Septic Drain Field Calculator
Drain field area and trench length from soil type and bedrooms.
Enter your values
Enter the bedrooms you are working with.
Enter the flow per bedroom in gal/day.
Choose one of: Sand (fast), Sandy loam, Loam, Silt loam, Clay (slow).
Enter the trench width in ft.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Drain field area and trench length from soil type and bedrooms. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Bedrooms, Flow per bedroom, Soil type, Trench width |
| Main output | Absorption area |
| Formula | area = daily flow / loading rate |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Bedrooms
- Enter the bedrooms you are working with.
- Flow per bedroom(gal/day)
- Enter the flow per bedroom in gal/day.
- Soil type
- Choose one of: Sand (fast), Sandy loam, Loam, Silt loam, Clay (slow).
- Trench width(ft)
- Enter the trench width in ft.
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.
What do I need to enter into the Septic Drain Field Calculator?
Just 4 values: bedrooms, flow per bedroom, soil type, trench width. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Septic Drain Field Calculator work out the answer?
Absorption area = daily wastewater flow divided by the soil loading rate. Trench length is that area divided by trench width. It applies the formula area = daily flow / loading rate and shows the working so you can check each step by hand.
Is the Septic Drain Field Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.