Abstract: The invention provides an apparatus and method for continuously measuring the length of linear material during winding of the material onto a rotating core to form a package. First means form an electrical signal whose frequency is proportional to the instantaneous diameter of the package being wound. Second means form at least one pulse of fixed length for each revolution of the package. A gating means allows the variable frequency signal to reach an output line only during the period of each said fixed length pulse and a counter algebraically counts at least a proportion of the individual cycles of said signal reaching said output line.