Septic Drain Field Size Calculator

How it works

Area = daily flow ÷ soil application rate (from percolation test or texture); trench length = area ÷ width; laterals = length ÷ max lateral length

The drain field is where the treatment really happens: septic effluent trickles from perforated pipes into gravel trenches and then into the soil, where bacteria and filtration finish the job. The soil’s ability to accept water — measured by a percolation test or estimated from texture — sets how many square metres of trench bottom are needed for the daily flow; fast sands take 40–50 litres per square metre a day, loams around 25, heavy clays too little for a conventional system at all. Regulators use long-term acceptance rates well below the raw perc rate, because a biological mat forms on the trench bottom over the years. Trench width and permitted lateral length then turn area into a layout, with generous spacing between trenches and setbacks from wells and buildings, plus a reserve area for the day the field eventually needs replacing.

Worked example

1,500 L/day (a US 3-bedroom design flow) into loam with a 20 min/inch perc rate (25 L/m²/day): 60 m² of trench bottom — 67 m of 0.9 m trench as three 22 m laterals, or about 47 m of chambers.

Frequently asked questions