Free Online Septic Tank Design Calculator

A septic tank settles and partially digests sewage from buildings not connected to a public sewer, and a soak pit disposes of the effluent to ground. This calculator sizes the tank and soak pit from the number of users, the sewage flow, the retention time and the desludging interval, following IPC.

Tank capacity is the sum of the settling volume (flow × retention time, usually 24-48 hours) and the sludge storage for the period between desludgings (typically one to two years). A minimum liquid depth and freeboard are added. The soak pit is sized on the soil's percolation rate - clayey soils need a much larger pit than sandy ones.

Example: For 10 users at 120 L/person/day, the daily flow is 1200 L. With a 1-day retention the settling volume is 1200 L, plus sludge storage of 10 × 30 × 2 years = 600 L, giving about 1800 L plus freeboard - a tank of roughly 2.0 × 0.9 × 1.2 m liquid depth. The soak pit is then sized on the local percolation rate.

Indicative septic-tank sizes for domestic use (IPC), cleaned every 1-2 years:

Soak Pit & Effluent Disposal

The soak pit disposes of the clarified effluent to ground; its size depends on the soil percolation rate - sandy soils absorb quickly, clay very slowly, sometimes needing a dispersion trench instead. Keep the tank and soak pit well away from any water source (typically 15 m minimum), provide two chambers with a baffle, and desludge on schedule to keep the tank working.

Indicative sizes - confirm against IPC Parts 1 & 2 and the soil percolation test.