Septic & perc test FAQ
Every answer lists its sources.
What is a perc test?
A percolation (perc) test measures how fast water soaks into the soil where a septic drain field would go. A tester digs holes, pre-soaks them, and times how fast the water level drops, usually reported in minutes per inch. Many states now also require (or instead require) a soil profile evaluation, where a licensed evaluator inspects the soil layers for seasonal wetness and restrictive layers.
Sources: U.S. EPA - How Your Septic System Works; Fixr - Perc test cost (Jan 2025)
How much does a perc test cost?
Published U.S. ranges run roughly $600 to $1,900. HomeAdvisor reports most tests cost $750 to $1,900 (about $1,300 on average, with extremes of $300 to $3,000); Fixr reports $600 to $800 (about $725 on average). Price depends on the number and depth of test holes, your state's rules, and county permit fees.
Sources: HomeAdvisor - Perc test cost (Apr 2025); Fixr - Perc test cost (Jan 2025)
Is this site a perc test?
No. SepticSoil is a free pre-check from public records: the USDA soil survey's septic rating for the mapped soil at your address, FEMA's flood zone, and nearby USGS groundwater wells. It helps you decide whether a lot is worth testing. Only an on-site perc test or soil evaluation, reviewed by your county or local health department, decides what system a lot can have.
Sources: USDA NRCS - Soil Data Access; FEMA - National Flood Hazard Layer; USGS - Water Data for the Nation
What do 'not limited', 'somewhat limited' and 'very limited' mean?
They are the USDA NRCS ratings for 'Septic Tank Absorption Fields'. Not limited: the soil is very favorable. Somewhat limited: limitations can be overcome by special planning, design or installation. Very limited: limitations generally cannot be overcome without major soil reclamation, special design or expensive installation. The ratings consider depth to the water table and bedrock, flooding, slope, and how fast water moves through the soil.
Sources: USDA NRCS - Web Soil Survey
If the soil is 'very limited', can I still build?
Often, yes, but usually with an alternative system. Mound, at-grade, drip and aerobic treatment unit (ATU) systems are designed for high water tables, shallow bedrock or poor soils. They cost more to build and maintain than a conventional gravity system, and some sites cannot be permitted at all. Your local health department decides.
Sources: U.S. EPA - Types of Septic Systems
How accurate is the soil survey for one lot?
The USDA soil survey is mapped at roughly 1:12,000 to 1:24,000 scale, so a map unit can include small areas of different soil that are not shown. Treat the rating as a strong hint about what testing is likely to find, not a measurement of your exact drain field spot.
Sources: USDA NRCS - Web Soil Survey
Who do I call to get a perc test?
Start with your county or local health department (often the environmental health or onsite wastewater program). They issue septic permits and will tell you whether the test is done by their staff, a licensed soil evaluator, or a professional engineer. A licensed septic installer can often coordinate the test and design.
Why does the flood zone matter for a septic system?
Flooding can saturate or wash out a drain field and push sewage back into the house. Many states restrict drain fields in FEMA Special Flood Hazard Areas or require extra measures such as watertight tanks and backflow prevention.
Sources: FEMA - National Flood Hazard Layer; U.S. EPA - How Your Septic System Works