Determine Your Need

• 1). Test the site's soil. The size of a septic leach field depends primarily on the condition of the soil it is sited in. The more absorbent the soil, the faster water percolates through it, and the smaller the leach field can be. The less absorbent the soil, the slower water percolates through it, and the larger the field must be. Soil testing can be performed by the county agricultural extension or a septic engineer.
  
  
• 2). Determine how much water your household will use each day. The more water you use, the larger your leach field needs to be. Water usage also determines how large a septic tank you need and how often it should be pumped.
  
  If you're not sure, check the water meter at your current residence or research your water bill. Be aware that water conversation measures can drastically cut the amount of water used. Low flow showerheads and toilets, on-demand water heaters, and water efficient appliances can make a huge difference.
  
  
• 3). Consider other factors. If your property is at the bottom of a slope, it may receive drainage and need a larger leach field to absorb all the water properly. If the property sees significant rainfall, the same applies.
  
  Zoning laws may require that you build the leach field a certain distance from roads, utilities, wells, streams or other leach fields.
  
  Environmental regulations will affect leach field placement, especially if there are wetlands or vulnerable environments on your property. Check with county, state and federal authorities to see if your property is subject to any such regulations.
  
  
• 4). Do the Math. Take the gallons of water your household uses each day and divide it by the permeability of the soil. An example is a four-person household using 2,000 gallons of water per day on a site that contains sandy loam with an absorption rate of 2.5 gallons per square foot per day.
  
  Divide 2,000 by 2.5, and the answer is 800. So, the minimum size of the necessary leach field is 800 square feet.
  
  
• 5). Overestimate Your Need. Your family may expand. You might not get the septic tank pumped as often as you should. You might receive 40 days and nights of rain. Having more leach field than you need is far preferable to having less leach field than you need.
  
  If your leach field is too small, the ground becomes saturated with sewage, contaminating nearby water supplies. Sewage can also back up into your house.