We had a cistern that is 15' by 15' by 12' high. We have never run out in the dry yet. We know to conserve heavy in the dry and have low volume flush toilets, old style wringer washers and such. That was where we used to live for several years. I am planning to build one the same at our new house. Above or below ground depends on how much space on your lot. I cannot build below ground here as my watertable is too high.