Early site planning
Septic Drain Field Size Calculator
Estimate the absorption area and trench layout from design flow and your local soil loading rate.
Calculation method
How drainfield area is estimated
Daily design flow equals bedrooms × design flow per bedroom. Minimum infiltrative area equals daily design flow ÷ the local soil loading rate; trench length equals area ÷ trench width.
Codes can require additional area, specific trench products, pressure distribution, or a full replacement field. Enter the local values instead of treating this model as a national rule.
Worked example
Septic field size and absorption trench calculations
The calculator keeps design flow, soil loading rate, trench width, lateral length, and reserve area separate so each local assumption remains visible.
Daily flow = bedrooms × design flow per bedroom Absorption area = daily flow ÷ soil loading rate Total trench length = absorption area ÷ trench width Example: three-bedroom planning layout
At 120 gpd per bedroom, design flow is 360 gpd. With a local loading rate of 0.60 gpd/ft², the minimum infiltrative area is 600 ft². At a 3 ft trench width, total trench length is 200 ft.
With a 100 ft maximum lateral, that example needs at least 2 laterals. A 100% replacement reserve protects another 600 ft², for 1,200 ft² total.
Sources and references
Reviewed July 28, 2026. Local permit rules and official records override planning values. How we select and review sources.
FAQ
Drainfield sizing FAQ
Can I use a perc rate directly?
Only after converting it through the loading-rate table required by your local authority. Perc rate and soil loading rate are not interchangeable.
Is this enough for a permit?
No. Site evaluation, setbacks, seasonal water table, slope, reserve area, and system type still require professional review.