20.3 If elongation is measured over a given fixed length, it can be converted to proportional gauge length, using conversion formulae or tables as agreed before the commencement of testing (e.g. as in ISO 2566-1 and ISO 2566-2).NOTE Comparisons of percentage elongation are possible only when the gauge length or extensometer gauge length, the shape, and cross-sectional area are the same or when the coefficient of proportionality, k, is the same.