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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 AshinsanaUpdated Jul 11, 2026
Size your septic tankdomestic
SLS 745 / IS 2470

Total occupants the tank serves (1–50).

years

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

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

TermWorkingLitres
Retention volume A5 × 100 × 1500 L
Sludge storage B5 × 25 × 3375 L
Computed V = A + Bsum of the two components875 L
Required volume1000 L minmax(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.

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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

  1. Retention A = 5 × 100 × 1 = 500 L
  2. Sludge B = 5 × 25 × 3 = 375 L
  3. V = 500 + 375 = 875 L → below 1,000 L minimum
  4. Required volume = 1,000 L = 1.0 m³ (minimum governs)
  5. Plan area = 1.0 ÷ 1.2 = 0.83 m²
  6. W = √(0.83 ÷ 2.5) = 0.58 m, L = 2.5 × 0.58 = 1.44 m
  7. 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

  1. Retention A = 8 × 130 × 1.5 = 1,560 L
  2. Sludge B = 8 × 30 × 3 = 720 L
  3. V = 1,560 + 720 = 2,280 L = 2.28 m³ (above minimum)
  4. Plan area = 2.28 ÷ 1.5 = 1.52 m²
  5. W = √(1.52 ÷ 2.5) = 0.78 m, L = 2.5 × 0.78 = 1.95 m
  6. 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

  1. Retention A = 2 × 60 × 1 = 120 L
  2. Sludge B = 2 × 25 × 3 = 150 L
  3. V = 120 + 150 = 270 L → far below the 1,000 L minimum
  4. Required volume = 1,000 L = 1.0 m³ (minimum governs)
  5. 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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