An example of an apparatus is provided. The apparatus includes a vacuum source to draw gas and a build material from a build process. The apparatus includes a filter to separate the build material from the gas. A filtered portion of the build material is to be deposited on the filter and a collected portion of the build material is to be redirected. The apparatus includes a storage container to receive the collected portion of the build material. The apparatus includes a mass estimation engine to determine a total mass of the build material. The mass estimation engine is to estimate a mass of the filtered portion.