How to Calculate Septic Tank Size?

You may wonder, What should be the required size of Septic Tank for a domestic or commercial building? We already explained how to calculate the Water Tank Size.  

In this post, we are going to see, how to calculate the size of Septic Tank?.

What is Septic Tank & it’s process – A Glance

Location – Mostly Septic tanks are suitable for rural & urban areas where proper sewer systems are not available.

At Present, every domestic/commercial building having the septic tank, in order to retain the sewage waste for sometime before it gets down to sewer line to avoid any blockage.

Basically, the septic tank is constructed of concrete/brick wall with the rectangular shape and it should have a minimum of two chambers.

Process – The effluent may be either Liquid or solid waste received from household to septic tank through the inlet pipe. The septic tank was designed to store the wastewater at the minimum period of 18 hours to 24 hours, it is called detention period.

During this time period, the sludge was decomposed by the biological action of anaerobic bacteria and the solid waste settled down as sludge. Then the surface water runs to the soak pit through an outlet pipe. Then the settled sludge shall remove after 1 or  2 years.

What is Soak Pit? – A pit dug under the natural ground, constructed in a circular shape with dry bricks or stone with the mortar joint free, where water can easily soak into the ground is known as soak pit. The top portion was closed by RCC Slab & accessible. It should not less than 900mm Dia,  1500mm Depth. At the same time, it should not below 2m from the groundwater. 


  1. The Tank should be constructed at the correct place & required size.
  2. Reduce the water usage to avoid the tank full in short period and also save the water.
  3. Do the sludge removal regularly at a certain time period.
  4. Inspect the septic tank regularly.
  5. Construct the septic tank without disturbing the groundwater.
  6. If any blockage has happened in septic tank act immediately to resolve the problem.
  7. Manhole should provide to access the tank easily.

Don’t Do’s:-

  1. Due to the insufficient tank capacity, the flushed wastewater may back up to our home so the capacity of the tank is very important.
  2. Don’t flush the garbage, sanitary items, paper or other solid material to avoid unnecessary blockage.
  3. The other source of water should not enter in the septic tank.
  4. The wastewater should not run in the natural ground to secure the environment.

Septic Tank Plan View

Septic Tank Section View

Septic Tank Size Calculation based Per User Consumption

As above said, the capacity of septic tank should be designed correctly because insufficient tank size may result in wastewater backup to the house or it coerces us to clean the water often and also it will increase the maintenance cost.

Let us estimate the Tank size for a Home – 5 Members.

Daily Water Usage for a Home – 5 Person

  1. Cooking  – 5 Liters
  2. Bathing & Toilet – 85 Liters/Person, So for 5 person  – 425 liters/Day
  3. Washing cloths & Utensils – 30 Liters
  4. Cleaning House – 10 Liters
  5. Other – 5 Litres

Total – 475 Litres/Day Approximately

We take detention period as 3 days. So the septic tank should have the capacity to retain the household wastewater at least 3 days.

Total wastewater in 3 days – 475×3 = 1425 Liters

Let us take 2000 Liters capacity minimum for a home. Minimum depth of Septic tank should not be less than 1.8m.

Sludge settled down per person – 30 liters/year. So here we take sludge removal 2 years once.

So total sludge = 30 litres x 5 persons x 2 years = 300 Litres

Total tank capacity = 2000+300 = 2300 Liters

We knew that 1 Cum = 1000 Liters = 2300/1000 = 2.3 Cum

Area required @ 1.8m depth = 2.3/1.8 = 1.2 Sqm

Length & breadth ratio of Septic Tank is 4:1 or 2:1

L:B ratio taken as 4:1

So 4BxB = 1.2 Sqm where B=0.54m

Note – Breadth of the tank should minimum 750mm, So that L = 4×0.75 = 3m

L – 3m; B – 0.75m; Depth = 1.8+0.3 = 2.1m (free board should be provided at least 300mm)

Tank capacity = 3 x 2.1 x 0.75 = 4.725 Cum = 4.725x 1000 = 4725 Liters

British Standard Calculation

As per British standard, they follow the below formula to calculate the wastewater flow for a septic tank.

C=A+P(rq+ns) Where

C – Capacity in liters

P – Number of People

A – 2000 Liters as constant

R – Detention period of Sewage in Days

Q – Sewage Flow in liter per day

N – Number of years

S – Sludge accumulation in liters per person/year

Simplification of (rq+ns) = 180 Liters

Now we rewrite the formula as C=A+180P

C = 2000+(180×5)

   = 2900 Liters

We have illustrated the British standard formula just for your knowledge.

Hope you found this article useful. Happy Learning 🙂