Septic Drainfield Calculator
Drainfield flow, area, and trench length from bedrooms and soil.
What it does
A drainfield fails when math meets mud: too much daily flow on too little soil, and sewage surfaces. Sizing starts from bedrooms and fixture load, then divides by what each square foot of soil can take.
The tool above takes bedrooms, dwelling class, and soil in US or metric units. It returns design flow, bottom absorption area, and trench length.
The default 3 bedroom class II home on loam runs 300 gpd over 500 ft2, or 167 lineal ft of 3 ft trench. The metric mirror reads 1136 L/day over 46.5 m2, or 52 lineal m.
Minnesota Rules chapter 7080 carries the numbers: Table IV class formulas and Table IXa percolation loading rates. EPA system guidance frames the train, tank settling solids before soil treats effluent.
Rules vary by county and soils must be confirmed by pit and perc test. Use this page to speak the inspector language, then let the licensed designer stamp the plan.
How to use
- Type the bedroom count.
- Pick dwelling class I, II, or III.
- Pick the soil class.
- Read design flow first.
- Read absorption area beside it.
- Read trench length last.
Formula
- bed
- Bedroom count, 1 to 10.
- class
- Dwelling water use class I, II, or III.
- rate
- Soil loading rate in gpd per ft2.
Minnesota Rules 7080.1860 Table IV fixes the three class formulas, with class I also flat rated at 150 gallons per bedroom. Table IXa fixes loading rates by percolation: 0.1 to 5 MPI at 1.2, 6 to 15 at 0.78, 16 to 30 at 0.6, 46 to 60 at 0.45, and over 60 to mounds only. EPA describes the tank plus drainfield train this math feeds.
Worked example
Example: 3 bedroom home on loam
Imperial defaults: 3 bedrooms, class II, loam.
- Home: 3 bedrooms, class II, loam soil.
- Flow: (3 + 1) x 75 = 300 gpd per Table IV.
- Rate: loam takes 0.6 gpd per ft2 per Table IXa.
- Area: 300 / 0.6 = 500 ft2 of bottom absorption.
- Trench: 500 / 3 ft wide = 167 lineal ft.
- Tool shows 300 gpd, 500 ft2, 167 lineal ft (3 ft wide).
Reference table
| Home and soil | Flow | Field |
|---|---|---|
| 3 bed II, loam 0.6 | 300 gpd | 500 ft2, 167 ft |
| 4 bed I, sand 1.2 | 600 gpd | 500 ft2, 167 ft |
| 5 bed III, clay loam 0.45 | 294 gpd | 653 ft2, 218 ft |
| 3 bed II metric | 1136 L/day | 46.5 m2, 52 m |
Limits
- Planning estimate, not a permit design.
- Needs pit and perc confirmation on site.
- Conventional trenches only; mounds separate.
- Local codes can exceed state minimums.
Common mistakes
- Sizing by bathrooms instead of bedrooms.
- Using class I flow for a compact cabin.
- Trenching tight clay deeper instead of mounding.
- Skipping setbacks to wells and lines.
- Driving over the finished absorption area.
Try it yourself
Quick checks. Answers sit tucked under each question.
4 bed class I on sand?
600 gpd over 500 ft2 at 1.2: 167 lineal ft of 3 ft trench.
5 bed class III on clay loam?
294 gpd over 653 ft2 at 0.45: 218 lineal ft of trench.
Tight clay lot?
Perc over 60 MPI needs a mound or advanced unit, not a plain trench.
Questions
What are dwelling classes?
Bedrooms plus water fixtures set the class: I over 800 sq ft per bedroom or 3+ fixtures, II 500 to 800, III under 500.
How is field area sized?
Flow over soil loading rate. Sand 1.2 down to clay loam 0.45 gpd per ft2 in this tool.
Tight clay lot?
Slow perc over 60 MPI needs a mound or advanced unit, not a deeper trench.
Setbacks?
20 ft from structures and 10 ft from property lines per Table VII, with well and water setbacks beyond that.
How deep must treatment soil run?
Three feet of unsaturated treatment zone below the media, kept out of floodways and saturation.
Does this replace a permit?
No. This sizes new conventional trenches for planning talks with the local inspector, who permits the real system.
Sources
- US EPA: how your septic system works
- Minnesota Rules 7080.1860: design flow Table IV
- Minnesota Rules 7080.2150: loading rates Tables IX and IXa