A grading method for a knitted product for obtaining knitting data of the knitted product for at least three separate sizes by means of a computer implemented grading system, the method comprising: a: a step for inputting pattern data defining at least 2D outlines of the knitted product as initial pattern data to the grading system; b: a step for inputting gauge data indicating a course directional size and a wale directional size of stitches to the grading system; c: a step for converting the initial pattern data into initial knitting data based on the gauge data by means of the grading system and for test knitting the knitted product on a knitting machine based on the initial knitting data; d: a step for comparing a size of the test-knitted product with a size indicated by the initial pattern data manually or by means of the grading system comprising an editing means capable of capturing an image of the test-knitted product; e: a step for correcting the knitting data manually or by means of the grading system and for test knitting the knitted product on a knitting machine based on the corrected knitting data, when an error between the size of the test-knitted product and the size indicated by the initial pattern data exceeds a predetermined range; and f: a step for repeating the steps d and e until the error between the size of the test-knitted product and the size indicated by the initial pattern data is within the predetermined range, characterized in that: g: the steps a to f are executed on at least two separate sizes of the knitted product, and the method further comprises: h: a step for determining correction amounts of outlines in the pattern data to the initial pattern data or to the initial knitting data for the outlines, in order to make the initial pattern data or the initial knitting data nearer to the pattern data of the test-knitted product test-knitted in the step f, by means of the grading system, when, in the step f, the error between the size of the test-knitted product test-knitted in the step f and the size indicated by the initial pattern data is within the predetermined range; and i: a step for correcting pattern data or knitting data for sizes of un-test-knitted product through interpolation or extrapolation of the correction amounts for the at least two sizes determined in the step h, by means of the grading system. a: a step for inputting pattern data defining at least 2D outlines of the knitted product as initial pattern data to the grading system; b: a step for inputting gauge data indicating a course directional size and a wale directional size of stitches to the grading system; c: a step for converting the initial pattern data into initial knitting data based on the gauge data by means of the grading system and for test knitting the knitted product on a knitting machine based on the initial knitting data; d: a step for comparing a size of the test-knitted product with a size indicated by the initial pattern data manually or by means of the grading system comprising an editing means capable of capturing an image of the test-knitted product; e: a step for correcting the knitting data manually or by means of the grading system and for test knitting the knitted product on a knitting machine based on the corrected knitting data, when an error between the size of the test-knitted product and the size indicated by the initial pattern data exceeds a predetermined range; and f: a step for repeating the steps d and e until the error between the size of the test-knitted product and the size indicated by the initial pattern data is within the predetermined range, h: a step for determining correction amounts of outlines in the pattern data to the initial pattern data or to the initial knitting data for the outlines, in order to make the initial pattern data or the initial knitting data nearer to the pattern data of the test-knitted product test-knitted in the step f, by means of the grading system, when, in the step f, the error between the size of the test-knitted product test-knitted in the step f and the size indicated by the initial pattern data is within the predetermined range; and i: a step for correcting pattern data or knitting data for sizes of un-test-knitted product through interpolation or extrapolation of the correction amounts for the at least two sizes determined in the step h, by means of the grading system.