What size septic tank do I need for a 5-person house in Sri Lanka?

For 5 people with all household wastewater draining to the tank, the computed liquid volume is about 875 L, so the SLS 745 / IS 2470 minimum of 1,000 L (1.0 m³) applies. A practical internal size is roughly 1.4 m × 0.6 m at 1.2 m liquid depth. Add a 0.3 m freeboard for a 1.5 m total depth.

How is septic tank capacity calculated?

The effective liquid volume is V = P × (q × Tr + G × N): persons P times per-capita flow q held for a detention time Tr (the retention volume), plus persons times sludge rate G accumulating over the desludging interval N (the sludge-storage volume). The two parts are added, then a 1,000 L minimum is enforced.

What is the minimum septic tank size for a Sri Lankan home?

The minimum domestic liquid capacity is 1,000 litres (1.0 m³), taken from IS 2470 Part 1, the standard SLS 745 is based on. For small households the computed volume often falls below this, so the 1,000 L floor governs and the tool flags when that happens.

How often should a septic tank be desludged?

Every 2 to 3 years is typical for a domestic tank in Sri Lanka, though it depends on tank size and load. A longer desludging interval needs a larger sludge-storage volume, so it makes the tank bigger. This calculator lets you set the interval (1–5 years) and sizes the sludge component accordingly.

What is the difference between a septic tank and a soakage pit?

The septic tank settles solids and lets sludge digest; its liquid effluent then flows to a soakage pit (soakaway) that lets the water percolate into the ground. This tool sizes the septic tank only. The soakage pit is a separate unit sized from an on-site soil percolation test, so it is out of scope here.

Does the type of wastewater change the tank size?

Yes. If only the WC (blackwater) drains to the tank, the per-capita flow is lower (about 60 L/person/day) than when all household wastewater, kitchen, bath and laundry, is included (about 100 L/person/day). Lower flow means a smaller retention volume, though the 1,000 L minimum still applies for small homes.

Can I use this for a hotel, factory or school?

No. This version is for domestic houses. Commercial and institutional buildings have different flow assumptions, higher peaks and often need a package sewage treatment plant rather than a simple septic tank. Consult a qualified engineer and the full SLS 745 text for non-domestic designs.

Will a Pradeshiya Sabha or UDA accept these figures?

The method follows SLS 745 / IS 2470, which local authorities reference for building-plan approval, and the breakdown is auditable term by term. Requirements still vary by authority, so confirm the flow figures and any local minimum with your Pradeshiya Sabha, Urban Council or the UDA before construction.

Sri Lanka · Utility

Sri Lanka Septic Tank Size Calculator

Size a domestic septic tank from the number of occupants, the type of wastewater and how often you desludge. Get the required volume in litres and m³ plus a suggested length × width to hand to your mason, using the SLS 745 / IS 2470 method, no signup, sources cited.

By Induwara Ashinsana· Co-Founder, Ryzera TechnologiesUpdated Jul 11, 2026

Size your septic tankdomestic

SLS 745 / IS 2470

People served

Total occupants the tank serves (1–50).

Desludge every

years

How often the tank is emptied (1–5 years).

Wastewater type

WC, kitchen, bath and laundry all drain to the tank (per-capita flow defaults to 100 L/person/day).

Household size

Required liquid volume

1.00 m³

1,000 litres · 1000 L minimum applied

Suggested internal size

1.44 × 0.58 m

Liquid depth 1.20 m · plan area 0.83 m²

Total tank depth

1.50 m

Liquid 1.20 m + 0.30 m freeboard

Formula breakdown

Term	Working	Litres
Retention volume A	5 × 100 × 1	500 L
Sludge storage B	5 × 25 × 3	375 L
Computed V = A + B	sum of the two components	875 L
Required volume1000 L min	max(875, 1,000)	1,000 L

Sizes the septic tank only. The soakage pit / soakaway that disposes of the effluent is a separate unit sized from a soil percolation test. Always confirm the final size against your local-authority approval requirements.

Cross-checked against the IS 2470 formula V = P × (q·Tr + G·N).

How it works

A septic tank has to do two jobs at once: hold incoming sewage long enough for solids to settle, and store the sludge that builds up until the next desludging. Its effective liquid volume is therefore the sum of a retention component and a sludge-storage component, per the SLS 745 code and the IS 2470 (Part 1) design method it is based on.

Retention (detention) volume A = P × q × Tr , persons P times per-capita wastewater flow q (litres/person/day) held for a detention time Tr in days. SLS 745 / IS 2470 set a minimum 24-hour (Tr = 1 day) detention.
Sludge + scum storage volume B = P × G × N , persons times the sludge accumulation rate G (litres/person/year, ~25 per the WHO on-site sanitation guide) over the desludging interval N in years.
Required liquid volume V = A + B, which is algebraically the IS 2470 combined form V = P × (q·Tr + G·N). The calculator computes both and confirms they agree to the litre.
Minimum-capacity floor. If the computed V falls below 1,000 litres, the IS 2470 minimum domestic liquid capacity of 1,000 litres (1.0 m³) governs instead, the tool flags when this happens.
Dimensions. At a chosen liquid depth D in metres, the plan area is (V/1000) ÷ D. For a length:width ratio r (IS 2470 permits 2:1 to 4:1), width W = √(area ÷ r) and length L = r × W. A 300 mm freeboard is added on top of the liquid depth to give the total tank depth.

Every input, flow, detention, sludge rate, depth and ratio, is shown in the “advanced” panel and reflected in the breakdown table, so no constant is hidden. The defaults are the SLS 745 / IS 2470 / WHO domestic figures; change them only where your local authority specifies otherwise. This tool sizes the septic tank alone, not the downstream soakage pit, which depends on a soil percolation test.

Worked examples

5-person house, all wastewater

P=5, q=100, Tr=1, G=25, N=3, D=1.2 m, ratio 2.5

Retention A = 5 × 100 × 1 = 500 L
Sludge B = 5 × 25 × 3 = 375 L
V = 500 + 375 = 875 L → below 1,000 L minimum
Required volume = 1,000 L = 1.0 m³ (minimum governs)
Plan area = 1.0 ÷ 1.2 = 0.83 m²
W = √(0.83 ÷ 2.5) = 0.58 m, L = 2.5 × 0.58 = 1.44 m
Suggested: 1.44 m × 0.58 m × 1.2 m liquid depth (1.5 m total)

8-person household, longer detention

P=8, q=130, Tr=1.5, G=30, N=3, D=1.5 m, ratio 2.5

Retention A = 8 × 130 × 1.5 = 1,560 L
Sludge B = 8 × 30 × 3 = 720 L
V = 1,560 + 720 = 2,280 L = 2.28 m³ (above minimum)
Plan area = 2.28 ÷ 1.5 = 1.52 m²
W = √(1.52 ÷ 2.5) = 0.78 m, L = 2.5 × 0.78 = 1.95 m
Suggested: 1.95 m × 0.78 m × 1.5 m liquid depth (1.8 m total)

Edge case, 2-person, WC only

P=2, q=60, Tr=1, G=25, N=3, D=1.2 m, ratio 2.5

Retention A = 2 × 60 × 1 = 120 L
Sludge B = 2 × 25 × 3 = 150 L
V = 120 + 150 = 270 L → far below the 1,000 L minimum
Required volume = 1,000 L = 1.0 m³ (minimum governs)
A minimum-size tank is still required even for a tiny household.

Frequently asked questions

Sources & references

The design method, the 1,000 L minimum liquid capacity, the 300 mm freeboard and the per-capita constants were cross-checked against these sources on 2026-07-11. Local-authority requirements can differ; always confirm before building.

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