A typical septic system has two main parts: a buried, watertight tank and a drain field of perforated pipe or chambers in the soil (EPA, June 10, 2026). Everything that goes down your drains flows into the tank, where solids sink and grease floats. The liquid in the middle flows out to the drain field, and the soil underneath does the actual cleaning, removing bacteria, viruses and nutrients before the water reaches groundwater (EPA, June 10, 2026).
It's natural to think of a septic system as a tank. In the calculator's own breakdown the tank is 12% of a conventional install and the drain field is 45%, and the field is also where an unpumped tank's problems end up.
Schematic, not to scale. Percentages and dollar ranges are this site's calculator breakdown for a 3-bedroom conventional system, average soil, national average; other systems and states split differently (the drain field or mound is 55–58% on aerobic and mound systems). Numbered parts link to the guide for each.
45%28%15%12%
To scale: drain field · digging & site prep · permits & perc test · tank. Total $4,000 – $8,000 for a 3-bedroom conventional system at the national average.
1Sewer linenot itemized
Carries everything from the house to the tank. Following it out is how you find the tank; EPA says the access riser is usually about 10 feet from the house.
The cheap part. A concrete tank can last 50 years or more, per EPA. Its job is to hold back solids, so the usual failure is not the tank breaking but the tank going unpumped.
Solids sink, grease floats, the liquid between flows on. When sludge and scum build up near the outlet, solids get pushed out into the drain field and clog it.
The expensive part, and where trouble ends up. The biomat at the bottom of the trenches thickens with age (EPA: fields over 25 to 30 years old), and solids from an unpumped tank, too much water, roots and vehicles can all ruin a field.
The soil under the trenches removes bacteria, viruses and nutrients before the water reaches groundwater. That is why the system needs enough dry soil above the water table.
All the wastewater leaves the house through one main pipe into the septic tank, usually concrete, fiberglass or polyethylene (EPA, June 10, 2026). The tank's job is to hold the water long enough for it to separate into three layers:
Sludge: heavy solids that sink to the bottom. Bacteria break some of it down; the rest builds up until the tank is pumped.
Scum: oil, grease and light solids that float on top.
Effluent: the partly clarified liquid in between.
Compartments and a T-shaped outlet keep the sludge and scum from leaving the tank (EPA, June 10, 2026). As new water comes in, an equal volume is pushed out to the field (UGA Extension, October 18, 2024), so the tank always holds liquid. What builds up is the sludge and scum; EPA's guidance on when to pump is based on how thick those layers get (EPA, August 17, 2026) (see how to tell your tank needs pumping).
Step 2: the drain field treats
The drain field is a shallow, covered excavation in unsaturated soil. Effluent is released through pipes or chambers and filters down through the soil, which "accepts, treats, and disperses" it. The soil removes harmful coliform bacteria, viruses and nutrients before the water reaches groundwater (EPA, June 10, 2026). This is why soil matters so much: a perc test measures whether your soil drains fast enough to do this job (see perc test cost).
If the field gets more water than the soil can take, it floods. Sewage can surface in the yard or back up into toilets and sinks (EPA, June 10, 2026). That's the failure the rest of the system's maintenance is designed to prevent.
When soil can't do the job alone
System design depends on household size, soil, slope, lot size, nearby water and local rules (EPA, May 4, 2026). Where soil can't do the job alone, alternative systems add a treatment or delivery step. EPA describes these (EPA, May 4, 2026):
Aerobic treatment units inject oxygen so bacteria break waste down more completely, much like a small sewage plant. They suit small lots, poor soil and high water tables, but EPA notes regular lifetime maintenance should be expected. See aerobic system cost.
Mound systems pump effluent up into a built-up mound of sand above the native soil, for shallow soil, high groundwater or shallow bedrock. See mound system cost.
Sand filters, drip distribution, evapotranspiration beds and constructed wetlands are other options for specific sites (EPA, May 4, 2026).
Mound and sand-filter systems use a pump chamber (EPA, May 4, 2026), and Washington's guide to mound systems describes a high-water alarm float in the pump chamber that sets off a light and buzzer when effluent rises too high, for example if the pump malfunctions (Washington Dept. of Health, January 2025). If yours is going off, see what to do when the septic alarm goes off.
What this means for you
Almost every septic rule of thumb follows from these two steps. EPA warns that a tank full of solids can push them into the drain field and clog it (EPA, March 11, 2026). Harsh chemicals can kill the organisms the system relies on, and too much water at once can flood the field (EPA, August 17, 2026). See septic tank maintenance.
Budgeting for a new or replacement system?
The calculator gives an itemized installed-cost estimate for your bedrooms, system type, soil and state.
Cost figures are other publishers' ranges, quoted with their dates, not quotes for your job. Septic rules are set by your state and county, so confirm requirements with your local permitting office. General information, not legal or engineering advice.