Septic Tank Size for a House in Nepal: Design and Calculation

Section of a two-chamber septic tank with inlet, scum, liquid, sludge, baffle wall and outlet

To size a septic tank for a house in Nepal, add two volumes: the wastewater that enters the tank in one day, and the sludge that builds up between two cleanings. With 80 litres of wastewater per person per day and 30 litres of sludge per person per year, a family of eight that empties the tank every three years needs about 1,360 litres of liquid volume. A tank of 1.6 m x 0.8 m inside with 1.2 m liquid depth, plus 0.3 m freeboard, gives 1,536 litres and fits that family.

Almost every house outside a sewer line in Nepal ends in a septic tank and a soak pit, and many municipalities now ask to see both on the drawings during naksa pass. Too small a tank sends solids into the soak pit, which then clogs and overflows. Too large wastes money and space. This guide gives the formula, a table by family size, a worked example, the internal layout and the site checks that matter.

How a septic tank works

A septic tank is a watertight box, usually of brick masonry plastered inside or RCC, that holds wastewater long enough for solids to settle. Heavy solids sink and form sludge at the bottom. Grease and light matter float and form a scum layer on top. Bacteria slowly digest part of the sludge. The clearer liquid in the middle flows out through a tee pipe to a soak pit or dispersion trench, where it soaks into the ground.

The tank does not clean the water fully. It only removes most of the solids, so the outflow still has to go into soil that can absorb and filter it, well away from any drinking water source.

The outlet sits about 50 mm lower than the inlet. Only the liquid depth counts as tank capacity; the freeboard above it is space for scum and gas.

Septic tank size formula

A common design method, based on the Indian Standard IS 2470 and the CPHEEO manual that most designers in Nepal refer to, is:

Liquid volume (litres) = P x q x T + P x S x N

P = number of people using the tank.
q = wastewater per person per day that goes into the tank. If kitchen and bath water also go in, take about 80% of the water supply. At 100 litres per person per day of supply, that is 80 litres. If only toilets drain to the tank, the figure is much lower.
T = detention time, usually 1 day (24 hours).
S = sludge built up per person per year. Published values run from about 25 to 40 litres; 30 litres is a common middle figure.
N = years between cleanings, usually 2 or 3.

Add a freeboard of at least 0.3 m above the liquid level. That space holds scum and gas and does not count as capacity. For the water supply figure, see the guide on water tank size for a house in Nepal, which explains where the 100 litres per person figure comes from.

Septic tank size by number of people

The table uses q = 80 litres per person per day, T = 1 day and S = 30 litres per person per year.

People	Cleaning every 2 years (litres)	Cleaning every 3 years (litres)
4	560	680
6	840	1,020
8	1,120	1,360
10	1,400	1,700
12	1,680	2,040

For small families the formula gives a very small tank. In practice, do not build smaller than the smallest size in the IS 2470 table for 5 users, which is about 1.5 m long, 0.75 m wide and 1.05 m liquid depth (around 1,180 litres). A small tank fills with sludge quickly and leaves too little room for settling. Plan for guests, tenants or a future extra storey as well; many houses in Nepal add a floor later.

Worked example: house for eight people

A two-storey house will have eight users. All wastewater goes into the septic tank. The owner wants to empty it every three years.

Daily flow = 8 x 80 x 1 = 640 litres.
Sludge storage = 8 x 30 x 3 = 720 litres.
Liquid volume needed = 640 + 720 = 1,360 litres = 1.36 m³.
Choose a liquid depth of 1.2 m. Plan area = 1.36 / 1.2 = 1.13 m².
With length = 2 x breadth: 2B x B = 1.13, so B = 0.75 m and L = 1.51 m.
Round up to 1.6 m x 0.8 m inside. Liquid volume = 1.6 x 0.8 x 1.2 = 1.536 m³ = 1,536 litres, which is more than 1,360. This also clears the 5-user minimum above.
Total internal depth = 1.2 + 0.3 freeboard = 1.5 m, measured from the tank floor to the underside of the cover slab.
Baffle wall: first chamber about 2/3 of the length, so about 1.05 m, and the second chamber about 0.55 m.

Add wall thickness, a base slab and the cover slab to get the excavation size. The excavation calculator gives the earthwork, and the concrete calculator gives the materials for the walls and slabs.

Internal layout and pipes

Shape: keep the length two to four times the breadth so the flow has a long path. Keep the inside width at least 0.75 m so a person can clean it.
Two chambers: a baffle wall splits the tank so that the second chamber catches solids that escape the first. Leave openings in the wall in the middle of the liquid depth, away from both sludge and scum.
Inlet and outlet: use tee pipes, usually 100 mm. The outlet invert is about 50 mm below the inlet invert so water does not back up into the house drain. The outlet tee reaches deeper into the liquid than the inlet tee so floating scum cannot leave.
Vent pipe: fit a vent, usually 100 mm, carried above the roof or well above head height, so gas does not smell at ground level.
Manholes: give each chamber an access cover so the tank can be emptied by a suction tanker.
Watertight: plaster the inside with cement mortar, round the corners, and test with water before backfilling.

Soak pit and location rules

The outflow goes to a soak pit (a pit lined with open-jointed bricks or stone and filled with brick bats or gravel) or to dispersion trenches. Its size depends on how fast the soil absorbs water, so a percolation test on the actual plot gives the best answer. In clay or where the groundwater is high, a soak pit may not work, and trenches or a connection to a sewer are needed.

Common guidance is to keep the soak pit at least 15 m from any well or spring used for drinking water, and more in gravel or fractured rock. Keep the tank a few metres away from foundations. Your municipality may set its own distances and may want a separate soak pit for rainwater, so check the local building bylaws and the drawings approved during naksa pass.

Mistakes seen on site

Building one long chamber with no baffle, so solids flow straight into the soak pit.
Putting the inlet and outlet at the same level.
No vent pipe, or a vent that ends below a window.
Sealed covers with no manhole, so the tank is never emptied until it overflows.
Draining rainwater from the roof into the septic tank, which washes the sludge out.
Letting the tank go ten years without emptying. Sludge then fills the tank and the soak pit fails.

Frequently asked questions

What size septic tank does a family of five need?

The formula gives 700 litres for a 2-year cleaning cycle (5 x 80 + 5 x 30 x 2), which is smaller than a practical tank. Build at least the IS 2470 minimum of about 1.5 m x 0.75 m x 1.05 m liquid depth, which holds about 1,180 litres.

How often should a septic tank be emptied?

Every 2 to 3 years for a typical house, or when the sludge and scum together take up about a third of the liquid depth. Size the tank for the interval you will actually follow.

Can kitchen and bathroom water go into the septic tank?

Yes, if the tank is sized for it, as in the example above. Kitchen water carries grease, so a grease trap before the tank helps. If only toilet waste goes in, the tank can be smaller, but the other water still needs a safe outlet.

How do I convert the tank size to cubic feet?

Divide litres by 28.32. A 1,536 litre tank is about 54.2 cft.