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SEPTIC SYSTEM INSPECT DIAGNOSE REPAIR
SEPTIC CARE INSTRUCTIONS
SEPTIC D-BOX INSPECTION
SEPTIC DRAINFIELD FAILURE DIAGNOSIS
SEPTIC DYE TEST PROCEDURE
SEPTIC FAILURE SIGNS
SEPTIC INSPECTION & TEST GUIDE
SEPTIC LIFE EXPECTANCY
SEPTIC SUPPLIES & PARTS
SEPTIC SYSTEM DESIGN ALTERNATIVES
SEPTIC SYSTEM DESIGN BASICS
SEPTIC SYSTEMS, HOME BUYERS GUIDE to
SEPTIC SYSTEM SAFETY WARNINGS
SEPTIC TREATMENTS & CHEMICALS
SEWAGE BACKUP PREVENTION
SEWAGE EJECTOR / GRINDER PUMPS
SEWER GAS ODORS
SEWER LINE REPLACEMENT
SINKHOLES, WARNING SIGNS
SOAKAWAY BED FAILURE DIAGNOSIS
SULPHUR & SEWER GAS SMELL SOURCES
TOILETS, INSPECT, INSTALL, REPAIR
TRAPS on PLUMBING FIXTURES
TREATMENTS & CHEMICALS, SEPTIC
VIDEO GUIDES: Septic Videos
WATER CONSERVATION MEASURES
WATER SOFTENERS & CONDITIONERS
WATER SUPPLY & DRAIN PIPING
WATER, WELLS, WATER TANKS: TESTING GUIDE
WASHING MACHINES & SEPTIC SYSTEMS
WASTEWATER TREATMENT BASICS
WINTERIZE A BUILDING
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.
Notes to Table:
SEPTIC TANK SIZE TABLE - Table of Required Septic Tank Size Based on Number of Bedrooms in a Home
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. For example, New Mexico 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 .
Notes to the septic tank table:
2. 750 gallons is smaller than the minimum 1000g size required for new construction in some jurisdictions.
Septic Tank Size Requirements May Be Set or Adjusted by Local or State or Provincial Ordinances, Lift Stations, Weather
In some jurisdictions government ordinance may simply specify the minimum allowable septic tank size by
Design, Code Requirements & Maintenance for Two-Compartment Septic Tanks
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 entire tank, both compartments, will need to be filled with wastewater before any effluent will begin to flow out of the septic tank and into the drainfield or soakaway bed.
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 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 SEWAGE LEVELS in SEPTIC TANKS 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 Filters
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.
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: Inspecting, Testing, & Maintaining Residential Septic Systems. Also see The Septic 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.
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. Also see our septic tank examples and data at
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