Weft and Bulk hair is measured by the length of the bundled hair. Bulk hair is loose and is not sewn together in tracks. It is typically used for fusion extensions or braids. Weft hair is tied into tracks for weaving. Both are sold by the bundle/pack, in 4 ounce bundles. Usually 8-12 ounces, or 2-3 bundles/packs, are needed for full head hair extensions.