The major reason for figuring infiltration rate and whether your soil is mostly clay is to consider your need for soil amendments. Soil amendments can significantly increase the cost; however, without them many areas would not absorb water quickly enough to properly function. If your infiltration rate is less than 5 inches per day or your soil is mostly clay (or likely both), it is almost essential that you amend the soil.
In amending the soil, the key is to add compost (organic matter) and sand. Most literature suggests a mix of 50-60 percent sand, 20-30 percent topsoil, and 20-30 percent compost, with no more than 10 percent clay. In building a mix for the garden use a convenient system such as two pails of sand for each pail of topsoil and compost.
If your infiltration rate suggests that your soil should be amended and you would rather not, another option is to increase the area of the garden and make it shallower. To figure the cost with this tool, still check that you will be amending the soil. This should give you still a rough estimate of the cost. However, the area suggested for that cost should be increased 30-50 percent. So for example, if by clicking that you will amend the soil, you get a size of 200 square feet. You can either build the garden at the size listed and amend the soil, or you can choose a larger size for the garden, between 260 and 300 square feet. (200 multiplied by .3 equals 60, and 60 plus 200 gives an increase of 30 percent. Multiplying by .5 and adding it to the original value gives an increase of 50 percent.)