# How to Calculate the Size and Capacity of Septic Tank | Septic Tank Size Calculation

In this Article today we will talk about the How to Calculate the Size and Capacity of Septic Tank | Septic Tank Size Calculation | What is Septic Tank | What is Septic Tank Process | Septic Tank Capacity Calculation Formula | Standard Size of Septic Tank | Size of Septic Tank for 5 Users | British Standard Calculation for Septic Tank | Septic Tank Capacity Per Person

## What is Septic Tank and It’s Process:

Location: Mostly septic tanks are suitable for rural and urban areas where proper sewer system are not available. What is Septic Tank Process

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

Basically, the septic tank is constructed of concrete/brick wall with the regular shape and it should have a minimum of two chambers. What is Septic Tank Process

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.

### Do.s:

1. The tank must be built in the proper location and size.
2. Reduce your water usage to keep your septic tank from filling up too quickly and to save water.
3. Sludge removal should be done on a regular basis and at a specific time.
4. Inspect the septic tank on a routine basis.
5. Build the septic tank while taking care not to disturb the groundwater.
6. If a septic tank blockage occurs, act quickly to resolve the issue.
7. The manhole should allow easy access to the tank. Septic Tank Capacity Calculation Formula

### Don’t Dos:

1. The flushed wastewater may back up into our home due to insufficient tank capacity, so tank capacity is extremely crucial or important.
2. To avoid unnecessary blockage, do not flush rubbish, sanitary items, paper, or other solid or hard material.
3. Other sources of water should not be allowed to enter the septic tank.
4. To protect the environment, wastewater should not be discharged into the natural ground. Septic Tank Capacity Calculation Formula

## Shape of Septic Tank:

The simplest form of septic tank is a single box, rectangular or cylindrical shape. The shape of the septic tank affects the flow velocity of the wastewater and sludge accumulation.

The dimension of septic tank should not be too deep. This is because it may cause a short circuit of the inlet and outlet flows. Size of Septic Tank for 5 Users

The septic tank has a large surface area and sufficient depth should always be desired. The rectangular single box tank (three times its width and length) is most favorable.

In some cases, a cylindrical shape with the appropriate size may also be the best choice.

 SIZE OF SEPTIC TANK DIMENSIONS WEIGHT (kg) ANCHOR WEIGHT (kg) SOIL COVER (IN) 5000 gal -18928 liter 17’ L x 8’ W x7.75’ H 41400 30850 24 3000 gal – 11356 liter 13.75’ L x 7. 67‘W x 6.41’H 20300 23320 23 2600 gal – 9842 liter 12.25’L x 7.5’W x 6.08’H 18100 20625 24 2000 gal – 7570 liter 13.5’L x 6.5’W x 5.33’H 16100 15675 19 1600 gal – 6056 liter 12.08’L x 6.5’W x 5.08’H 14000 11270 16 1200 gal – 4542 liter 9.25’L x 6.5’W x 5.08’H 11400 9532 17 1000 gal – 3785 liter Low Profile 10’L x 67’W x 57’H 9500 8705 17 1000 gal – 3785 liter Heavy Duty 8’L x x 6.5’W x 5.08’H 9200 8945 18 800 gal – 3028 liter 8’L x 67’W x 57’H 8000 6560 16 600 gal – 2271.25 liter 6.5’L x 56’W x 60’H 6600 3810 14 1600/1400 gal – 6056/5299 liter 14.5’L x 90’W x 73’H 23000 22410 22 1250/750 gal – 4730/2839 liter 13.5’L x 6.5’W x 5.33’H 16400 15725 19 1000/600 gal – 3785/2271 liter 12.08’L x x 6.5’W x 5.08’H 14700 12705 17

## How to Calculate the Size and Capacity of Septic Tank in Gallons?

How do we calculate the gallon volume a septic tank can deliver based on the simple measurements we can make? British Standard Calculation for Septic Tank

Generally, the residential septic tank is typically 4.5 feet wide x 8.0 feet long x 6 feet high. Your septic tank may be different size. The best practice is to find and measure the septic tank and to make correct calculations.

