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How to use the Septic Tank Size Estimator

The Septic Tank Size Estimator gives a preliminary internal size for a household septic tank (up to 20 users) from the number of users and the cleaning interval, using the sizes commonly given in CPHEEO guidance based on IS 2470 Part 1.

Open the Septic Tank Size Estimator ›

On this page
  1. Quick start
  2. Inputs
  3. Your estimate
  4. Questions people ask

Quick start

  1. Enter the number of regular users (up to 20).
  2. Choose a cleaning interval of 2 or 3 years.
  3. Press Estimate tank size and note the internal size; add 300 mm freeboard and the wall thickness.

Inputs

FieldWhat to enterTypical valueTip
Number of usersEveryone who uses the toilets regularly.5 for a family; max 20The tool adopts the next category: 5, 10, 15 or 20 users.
Cleaning / desludging interval2 or 3 years.2 yearsA longer interval needs more liquid depth.

Your estimate

ResultMeaningExampleTip
Internal tank dimensionsLength × breadth × liquid depth.5 users: 1.50 × 0.75 × 1.00 m (2 yr)
Liquid volume / capacitym³ and litres.1.13 m³ = 1,125 L
Freeboard to addAir space above the liquid.0.30 m
Total inside depthLiquid depth + freeboard.1.30 m

The Reference sizing table lists the sizes for 5, 10, 15 and 20 users at 2- and 3-year intervals. The sizes are internal: add wall and base thickness separately.

This is a preliminary size. The soak pit or other effluent disposal is not designed here — it depends on soil, ground water and local rules.

Questions people ask

What size septic tank for a family of 5?

About 1.50 m × 0.75 m inside, 1.00 m liquid depth for a 2-year cleaning interval (1.05 m for 3 years), plus 0.30 m freeboard.

Can I use it for more than 20 users?

No — this version is for small residential tanks up to 20 users.

Does it design the soak pit?

No. Soak pit or other disposal must be designed separately for the soil and ground-water level.

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