Size a septic tank and drain field from occupancy and soil percolation rate.
Tanks are sized on design occupancy rather than current occupancy, because bedrooms outlast households. Drain field area depends on how fast the soil accepts water, so a slow percolation rate can more than double the land needed. Percolation testing governs whether a conventional system is permitted at all, which makes it the first thing to establish on an unserviced plot.
Septic Tank
Tank volume = daily flow × retention days + sludge allowance; field area = flow ÷ loading rate
Tank volume = daily flow × retention days + sludge allowance; field area = flow ÷ loading rate Tanks are sized on design occupancy rather than current occupancy, because bedrooms outlast households. Drain field area depends on how fast the soil accepts water, so a slow percolation rate can more than double the land needed.
Percolation testing governs whether a conventional system is permitted at all, which makes it the first thing to establish on an unserviced plot.
This calculator takes 5 inputs: Number of bedrooms, Occupants, Water use per person, Percolation rate, Tank retention time. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.