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Septic Tank Calculator

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Septic tank size for a household, following IS 2470 — detention volume plus sludge.

Runs entirely in your browser — nothing is uploaded

Septic capacity is two things added together, and the second surprises people. There is the sewage held long enough for solids to settle — a day or two of flow. And there is the sludge that accumulates between desludgings, which is years of it. On a small household the sludge allowance is often the larger share, so a tank sized on detention time alone silts up and stops settling long before anyone expects to empty it.

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Features

  • Detention volume and sludge accumulation costed separately.
  • IS 2470 minimum sizes applied as a floor for small households.
  • Internal dimensions from the length-to-width ratio and liquid depth.
  • Freeboard added above the liquid level.
  • Shows what proportion of the tank is sludge allowance.
  • Runs entirely in your browser — nothing is uploaded.

How to use the Septic Tank Calculator

  1. 1Enter how many people will use it.
  2. 2Set the sewage flow and the detention time.
  3. 3Choose how often you intend to desludge.
  4. 4Read the liquid capacity and the internal dimensions.

Frequently asked questions

What size septic tank do I need for a family of five?

IS 2470 tabulates a minimum of about 1.5 × 0.75 m with a 1 m liquid depth for five users — roughly 1.1 m³ of liquid capacity. A calculation from flow alone gives less than that, which is why the standard sets a floor: a very small tank short-circuits, letting sewage pass through faster than it can settle whatever the nominal detention time says.

How is septic tank capacity calculated?

Sewage flow × detention time, plus sludge accumulation × the desludging interval. At 120 litres per person per day with one day of detention, ten people produce 1.2 m³ of detention volume; at the IS allowance of 30 litres per person per year over two years they add another 0.6 m³ of sludge. The total is what the tank has to hold.

How often does a septic tank need emptying?

Every two to three years for a domestic tank sized on that basis — and the tank has to be sized for the interval you actually intend. Choosing a five-year interval means a noticeably larger tank. Leaving it far longer than designed means solids carry over into the soak pit and clog it, which is a much more expensive problem than desludging.

Why is the tank long and narrow rather than square?

Because settling depends on the path the flow takes. A long narrow tank gives sewage a longer journey from inlet to outlet, so solids have more time and distance to drop out. A ratio of 2:1 to 4:1 is usual, with baffle walls to stop flow short-circuiting straight across.

Does this include the soak pit?

No. The soak pit or dispersion trench that takes the effluent depends entirely on how fast your soil percolates, which needs a test pit rather than a formula. Siting is also a public health matter — minimum distances from wells, boreholes and boundaries are set locally, and a septic tank in the wrong place contaminates drinking water.