A method and device for automatically measuring the yarn length absorbed by a rectilinear machine adapted to produce an article and including a carriage movable to-and-fro over a needle bed between two ends of a working path, needles moving towards the carriage to cooperate with the yarn (F) and form an article, the yarn unwinding from a feeder with elements for measuring the yarn quantity fed at constant tension towards the needle bed. The feeder measures and constantly monitors the yarn feed velocity, this monitoring enables the fed and knitted yarn quantity to be obtained in real time and each measurement to be associated with the respective carriage travel stroke, this information enabling precise regulation of the knitting cams.