Early site planning
Septic Drain Field Size Calculator
Turn a bedroom-based design flow and local soil loading rate into a planning area and trench layout.
Planning estimate only. A licensed designer and local permitting authority must confirm soil, setbacks, slope, reserve area, distribution method, and final dimensions.
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.
Sources and references
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.