This International Standard specifies four test methods which may be used for the determination of the diameter and cross-sectional area of single carbon fibre filaments. It is important to note that the shape of the cross-section of the filaments from different suppliers may vary significantly. The term "diameter" used in this standard applies to all cases, from a "true" diameter, where the filament is exactly circular in cross-section, to an "apparent" diameter where the filament is not circular. The methods proposed may not be directly applicable to all types of filament. The product specification should specify which method should be used. If there is no specification, the selection of the appropriate method is a matter of judgement. The details given here are considered to be sufficiently precise to enable this choice to be made.