Septic tank size specifications & requirements:
This article provides a septic tank size table to determine the required size or capacity of a septic tank. The typical residential septic tank size required for a given average daily sewage wastewater flow in gallons is provided in a table of septic tank sizes.
We also include tables of typical septic tank size dimensions and capacities for concrete septic tanks, plastic or fiberglass septic tanks & steel septic tanks.
This chapter also explains how to calculate septic tank volume based on septic tank inside dimensions measured in feet, and we discuss the sizing, installation, and functions of septic tank tees to prevent septic system clogging. Septic tank size requirements and how to calculate septic tank capacity are explained.
Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow & How to Calculate the Size (in gallons) of a Septic Tank are reflected in the table. This chapter summarizes guidelines on the required septic tank size based on anticipated level of daily gallons of sewage wastewater flow.
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How big does our septic tank need to be? Typically the septic tank volume for a conventional tank and onsite effluent disposal system (such as a drainfield) is estimated at a minimum of 1000 gallons or 1.5 x average total daily wastewater flow.
Here we also discuss the smallest recommended septic tank sizes based on building occupancy or wastewater volume. One should also ask, how does weather affect the necessary septic tank size?
Table of Required Septic Tank Size Based on Daily Waste-Water Volume in Gallons
|Average Sewage Wastewater
Flow - Gallons Per Day
|Minimum Septic Tank Size in Gallons
of Effective Capacity Needed (1)
Notes to Table:
How big does our septic tank need to be based on the number of bedrooms in the home? Some jurisdictions use the number of bedrooms rather than number of occupants or estimated daily wastewater flow to guide homeowners and septic installers in choosing a septic tank size.
Septic Tank in Gallons Size Based on Number of Bedrooms
|Number of bedrooms||Minimum Septic Tank Capacity (Gallons)||Minimum Septic Tank Liquid Surface Area (sq.ft.)|
|0 bedrooms||750 gal. (2) - obsolete in NYS|
|1, 2, or 3 bedrooms||1,000 gallons||27 sq.ft.|
|4 bedrooms||1,200 gallons||34 sq.ft.|
|5 bedrooms||1,500 gallons||40 sq.ft.|
|6 bedrooms||1,750 gallons||47 sq.ft.|
Notes to the septic tank sizing table:
Original source: New York State NYS75-A. Wastewater Treatment Standards,[image] Update 12 January 2015 to current table that lumps 1-3 bedrooms together and adds 6-bedroom homes.
The current standard document is provided as a PDF and can be seen in its entirety at REFERENCES.
The older version of this table permitted 750 gallon septic tanks in some jurisdictions and permitted 1,500 gallon septic tanks for 6-bedroom homes. Some current standards such as New Mexico still permit these smaller septic tank sizes).
For example, New Mexico [reference citation] uses this standard. Other experts estimate that occupants use between 50 and 100 gallons of water per person per day in a home in the U.S. We can use that guesstimate to compare different septic tank size guidelines.
Also see WATER USAGE TABLE [web page].
The current and somewhat more stringent septic tank size used in the U.S. by New York (Public Health Law 201(1)(1) Section 75-A) standard includes these comments on septic tank sizing:
Tank size requirements for more than six bedrooms shall be calculated by adding \
250 gallons and seven square feet of surface area for each additional bedroom.
A garbage grinder shall be considered equivalent to an additional bedroom for determining tank size.
2. Watch out: 750 gallon septic tanks are smaller than the minimum 1000g size required for new construction in many jurisdictions world-wide.
In some jurisdictions government ordinance may simply specify the minimum allowable septic tank size by
Two compartment septic tanks do a somewhat better job of removing suspended solids from wastewater than single compartment septic tanks, and some jurisdictions, including Alaska (18 AAC 72), require that two compartment septic tanks be used. But we have not found regulations that translate that design difference into different septic tank size requirements.
In some states (Connecticut since January 1991) septic tanks now consist of two compartments in order to do a more effective job, and increasingly other jurisdictions (Alaska, Pennsylvania) require that new and up-graded onsite wastewater disposal systems use two-compartment septic tanks.
