The present disclosure relate to a bit-width control method of bi-directional optical flow (BDOF) for coding a video signal. The method includes obtaining a first reference picture I(0) and a second reference picture I(1) associated with a video block, obtaining first prediction samples I(0)(i,j) of the video block from a reference block in the first reference picture I(0), obtaining second prediction samples I(1)(i,j) of the video block from a reference block in the second reference picture I(1), controlling internal bit-widths of the BDOF by deriving internal bit-widths of intermediate parameters, obtaining motion refinements for samples in the video block based on the BDOF being applied to the video block based on the first prediction samples I(0)(i,j) and the second prediction samples I(1)(i,j), and obtaining bi-prediction samples of the video block based a the motion refinements.