6.1 Grammage or Basis Weight

Grammage and basis weight are synonymous terms reflecting on the weight per unit area of the media. Grammage usually follows the metric (SI) system. Its units are g/m ². Basis weight usually refers to US customary units such as reams of material, lb/1,000 ft ², or g/yd ². Ream units are used by the American paper industry. The most common form for a filter medium is lb/3,000 ft ². This is developed from the weight of a ream containing 500 sheets, 24 36in. (24 36 500). Filter medium basis weights have also been based on ream sizes of 2,880 ft ² (24 36-480). Sheeted filter media used for industrial filtration such as plate and flame filters are often listed at ream sizes of 1,389 ft ² (20 20 500). In referring to basis weights based on ream sizes, one must make sure the ream size area is understood. The basis weight unit of lb/1,000 ft ² is used by the American paperboard industry. The unit of oz/yd ² and g/yd ² are popular with the American textile industry. These are units often applied to nonwovens produced by dry formed processes. Because of the confusing nature in which basis weights and grammages are reported in the US, the following Table 6.1 is offered. As much as possible, grammage and the units of g/m ² will be used in this handbook.