One item that every swimming pool must have is a pool filter. A swimming pool filter is the main process that keeps your swimming pool clean of the various debris that are brought in by wind, rain, cleaning supplies, toys, and swimmers. A pool filter works to keep the water clear and promotes safety and health while in the water. The pool filter has three main components that allow it to keep your water clear and safe for swimmers. First, there is a motor and pump that work to bring the water in and push it out during filtration. Then the filter itself falls into three categories DE (Diatomaceous Earth), cartridge and sand. A DE (Diatomaceous Earth) filter is probably the best for filtering even the minutest particles that could be in the water. Diatomaceous Earth is a natural substance that is found in the soil. It has the consistency of a fine, white powder. A filter made of Diatomaceous Earth comes in two different types of filters, a spin filter and a vertical grid filter. These two terms simply signify how the filter is set up. The spin filter does not work as well as the vertical and is virtually obsolete today. A sand filter makes use of sand to filter the water of particles and allows filtered water to run through easily. The cartridge filter works in the same manner as a Diatomaceous Earth filter but without the use of any substance other than fabric mesh to filter the water. Here are a few things you should consider before purchasing a swimming pool filter: First, find the volume of the entire swimming pool. To find the volume you must consider the shape of your pool: If your pool is rectangular ? Multiply the following ? Length, width, average depth If your pool is circular ? Multiply the following? Radius, 3.14, average depth Next, figure the capacity of your pool. This will allow you to calculate how much water your swimming pool holds. This is an important factor in purchasing a pool filter. To find the capacity, multiply the volume (you found in step one) by 7.48. You will then have to determine the type of filter you want. Consider if your pool would benefit from a cartridge, DE, or sand filter. You should also consider if you want the filter to be vacuum or pressure. It is suggested that you should make the filter slightly larger than your initial calculation to allow for better filtration. There are a lot of different aspects that must be taken into consideration when purchasing a swimming pool filter. For the entire filtration system you can expect to pay anywhere from $20.00(USD) to $300.00 (USD). Replacement filters are slightly cheaper in some aspects, depending on the type of filter your system uses, you can expect to pay anywhere from $4.00 (USD) to $200.00 (USD). It is important that you follow all recommended procedures outlined by the swimming pool manufacturer. This will help you in maintaining clear and healthy pool water that can be enjoyed by all swimmers.