Drainfield Size Calculator
Enter the number of bedrooms, the soil percolation rate from a perc test and the trench width. The calculator works out the daily flow, the application rate and the drainfield area and trench length to plan for.
- Drainfield area
- Trench length
- Daily flow
The percolation rate comes from a perc test, in minutes for the water to drop one inch.
The drainfield must soak away a day's wastewater. Allow 150 gallons a day per bedroom. A three bedroom house makes 450 gallons a day. In soil that percs at 20 minutes an inch, each square foot takes 0.6 gallons, so the field needs 750 sq ft, or 250 ft of 3 ft trench.
Drainfield size chart
Three bedrooms, 450 gal a day. Search for a perc rate.
| Perc (min/in) | Rate (gal/ft²/day) | Area (sq ft) | Trench (ft) |
|---|---|---|---|
| 3 | 1.2 | 375 | 125 |
| 5 | 1.2 | 375 | 125 |
| 10 | 0.8 | 562 | 188 |
| 15 | 0.8 | 562 | 188 |
| 20 | 0.6 | 750 | 250 |
| 30 | 0.6 | 750 | 250 |
| 45 | 0.45 | 1000 | 333 |
| 60 | 0.45 | 1000 | 333 |
3 ft trenches.
Soil and field size
Three bedroom house. Slide the perc rate.
The trenches lengthen as the soil drains more slowly.
How to size a drainfield
House to trench. Step through it.
Bedrooms
Count them.
Keep cars, sheds and trees off the drainfield. Weight and roots damage the pipes.
Trench, bed or mound
Field types. Compare them.
Most common.
One wide area.
Built up.
Mound systems are used where soil is too slow or the water table too high.
Before you plan
Drainfield checks. Tick each item.
Tick each item as you go.
Keep the field well away from wells and streams. Local rules set the distances.
What you get
Drainfield sizing. Each tile explains one result.
Square feet.
Length.
Gallons a day.
Soil.
Area.
Many areas require a reserve area the same size for a future replacement field.
Frequently Asked Questions
No question matches that search.
How big should a drainfield be?
Divide the daily flow by the soil application rate from a perc test.
How much wastewater does a house make?
Design rules usually allow one hundred and fifty gallons a day per bedroom.
What is a perc test?
A test of how fast water soaks into the soil, in minutes per inch.
What if my soil drains too slowly?
A mound or other engineered system may be needed.
How long are drainfield trenches?
Usually no more than one hundred feet each.
Can I drive over a drainfield?
No. The weight can crush the pipes and compact the soil.
Do I need a permit?
Almost always. Local health rules set the final design.
Plan the space
Size the drainfield early and keep room for it, and the septic system can be designed without surprises.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.