This document specifies five test methods used for the determination of the diameter and cross-sectional area of single carbon fibre filaments.

The shape of the cross-section of the filaments from different suppliers can vary significantly. The term "diameter" used in this document 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 are not necessarily directly applicable to all types of filament. The product specification determines the method to 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.