Selecting the correct filter paper in laboratory science is dependent upon the requirements involved in the use of the paper. These requirements will determine what type of particle retention and paper grade will be needed. Porosity and flow-rate are two factors that determine the particle retention of a paper. Particle retention is measured in relation to the size of the particle being retained. A smaller retention value indicates greater particle retention. This should be used as a guide to select a paper based on the amount and size of the filtrate in question. Generally, Qualitative grade papers are used in most laboratory applications that do not require gravimetric or vacuum filtration. In laboratories using these procedures, a Quantitative grade paper should be used.