Method and system for the weighing of sheets (6) of wood material on a sheet forming line, on which line the sheets are carried on a conveyor from a sheet forming apparatus and cooled down in a cooling turn gear comprising a cooling wheel (3) provided with radial supporters (5). The sheets (6) are weighed using a weighing system (8) which has been fitted in the cooling turn gear and which weights the sheets (6) when they are turned to an upright position or at least to a nearly upright position. The invention makes it possible to leave out the conveyor weight meter from the press line, thus allowing the press line to be made shorter. The weighing accuracy is substantially better than in a conventional conveyor weight meter solution because only the sheet itself is weighed instead of weighing the relatively heavy conveyor together with the light sheets on it.