Image adapted from Alaska DEC  The dashed lines illustrate the liquid level (red) and the difference in elevation (green) between the inlet and outlet septic tank pipe connections.
More about these measurements is at SEPTIC TANK TEES where we discuss repair procedures and backwards septic tanks.
In a two compartment septic tank the wall separating the two compartments will have an opening that allows liquid effluent to flow into the second compartment, keeping floating scum and settled sludge in the first compartment (mostly).
The septic entire tank, both compartments, will need to become filled with wastewater before any effluent will begin to flow out of the septic tank and into the drainfield or soakaway bed. That happens naturally during normal building plumbing system usage over the course of several days for typical septic tank sizes.
So when you observed about two feet of waste in the septic tank, then left the system unused, you'd expect to find exactly the same amount in the tank weeks later. Only a very slight drop in level might occur, less than an inch - caused by evaporation - because you left the tank open (and dangerous).
Watch out: if your tank is a two compartment type the solids, floating scum and settled sludge are accumulating at the inlet portion of the tank. Inspecting at the final septic tank outlet end will not discover sludge and scum early enough to prevent septic system damage.
Such septic tanks may have a center inspection port which admits tank access at the outlet of the sludge/scum containing compartment. That's where to test in two-compartment septic tanks.
Please see SEPTIC TANK LEVELS of SEWAGE for details about how to interpret abnormal levels of sewage found in the septic tank (too high or too low).
Septic media filter systems also require either two septic tanks or a two compartment tank - see SAND FILTER SEPTIC DESIGNS
How do we compute the volume in gallons that a septic tank provides based on simple measurements we can make?
Septic Tanks are usually about 4.5 feet wide x 8.0 feet long x 6 feet tall. Tanks are typically buried 4 inches to 4 feet deep depending on local site conditions, shape, slope, and other factors.
Here is the basic math for computing septic tank capacity (volume) in gallons. Measurements are in feet, taken of inside dimensions of the septic tank.
How to Calculate the Septic Tank Capacity in Gallons
|Round Septic Tanks||3.14 x radius squared x depth (all in feet) = cubic capacity. Cubic capacity x 7.5 = gallons capacity.|
|Rectangular Septic Tanks||Length x Width x Depth in feet x 7.5 = gallons|
|Rectangular Septic Tanks
(alternative method 1)
|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|
|Rectangular Septic Tanks
(alternative method 2)
|Length x Width x Depth in feet / .1337 = gallons|
One gallon of water has a volume of .1337 cubic feet. For a rectangular septic tank, multiply depth (or inside "height") in feet times width times length. Divide this figure by .1337 to establish the number of gallons in the septic tank.
Example 1: how many gallons is held in a a 4ft. deep x 5ft. wide x 8 ft. long septic tank? If the tank dimensions were 4ft. x 5ft. x 8ft. = 160 cubic feet. Using the conversion factor to convert cubic feet to gallons, 160 / .1337 = 1196 - or about a 1200-gallon tank.
One cubic foot of volume can contain 7.481 gallons of liquid. So a second approach to calculating septic tank actual size or capacity in gallons is to multiply the septic tank volume in cubic feet by this constant, which we round up to 7.5 gallons/cubic foot.
Example 2: how many cubic feet and how many gallons are held in a septic tank of typical dimensions of 4.5 ft. wide x 8.0 ft. long x 6 ft. high. (4.5 x 8 x 6) = 216 cubic feet. Since one cubic foot can contain 7.481 gallons, which we round up to 7.5 gallons per cubic foot: 216 x 7.5 = 1620 gallons of septic tank capacity - this is probably nominally a "1500-gallon septic tank".
Note that if the dimensions given by your septic contractor are the external dimensions of the tank rather than the internal dimensions then the volume given by this calculation will come up with a septic tank size estimate that is higher than the actual tank capacity - the error is due to failure to allow for the thickness of the septic tank walls.
So for fitting a septic tank into a tight spot, the outer dimensions of the septic tank are important. But for accurate calculation of the capacity of a septic tank you need to use the septic tank internal dimensions.
This article is part of our series: SEPTIC SYSTEM INSPECTION & MAINTENANCE COURSE.
