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.
Others Post: How to Calculate Water Tank Capacity for Building
Do.s:
 The tank must be built in the proper location and size.
 Reduce your water usage to keep your septic tank from filling up too quickly and to save water.
 Sludge removal should be done on a regular basis and at a specific time.
 Inspect the septic tank on a routine basis.
 Build the septic tank while taking care not to disturb the groundwater.
 If a septic tank blockage occurs, act quickly to resolve the issue.
 The manhole should allow easy access to the tank. Septic Tank Capacity Calculation Formula
Don’t Dos:
 The flushed wastewater may back up into our home due to insufficient tank capacity, so tank capacity is extremely crucial or important.
 To avoid unnecessary blockage, do not flush rubbish, sanitary items, paper, or other solid or hard material.
 Other sources of water should not be allowed to enter the septic tank.
 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 radius^{2} x depth
Where,
Radius = half the diameter
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
 Cooking – 5 Liters
 Bathing & Toilet – 85 Liters/Person, So for 5 person – 425 liters/Day
 Washing cloths & Utensils – 30 Liters
 Cleaning House – 10 Liters
 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
We have illustrated the British standard formula just for your knowledge. After Reading this Article you must read about: How To Calculate the Estimate of Masonry Water Tank
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FAQs:
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.
8 Comments
So much thanks
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Its real helpful buddy, all good but sometime you should do it in English version in addition to the Americans version existed.
I will try, Thanks for the comments please visit more articles by clicking on “All Posts” tab of the Menu Bar
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Thanks for your help and I want to know how to construct a septic tank in Waterlogged area of the city and the depth is not exceed 1000mm high and the more excavation past the point the water is flooded. 2.what of rocky area too? Thanks Mohammed Danchua (Qs) from Nigeria.
Sir, Kindly post in Pdf…tnx
I will, Thanks for the comments please visit more articles by clicking on “All Posts” tab of the Menu Bar