The paper describes an optimised strategy to measure the porosity in natural bre composites. Particular emphasis is placed on thermoformed flax fibres reinforced poly-lactic acid (FFRPLA) laminates. Six different methods to measure the porosity were evaluated and compared: gravitometry in ethanol and in water, scanning electron microscopy (SEM), computed tomography (CT) scanning, optical microscope measurements and inverse identication from tensile tests. The measurements indicate porosity levels ranging from 3.5% to 21%. A measurement approach consisting of defining lower and upper bounds of porosity provided by CT-scanning and gravimetric measurements in water was identified. The lower bound of this interval represents the best approximation of the effective porosity, which is 8.3±2.3% for the FFRPLA tested in this work. The approach could be also extended to other similar classes of natural fibre reinforced composites.