This document specifies a method of measuring the air permeability and the porosity of a packed bed of metal powder, and of deriving therefrom the value of the envelope-specific surface area. The permeability is determined under steady-state flow conditions, using a laminar flow of air at a pressure near atmospheric. This document does not include the measurement of permeability by a constant volume method.Several different methods have been proposed for this determination, and several test devices are available commercially. They give similar, reproducible results, provided that the general instructions given in this document are respected, and the test parameters are identical.This document does not specify a particular commercial test device and corresponding test procedure. However, for the convenience of the user, an informative annex has been included (see AnnexA) which is intended to give some practical information on three specific methods: the Lea and Nurse method, involving a test device which can be built in a laboratory (see A.1); the Zhang Ruifu method, using a similar test device (see A.2); the Gooden and Smith method, involving a test device which can be built in a laboratory but for which a commercial test device also exists. (Two types of commercial test device exist; one of these is no longer available for purchase, but is still being used, see A.3.) These methods are given as examples only. Other test devices available in various countries are acceptable within the scope of this document.This testing method is applicable to all metallic powders, including powders for hardmetals, up to 1000µm in diameter, but it is generally used for particles having diameters between 0,2µm and 75,0µm. It is not intended to be used for powders composed of particles whose shape is far from equiaxial, i.e. flakes or fibres, unless specifically agreed upon between the parties concerned.This testing method is not applicable to mixtures of different metallic powders or powders containing binders or lubricant.If the powder contains agglomerates, the measured surface area can be affected by the degree of agglomeration. If the powder is subjected to a de-agglomeration treatment (see AnnexB), the method used is to be agreed upon between the parties concerned.