Septic Drainfield Size Calculator (Perc Test)

How it works

Design flow = bedrooms × gpd; area = flow ÷ application rate (from perc table); trench length = area ÷ effective width; laterals = ceil(length ÷ max lateral)

A percolation test times how fast water soaks into a pre-saturated hole at the depth of the proposed trenches; slower rates mean the soil accepts less effluent per square foot each day, so the field must be larger. State and county codes tabulate the allowable loading rate against the perc rate — around 1.2 gal/ft²/day in sand down to 0.2 in heavy soils — and set design flow by bedroom count rather than actual occupancy, because bedrooms define the house's potential. Dividing flow by loading rate gives the required bottom area, which is then laid out as trenches of limited length (to keep distribution even) with undisturbed soil between them. Codes also demand a replacement area of equal size, separation from wells, property lines and groundwater, and a tank large enough for two to three days of retention.

Worked example

A 3-bedroom house at 150 gpd per bedroom generates 450 gpd. With a 30 min/inch perc rate the loading rate is 0.6 gpd/ft², so 750 ft² of absorption area is needed — 250 ft of 3 ft trench, laid as three 83 ft laterals in a field about 21 × 87 ft — with a 1,000-gallon tank.

Frequently asked questions