A method for measuring defects in a silicon substrate obtained by silicon ingot pulling, wherein the defects have a size of less than 20 nm. The method includes applying a first defect consolidation heat treatment to the substrate at a temperature of between 750° C. and 850° C. for a time period of between 30 minutes and 1 hour to consolidate the defects; applying a second defect enlargement heat treatment to the substrate at a temperature of between 900° C. and 1000° C. for a time period of between 1 hour and 10 hour hours to enlarge the defects to a size of greater than or equal to 20 nm, with the enlarged defects containing oxygen precipitates; measuring size and density of the enlarged defects in a surface layer of the substrate; and calculating the initial size of the defects on the basis of the measurements of the enlarged defects.