Septic System Size Calculator

About the Septic System Size Calculator

A septic tank only does half the job. The liquid that leaves it still carries bacteria and nutrients, and it is the drain field, also called a leach field or absorption field, that finishes the treatment. Perforated pipes in gravel-filled trenches spread the effluent over a large area of soil, where it soaks in slowly and is cleaned by the soil itself. Getting the drain field size right is the most important part of a septic system design: too small and it floods, too large and the cost rises for no benefit.

This septic system size calculator works out the drain field you need. It starts from the design flow for your number of bedrooms, applies a long-term application rate based on your soil's percolation (perc) test, and gives the absorption area in square feet, the total length of trench at your chosen width, and how many separate trenches that makes.

How to Use the Septic System Size Calculator

Enter the number of bedrooms, which sets the design flow.

Enter the perc rate in minutes per inch from your percolation test.

Enter the trench width, usually 2 or 3 feet, and the longest trench your code allows.

The Formulas

  daily flow        = bedrooms × 150 gallons a day
  application rate  (gallons per ft² per day, from the perc rate)
      up to 5 min/in    1.2
      6–15 min/in       0.8
      16–30 min/in      0.6
      31–60 min/in      0.45
  absorption area   = daily flow ÷ application rate
  trench length     = area ÷ trench width

Step-by-Step Example

A three-bedroom house on soil that percs at 20 minutes per inch, with 3-foot trenches.

  daily flow   3 × 150 = 450 gallons a day
  rate         20 min/in → 0.6 gal/ft²/day
  area         450 ÷ 0.6 = 750 ft² of trench bottom
  length       750 ÷ 3 = 250 ft of trench
  trenches     250 ÷ 100 ft max → 3 trenches of about 83 ft

On faster soil at 10 minutes per inch, the rate rises to 0.8 and the area falls to about 563 square feet.

What the Perc Test Measures

A percolation test measures how quickly water drains into the soil where the field will go. Holes are dug to trench depth, soaked, and then the drop in water level is timed, giving a rate in minutes per inch. Sandy soil may perc in a few minutes per inch; dense clay can take an hour or more. The rate tells the designer how much effluent each square foot of trench can absorb day after day without the soil becoming saturated, which is what the application rate captures.

Soil That Is Too Fast or Too Slow

Very slow soil, beyond about 60 minutes per inch, cannot absorb wastewater fast enough, and the effluent would pool or break out at the surface. Very fast soil, quicker than about one minute per inch, lets it reach groundwater before it has been treated. Neither suits a conventional gravity drain field. Alternatives include mound systems, which build a bed of suitable sand above the natural ground; sand filters; drip distribution; and aerobic treatment units that clean the effluent further before it reaches the soil. These usually need an engineer's design.

Trench Layout

Trenches are typically 18 to 36 inches wide and dug on the contour so the pipe is level, letting effluent spread evenly. Codes often limit a single trench to about 100 feet, require a minimum spacing of six feet or more between trenches, and set distances from wells, property lines, buildings and streams. A distribution box or pressure dosing system shares the flow between trenches. Many jurisdictions also require a reserve area of the same size, left undisturbed, in case the first field ever fails.

Protecting the Drain Field

A drain field can last 20 to 30 years or more if it is looked after. Pump the septic tank regularly so solids never reach the field, spread laundry over the week, fix leaking toilets and taps, and never drive or park on the field, which compacts the soil. Keep trees and shrubs with aggressive roots away from the trenches, and direct roof and surface water elsewhere so rainfall does not saturate the soil. Grass is the ideal cover.

Cost Factors

The drain field is often the largest part of a septic budget. Its cost rises with the area needed, so slow soil that needs a bigger field costs more, and engineered alternatives such as mounds can cost two to three times a conventional field. Access, rock, slope and the distance from the tank also matter. Knowing the approximate area helps you check quotes and judge whether a lot is practical before you buy it.

Water Use and Field Life

Low-flow toilets and showers reduce daily flow, easing the load on the field and extending its life.

Understanding Your Result

The headline is the absorption area in square feet.

The absorption area line shows the daily flow, the application rate and the area in square metres.

The trenches line gives the total trench length and how it splits into separate runs.

When Should You Use This Calculator?

Use it after a perc test to understand the size of field your lot must accommodate.

Use it to check whether a design or quote is in the right range.

Use it to compare how much room different soil areas on a lot would need.

Common Mistakes

Using the tank size to size the field. The field depends on flow and soil, not tank gallons.

Forgetting the reserve area. Many codes require space for a replacement field.

Ignoring setbacks. Wells, streams and property lines limit where trenches can go.

Building a conventional field in unsuitable soil. Very slow or fast soil needs an engineered system.