Also see our home page: THE SEPTIC SYSTEM INFORMATION WEBSITE
Also see the basic septic system design information links at SEPTIC SYSTEM DESIGN BASICS: Choosing Septic Tank Size, Absorption System Size - basic septic system volume and absorption system design guides.
I am working on a project that will utilize septic tanks (several in parallel and series). The WW flows are 20,000 GPD, and influent WW BOD strength is about 450 mg/L and it will be permitted with an effluent strength of 100 mg/L.
The effluent will be disposed of via surface irrigation. I have not been able to find any literature on how to size a septic tank using influent and effluent BOD loadings. All literature I have found is based on number of bedrooms. This OSSF system will serve a Kid's Summer Camp.
Thanks and I look forward to your response. Robby
This question was posted originally at SEPTIC SYSTEM DESIGN BASICS
On 2017-08-23 by (mod) - how to size a septic tank using influent and effluent BOD loadings
Right. Yet there has to be an engineering assumption underneath the "number of bedrooms" that translates to "number of occupants" that in turn translates to average daily wastewater flow. That's what we both should look for and report back here.
See for starters WATER QUANTITY USAGE GUIDE https://inspectapedia.com/septic/Water_Quantity_Usage_Tables.php
and be sure to see SEPTIC TANK SIZE http://inspectapedia.com/septic/Septic_Tank_Size_Tables.php
In my OPINION the rule of thumb of 50 to 100 gallons of water per person per day may not be the right match for a kids' summer camp
but it would be really interesting to calculate:
- toilet flushes
- use of laundry facilities
- dishwashing and cleaning by camp staff
The volume of wastewater in the net free area between the top of sludge and bottom of scum determines the treatment time in the tank as new wastewater flows into the system. That in turn will be an important determinant of BOD measurement results.
Thanks for the septic tank capacity question. Please hop over to SEPTIC TANK SIZE
Also see SEPTIC SYSTEM DESIGN BASICS at http://inspectapedia.com/septic/Septic_System_Design_Basics.php#FAQ
where in reply to a reader question we discuss average daily wastewater flow per person design assumptions and refer to a companion article WATER QUANTITY USAGE TABLES
On 2017-08-15 y email@example.com - computation of septic tank capacity per person
please provide proper computation of septic tank capacity per personThis question and answere were posted originally at SEPTIC TANK PUMPING MISTAKES
While septic tank sizes are normally given in gallons or liters of wastewater capacity, the actual physical dimensions of a septic tank can be important for some considerations.
For example on a very rocky site where excavation to bury a septic tank runs into trouble the installer may decide to choose a shallow but large "flat" septic tank.
In my limited experience with shallow septic tanks they don't work so well and need to be pumped more often. I suspect that the net free area in total volume may meet spec but effluent settlement time may still be too little.
The septic tank under the pile of straw at the top of the septic system failure photo at left was just such a shallow or "flat" septic tank. Here is a table of some typical septic tank dimensions and other properties.
Table of Concrete Septic Tank Outside Dimensions, Capacity, & Other Data
Septic Tank Size
| Tank Width
|Tank Depth (Height)
SEPTIC TANKS, CONCRETE
|600 gallons||78||56||60||Not legal in many jurisdictions|
|800||96||67||57||Not legal in many jurisdictions|
|1000 heavy duty||96||78||61|
|1000 low profile||120||67||57||low profile septic tank|
|1000 /600||145 in.||68 in.||61 in.||two compartment tank|
|1250/750||162||78||64||two compartment tank|
|1600/1400||174||90||73||two compartment tank|
| Plastic / Fiberglass
Septic Tank Size
| Tank Width
|Tank Depth (Height)
SEPTIC TANKS, FIBERGLASS / PLASTIC
|500 spherical||64||64||64||holding tank or pumping tank, Norwesco|
|750 linear||96||60||51||sold as single or dual compartment|
Septic Tank Size
| Tank Width
|Tank Depth (Height)
/septic/Steel_Septic_Tanks.phpTanks.htm">SEPTIC TANKS, STEEL
|500 gallons round||46||46||73||
Cylindrical, laid horizontally.
In the U.S. older round cylindrical septic tanks were placed end-wise, that is with a flat end down and a flat cover on top. Recangular steel septic tanks are installed in low-profile forms in problem sites in some jursidictions.
