Septic Holding Tank Size

Hello all. Do any of you have a way you can roughly calculate the size of an exsisting septic holding tank? I remember seeing something on the internet that gave a rough idea based on the top dimensions of the tank, but I can’t seem to find it. Any help would be appreciated…

Clive Peacock
ProbeComm Home Inspections

Tell me how you can be certified to do septic inspections and you have no clue as to how to estimate the size of a holding tank?

5.1 Who Does the Assessment: Inspector Qualifications
One factor contributing to the effectiveness of a septic system re- inspection program is determining the number of re-inspections which a program intends to conduct per year.
In determining this capacity to conduct re-inspections, consideration should be given to such factors as:
•= The number of sufficiently-trained staff to provide timely follow-up with property owners for the anticipated number of systems which may have problems.
•= The impact a program will have on municipal building permit issuance, inspectors and local staff resources.
•= The capacity of local septic system installers to meet the demand for an increase in repairs, pump-outs and new system installations.
The phasing of inspections (e.g., beginning with high risk systems) will serve to assist in mitigating any impacts on the local industry and municipal resources in meeting the demands for their services.
Given the relationship of inspections to enforcement authority under the BCA and OBC, it would be difficult for a program to be established which is not linked directly to the local septics enforcement agent. The regulatory authority for inspectors is given in BCA s. 3(1), which provides for municipal responsibility for the enforcement of the Act and the ability to appoint inspectors if they meet the qualifications specified in the Building Code. BCA s. 3.1(2) and 32.1 also provides for the appointment of sewage system inspectors by a board of health, conservation authority or planning board, where applicable.
Whether seasonally employed or not, those people conducting inspections should be appointed building inspectors pursuant to the BCA and certified as on-site sewage inspectors. Training and certification for On-Site Sewage Inspectors is administered through the Ministry of Municipal Affairs and Housing’s Housing Development and Buildings Branch. A course in the BCA itself, may also prove useful in understanding enforcement issues.

Thanks for the tongue lashing - Not very helpful.
I’m a new home inspector, and just completed the Ontario Ministry of Municipal affairs and Housing exam for septic inspections as an additional service to my clients. Unfortunately the course only covers inspections of new systems. This is my first septic inspection, and estimating the tank size is not required, just whether the system functions properly - All I’m looking for is a way to estimate the size of an exsisting septic holding tank so I have additional information.
Best Regards,

Simple Formula is Length times width times depth times in feet times .80 times 6.25 gives you Canadian Gallons or use 7.5 for USA Gallons .
This is close and can usually be done in your head .
Sorry I am from old school have trouble with metric .
You can frequently have the answer before the engineer does with his Calculator .
This if for a Cylindrical tank If it is Square do not use the 80% ( .8 ) Remember 6.25 Gal per Cubic ft = Can Gal or 7.5 US Gal

W X L X D X 7.48

Do you take a sludge sample? How about hydraulic loading of the system?

You measure the liquid level. You are looking for the holding capacity of the tank so you can estimate the capacity of the tank to hold the sewage long enough for complete digestion. There should be a minimum 1 foot free board, that is 1 foot between the top of the liquid and the ceiling of the tank. This space is for the scum or foam to float on top.
You should take the InterNACHI course of Private sewage disposal systems.
Then you should check with your local jurisdiction for local codes.
Septic system is a misnomer. Septic tank refers to the first tank or primary digestion tank. The next tank is the effluent tank and holds the liquid until there is enough the fill the field. All together it’s called a Private sewage disposal system or Public sewage disposal system if it’s connected to a municipal system.
The size of the tank is dependent on the anticipated volume, ground conditions and your local codes.
The national average is 50 imp gal per person per day.
The Alberta government made a substantial increase in size in the 2009 regulations.
What is a Septic holding tank. It’s a Holding tank and is not connected to any other tanks or drains. or it’s a Septic tank.

What jurisdiction are you talking about. Maybe you could put your location State or province in the location box. The same to Clive Peacock.

It is. Can’t you read English? à la maison :wink:

Have you had a look at this…

“Only the top few percent of all home inspectors reach InterNACHI status. (HUH?) These Master inspectors have all successfully passed InterNACHI’s Inspector Examination, adhere to a Standards of Practice, abide by a Code of Ethics, attend required continuing education courses, and all are InterNACHI Certified.”
Someone’s a bit confused!

Take this free course:

  • The sewage waste does not under go complete digestion in the tank. Good tank design allows for waste liquids to remain in the tank for at least 2 days so that solids settle out as sludge and grease/oils separate to the top- you do not want either of these reaching the disposal field to shorten its useful life.
    *** "In 1999, the average Canadian used 343 litres ((75.6 Imp gal) a day in domestic water use alone, **ranking second only to the average American who used 382 litres a day in terms of **most consumption in a selection of developed countries with comparable living **standards. In 2001, this average residential water use dropped to 335 l(73.8 Imp gal)itres per day. **Nonetheless, Canadians still rank as one of the world’s most profligate people in *terms of water consumption."

*“In 2004, the average Canadian daily domestic use of fresh water per capita was 329 litres (72.5 Imp gal).”

Interesting Clive just asked a simple question and he got many who posted a lot of things .

Thanks Clive

Getting back to sludge, a sample should be taken. The tank should be pumped when the sludge and scum layer equal 2/3 of the tank volume. Also, the color of the sludge can determine how healthy a tank is. Homeowners who are on heavy medications such as chemotherapy can kill the organisms that break down sludge thus increasing the volume in the tank.
Nicks septic course is pretty good, and free!!!

Picture of a sludge sampler.

Thanks to those that provided a constructive answer to my question. It’s the function of our message board to help each other be better inspectors.
To those that criticize and try to belittle fellow inspectors, shame on you. By posting negative responses in a open and public forum, you only make public viewers of the InterNACHI web site question the validity of home inspections.

Best Regards,
Clive Peacock

“Thanks to those that provided a constructive answer to my question. It’s the function of our message board to help each other be better inspectors.”

Not anymore…now it is just some place for little boys to compare pecker size. Your best bet is go to the educational section and look it up yourself.

Clive, I hope I didn’t come across as criticizing. I’m curios how other inspectors are inspecting septic systems. I recently took a course here in NH and learned some very interesting conditions to look for when inspecting septic systems.

I’d like to hear how you guys up north do them.


I have not heard or seen any plastic septic tanks for sale. But what i heard is that there are septic tanks wherein it can recycle your waste water and also septic tanks with different sizes.