Work out septic tank sizing instantly with clear inputs, formula shown and shareable results.
Septic tank capacity is the daily wastewater flow multiplied by the retention time needed for settlement, usually one to two days, plus an allowance for accumulated sludge between desludging visits. Dimensions follow from a liquid depth of about 1.5 metres and a length to width ratio of two to one, which promotes plug flow rather than short-circuiting.
Tank volume
Litres = users x litres per person per day x retention days + sludge allowance
Dimensions
Plan area = volume / 1.5 m depth, with length = 2 x width
Preliminary estimate only. On-site wastewater systems must be designed to local health and environmental regulations and sized from percolation test results.
Litres = users x litres per person per day x retention days + sludge allowance. Septic tank capacity is the daily wastewater flow multiplied by the retention time needed for settlement, usually one to two days, plus an allowance for accumulated sludge between desludging visits.
No. The tank only settles solids and provides anaerobic treatment. The effluent still needs a soakaway or drainage field sized on soil percolation to complete disposal.
This calculator takes 4 inputs: Number of users, Wastewater per person per day, Retention time, Sludge storage allowance. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.