Drainfield size comes from two measured inputs: the design flow the house will generate, based on bedroom count, and the soil’s application rate, measured by a percolation test or soil evaluation. A house with more bedrooms needs more field area, and so does a house sitting on slower-absorbing soil, even at the same bedroom count.
Application rate is usually expressed as how much effluent a square foot of that specific soil can safely absorb per day, and it comes directly from the site’s own soil evaluation, not from a table based on location alone. Coarser, well-drained soils absorb faster and need less field area for the same flow; tighter, clay-heavy or poorly drained soils absorb slower and need more.
Field size is calculated by dividing the required daily flow by that measured application rate, which is why two houses with the same number of bedrooms can need very different field footprints depending entirely on what the soil evaluation found. This is also why an accurate soil evaluation matters more to system cost and footprint than almost any other single decision in the design.
Dispersal method also affects the footprint for a given flow and soil: some methods spread effluent more efficiently per square foot than a conventional gravel trench, which is one reason a mound, chamber, or drip system might be chosen on a constrained lot even where a conventional field would technically work.
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The house's design flow, based on bedroom count, and the soil's measured application rate from a percolation test or soil evaluation.
Because the soil under each lot absorbs effluent at a different rate; slower-absorbing soil requires a larger field area for the same design flow.
Yes, some dispersal methods use the available soil area more efficiently than a conventional gravel trench, which can reduce the footprint needed for the same flow and soil.
Guide last reviewed 2026-08-31.