Septic systems · drain field
Septic drain field size calculator.
Turn design flow and a soil loading rate into the absorption area and trench length of a drain field. Look up a Minnesota loading rate from a percolation test or enter your own local rate. Runs in your browser; nothing is sent anywhere.
Planning aid: codes and soils vary by location. Confirm septic and well work with your local health department and a licensed professional.
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Worked example
A 3-bedroom house: 3 × 150 = 450 gpd. In Minnesota Table IXa a 20 min/inch soil under Level C has a loading rate of 0.6 gpd/ft², so absorption area = 450 / 0.6 = 750 ft². With 3 ft wide trenches, trench length = 750 / 3 = 250 ft, for example 3 trenches of about 83 ft. Maryland requires 9 ft of undisturbed soil between 3 ft wide trenches.
Formulas, sources, and limits
Formulas: Absorption area = design flow / loading rate. Standard trench length = absorption area / trench width (Maryland COMAR 26.04.02.05L and M). Residential design flow is at least 150 gpd per bedroom (COMAR 26.04.02.05I). Minnesota Table IXa loading rates in gpd/ft² (Level C | Level A/A-2/B/B-2): 0.1 to 5 min/in 1.2 | 1.6 (fine sand and loamy fine sand 0.6 | 1.0); 6 to 15: 0.78 | 1.0; 16 to 30: 0.6 | 0.78; 31 to 45: 0.5 | 0.78; 46 to 60: 0.45 | 0.6; 61 to 120: not allowed | 0.3; under 0.1 or over 120: not allowed.
Sources (retrieved 2026-10): Maryland COMAR 26.04.02.05, sections I, L, M, N; Minnesota Rules 7080.2150, Table IXa.
Limits: The loading rate drives everything, and it varies by state, soil texture, treatment level and system type. The Minnesota table is offered as one published reference, not a national standard; this tool does not apply Minnesota’s mound ratios or trench-depth rules. Level C/A/B definitions are in the Minnesota rule. Gravel-less chambers, pressure distribution, beds, mounds and setback distances are outside scope.
Frequently asked questions
How big should a septic drain field be?
Divide design flow in gpd by the soil loading rate in gpd per square foot to get the absorption area, then divide by trench width for the length.
How many gallons per day per bedroom?
Maryland’s rule uses at least 150 gpd per bedroom for residential design flow. Other jurisdictions differ.
What is a percolation test?
A hole is filled with water and the drop rate is timed in minutes per inch; slower soil needs more absorption area.
Can I use this for permits?
No. Permits require a site evaluation and a design that meets your local code.