Why include sludge volume separately?

Because sludge accumulates and reduces the settling volume. A tank sized only on retention gradually stops working and pushes solids into the soakaway, which is the expensive failure.

How often should it be desludged?

Every two to three years for a typical household. The sizing here assumes the interval you enter, so a longer interval needs a larger tank.

Construction & Homeplumbing

Septic Tank Size Calculator

Septic tank sizing from household size and retention time. The tank holds two things: the sewage retained long enough to settle, and the sludge that accumulates between desludgings, and undersizing either causes failure.

Also called: septic tank size calculator, soak pit sizing.

Tank capacity

3.49

3.49 m³, which is 3,485 litres. That is 2.03 m³ of 3 day retention plus 1.46 m³ of sludge storage for 2 years. Suggested dimensions 2.41 m by 0.96 m by 1.5 m deep. The sludge allowance is the part usually omitted. Without it the tank silts up and pushes solids into the soakaway.

In litres

3,485

Retention volume

2.03

Sludge storage

1.46

Daily flow

675

Suggested dimensions

2.41 m by 0.96 m by 1.5 m deep

Liquid surface area

2.32

On retention

The sludge allowance is the part usually omitted. Without it the tank silts up and pushes solids into the soakaway.

How this is calculated

Method and background

How this is calculated

Retention volume is the daily flow times the retention period, usually one to three days, which is what allows solids to settle and separate. Sludge volume is what accumulates between desludgings and is the part people forget: a tank sized only on retention fills with sludge and stops settling, sending solids to the soakaway and destroying it. The suggested dimensions use a length to width ratio of 2 to 3, which promotes settling by keeping flow slow and unmixed.

the tank holds the daily flow for the retention period plus accumulated sludge between desludgings

Q: Daily flow
t: Retention days
s: Sludge rate

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a five person household

People served: 5
Sewage per person: 135 litres/day
Retention time: 3
Years between desludging: 2
Sludge accumulation: 0 m³/person/day
Liquid depth: 1.5 m

Tank capacity3.49

675 x 3 / 1000; 5 x 0.0004 x 365 x 2

Open this example

a shorter desludging interval needs less storage

People served: 5
Sewage per person: 135 litres/day
Retention time: 3
Years between desludging: 1
Sludge accumulation: 0 m³/person/day
Liquid depth: 1.5 m

Tank capacity2.76

boundary: halving the interval halves the sludge allowance

Open this example

Method and limits

What it assumes

Domestic sewage with typical sludge accumulation rates.

What it deliberately does not model

Local regulation governs minimum sizes, materials and setbacks, and it overrides any calculation.
Soakaway sizing depends on percolation testing and is a separate exercise.
This is a preliminary estimate, not a design for construction.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why include sludge volume separately?: Because sludge accumulates and reduces the settling volume. A tank sized only on retention gradually stops working and pushes solids into the soakaway, which is the expensive failure.
How often should it be desludged?: Every two to three years for a typical household. The sizing here assumes the interval you enter, so a longer interval needs a larger tank.