Tanks are generally buried 4 inches to 4 feet deep, depending on local site conditions, shape, slope and other factors.

Here is the basic math for calculating the capacity (volume) of the septic tank in gallons. Measurements taken on the internal dimensions of the septic tank are in feet.

### HOW TO CALCULATE THE SEPTIC TANK CAPACITY IN GALLONS

Septic Tank In Rectangular Length x Width x Depth in feet x 7.5 = gallons
Septic Tanks In Round 3.14 x radius squared x depth (all in feet) = cubic capacity.

Cubic capacity x 7.5 = gallons capacity.

Septic Tank In Rectangular
(alternative method 1)
Length x Width x Depth in feet / .1337 = gallons
Septic Tanks In Round
(alternative method 2)
Length x width in inches / 231 = gallons per inch of septic tank depth.

Multiply this number by septic tank depth in inches to get gallons

### Notes To The Table Above:

These simple constants and the formula for sizing a rectangular or round tank can help convert the size of a septic tank into cubic feet of sewage:

• One gallon = 231 cubic inches
• One gallon = 0.1337 cubic feet.
• 1 cubic foot = 7.48 gallons
• Volume of a cylinder = Pi x radius2 x depth

Where,

Pi = 3.1416

• Volume of a rectangle = Length x Width x Depth

## Septic Tank Size Calculation based Per User Consumption:

As previously stated, the capacity of a septic tank should be properly designed because insufficient tank storage capacity may result in wastewater backup to the house or force us to clean the water frequently, as well as increase maintenance costs.

### Size of Septic Tank for 5 Users:

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 Liters

Total – 475 Liters/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 x 3 = 1425 Liters

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

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

So Total Sludge = 30 Liters x 5 Persons x 2 Years = 300 Liters

Total Tank Capacity = 2000 + 300 2300 Liters

We know that,

1 Cum = 1000 Liters = 2300/1000 = 2.3 Cum

Area Required @ 1.8 m 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 4B x B = 1.2 Sqm where B = 0.54 m

Note: Breadth of the tank should minimum 750 mm, so that L= 4 x 075 = 3 m

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

Tank Capacity = 3 x 2.1 x 075 = 4.725 Cum = 4.725 x 1000 = 4725 Liters

## British Standard Calculation for Septic Tank:

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

## FAQs:

### What is the standard size of septic tank?

Standard size of septic tank in feet:- standard size of septic tank should be 5 feet long by 2.5 feet wide by 3.3 feet in depth. This septic tank has capacity of 1000 litres of liquid wastages ideal for 5 users of house hold.

### What is minimum capacity of septic tank?

Septic tank shall have minimum width of 750 mm, minimum depth of one metre below water level and a minimum liquid capacity of 1 000 litres.

### What is the smallest size septic tank?

If you're looking to install a septic system, the smallest tank size you're likely to find is 750-gallon, which will accommodate one to two bedrooms. You can also opt for a 1,000-gallon system, which will handle two to four bedrooms.

### How do I calculate tank capacity?

To find the capacity of a rectangular or square tank: Multiply length (L) by width (W) to get area (A). Multiply area by height (H) to get volume (V). Multiply volume by 7.48 gallons per cubic foot to get capacity (C).

## Conclusion:

Full article on How to Calculate the Size and Capacity of Septic Tank | Septic Tank Size Calculation | What is Septic Tank | What is Septic Tank Process | Septic Tank Capacity Calculation Formula | Standard Size of Septic Tank | Size of Septic Tank for 5 Users | British Standard Calculation for Septic Tank | Septic Tank Capacity Per Person. Thank you for the full reading of this article in “The Civil Engineering” platform in English. If you find this post helpful, then help others by sharing it on social media. If any formula of BBS is missing from this article please tell me in comments.

Hi! Welcome to my blog. My name is Engr Waseem Raja and I’m the author of TheCivilEngineerings.com. I am a Civil Engineer by profession but I’ve specialized and taken the journey in the field of Quality Engineering. I’ve worked as a Quality Engineer & Manager in the well-known companies in the Gulf region for almost Ten years.