The purpose of this paper is to forecast the maximum solar flare magnitude in an active region and its uncertainty from photospheric magnetic field data. We analyzed 21 flare samples covering X‐ray flare magnitudes from A5.0 to X17.0. Photospheric magnetic parameters are obtained from vector and line of sight magnetograms observed with the Solar Flare Telescope and the Michelson Doppler Imager. Magnetic parameters are averaged axial field strength, averaged longitudinal field strength, and characteristic scale length. These parameters show linear relations with the X‐ray flare magnitudes. This enables us to forecast the maximum flare magnitude an active region will produce. The uncertainty in the forecast is estimated with a simultaneous tolerance interval analysis under 95% confidence level and probability. The minimum uncertainty is factor 3.7 with the data from high‐shear regions (shear angles larger than 60°). Therefore, log F _X of the regionally largest flare is forecasted with an uncertainty of log (3.7) = 0.57. For example, an X7.0 flare can be predicted with an uncertainty between X1.9 and X25.9.