Septic Drain Field Calculator
Drain field area and trench length from soil type and bedrooms.
Enter your values
ⓘ Live results
Results
- Absorption area
- 750 sq ft
- Trench length
- 250 ft total
- Suggested layout
- 3 trenches x 100 ft
- Daily design flow
- 450 gal/day
- Minimum tank size
- 1,250 gallons
Working
- 1450 gal/day / 0.6 gal/ft2/day = 750 ft2
Quick answer
Absorption area = daily wastewater flow divided by the soil loading rate. Trench length is that area divided by trench width.
area = daily flow / loading rate
How to use the Septic Drain Field Calculator
- 1Count bedrooms - regulators size systems by bedrooms, not occupants.
- 2Enter your perc test result as a soil loading rate.
- 3Check the result against your county's minimum sizing table.
Worked example
Using the values the calculator loads with:
Inputs
- Bedrooms3
- Flow per bedroom150 gal/day
- Soil typeLoam
- Trench width3 ft
Results
- Absorption area750 sq ft
- Trench length250 ft total
- Suggested layout3 trenches x 100 ft
- Daily design flow450 gal/day
- Minimum tank size1,250 gallons
- 450 gal/day / 0.6 gal/ft2/day = 750 ft2
Frequently asked questions
Do I need a perc test?
Yes for any new or replacement field. The soil loading rate drives the whole design and no permit is issued without it.
How long does a drain field last?
25 to 30 years with good maintenance. Pumping the tank every 3 to 5 years is what keeps solids out of the field.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.