Manhole risers: for septic tanks buried at depth that prevents ready access-to and removal of cleanout & inspection covers a manhole riser with a concrete or steel cover is recommended to permt proper inspection & cleaning. Covers must be of sufficient strength & security to prevent collapse & to prevent a septic tank fall-in hazard.
Concrete & some steel tanks are usually rectangular; plastic & fiberglass septic tanks may be roughly rectangular, oval, or even round spherical. Spherical tanks are usually used for pumping stations not for primary septic tank applicatins.
Dimensions & properties taken from surveys of septic tank manufacturers (concrete & other materials).
On 2017-07-15 by (mod) - Can a 550 gal water tank be used as septic tank
Not in any useful nor effective way
A water tank is the wrong shape, lacks the appropriate access connections for clean-out or connections to the Sewer Inlet or a fluid outlet, and by virtually all contemporary standards 550 gal would be too small. A water tank is also not designed to be buried nor withstand corrosion, earth pressure Etc.
It's a great question but IMO not a great idea.
On 2017-07-15 by Brandon
Can a 550 gal water tank be used as septic tank
On 2017-05-25 by (mod) - why can't I do some separate treatment of urine and sewage that will permit installing a septic system on a site that the authorities say is too small.
Regarding your question of treatment of urine and water mixture, I can't answer without knowing the intended application.
Normally urine and other wastewater flows either into a public sewer and on to a sewage treatment plant or into a private septic system where the wastewater in a conventional system is treated by bacteria in the septic tank (45%) and by both aerobic and anaerobic bacteria in the soil of the soakaway bed (55%).
Aerobic septics and other designs treat wastewater to different levels at different states of the process. Still other systems may use a disinfection step combining UV light or chlorine injection.
But ultimately the septic system components that dispose of wastewater effluent have to be able to do that effectively in the available space.
On 2017-05-25 by (mod) - - how is the volume of a septic tank calculated?
The volume of a septic tank is calculated based on the shape of the tank - calculating the volume of a rectangular shape Vol = L x W x H or of a cylindrical shape
The formula and discussion are at STATIC HEAD, WELL DEFINITION
The actual volume of wastewater in a properly-working rectangular septic tank is easier to calculate than a cylindrical one since we have to consider just the volume of the container up to the bottom of the outlet pipe.
If you want to calculate just the "liquid" volume of the septic tank you need to measure the current thickness of the floating scum layer and bottom sludge layer and thus to subtract those from the first volume that you calculated.
The "freeboard" in the septic tank has to be the volume between the bottom of the septic tank outlet pipe (assuming again a properly-working septic system with no blockages in the outlet or drainfield) and the top of the septic tank baffle or tee. That's because any depths higher than the tee top mean that the outlet is blocked and the system is not working normally.
This sketch http://inspectapedia.com/septic/Image4e.jpg used in several of our articles inculding SEPTIC TANK LEVELS of SEWAGE
Starting at SEPTIC TANK INSPECTION PROCEDURE you can see examples of tools used to measure the scum and sludge layers.
On 2016-01-23 by Mark - size for a septic tank in a population area of 650 people
What can be the appropriate size for a septic tank in a population area of 650 people?
Posted originally at SEPTIC SYSTEM DESIGN BASICS
On 2016-01-24 by (mod) - for 650 people - use an onsite wastewater treatment plant
No conventional septic tank would be reasonable for 650 people; the septic tank would have to be millions of gallons, and even then you'd need a thousand acres or so for a conventional drainfield.
For a population of 650 people, you'd need an onsite wastewater sewage treatment plant.
References & authorities for large capacity onsite wastewater treatment plants
Continue reading at SEPTIC DRAINFIELD SIZE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see SEPTIC TANK SIZE FAQs
Or see SEPTIC TANK, HOW TO FIND
Or see SEPTIC DRAINFIELD LOCATION for help in finding an existing septic soakaway bed, leach field or drainfield
Or see SEPTIC TANK PUMPING SCHEDULE
Or see SEPTIC TANK LEVELS of SEWAGE
Or see these
Also see our septic tank examples and septic tank properties or data at
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