How to Calculate Septic Drain Field Size | Upgraded home (2023)

Ready to turn your field into a septic drain? Before you start digging, it's important to know the size of the area you need. Let's take a look at the process of selecting the right amount of space for your septic drain to avoid a potential messy cleanup situation.

How to Calculate Septic Drain Field Size | Upgraded home (1)

A septic tank needs enough space to drain. And withoutknow exactly how big your field isshould be, at worst it can lead to soil contamination and at best to tedious cleaning work. Taking the proper precautions beforehand will save you a lot of headaches in the long run.

So, how do you determine the field size of the septic drain?First you need to determine the size of the septic absorption field and calculate how large the drain field should be. After that, you need to know the positioning of the trench line and combine them all to determine the size of your septic drain field.

This guide will show you how to calculate all of these aspects of your septic drain field so you can accurately determine the right size. We also provide some more information about septic drain fields, so that you know exactly what you are dealing with.

Related content:How to locate septic tank field lines|Dimensions septic tank| Do you need a permit to dig a trench?|Why is there water around my septic tank?|Can you build a garage over a septic field?

Do you need a septic system installation or replacement?

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How to Calculate Septic Drain Field Size | Upgraded home (2)

More about septic drain fields

There are many different things to consider when sizing yourseptic drainfield. The septic drain field is also known by a number of different names. You may have heard it called absorption beds, leach beds, leach fields, soak beds, or soil absorption systems.

However you know it, the reasoning for it is simple. They are intended to treat and dispose of wastewater on site. These systems are also known as your septic systems. This guide will take you through the different absorption systems out there. These are deep trenches, cut-and-fill, shallow trenches, conventional trenches, and septic systems without gravel.

How to determine the size of the septic absorption field

In general, a conventional septic tank will perform about 45% to 50% of the sewage treatment for a private home with a septic system. This may be less depending on the treatment system used.

What's left? The rest is wastewater treatment and the discharge of this wastewater is intended to take place in your septic drain field. Also keep in mind that there is no "right size"; the size may depend on a few variables, such as the daily wastewater volume and the rate at which the soil can absorb that liquid.

(Video) How To Calculate Septic Tank Drain Field Size

There are different types of systems that can get the job done, and one is likely to be a better fit for your situation than the other.

How big should the drain field be?

To know what size drain field you need (generally square footage), it may depend on the previously mentioned soil absorption or "soil seepage".Batches with excellent percolation rate are approximately 2.5 cm in three minutes. This would require about 450 square feet of drain field for an average three bedroom home.

Take that same home, but it has poor soil leaching; call it an hour per inch. In this case, you will need about 900 square feet for the drain field. This is intended to counteract the poor drainage in that soil.

How septic leach fields are built

The drain field - or one of the names mentioned above - is generally built by using perforated sewage distribution pipes and placing them in a field or gravel bed. A leach field will beseveral trenchesthat can be as large as 100 feet long and maybe 1-3 feet wide. In general you would want them to be about six feet apart, maybe more depending on what your local requirements are.

When constructing a leach field, it is a good idea to leave some space between the original lines so that replacement leach lines can be installed if necessary. In fact, it leaves room for error if the first few lines don't work correctly.

Specifications of a septic trench line

Typically, drainage fields have a series of perforated pipes buried in trenches filled with gravel. There are other ways to construct a drain field, but this is perhaps the most common way.

If you have a sloped site, make sure it is installed over the slope line. The point here is to make sure the waste water doesn't flow downhill and simply leaks out at the end of your drain line.

Most professionals will say that the slopes of these trench drainage pipe systems are "flat." Still, they will have a slight slope, somewhere near 1/8-inch per foot and less in some cases.

Position up or down?

The drains that run into the septic trench line are placed on top of that bed of gravel and then covered with more gravel.Be sure not to place the pipe at the bottom of the trench until you have lined it with a gravel bottom.

When placing perforations in the pipe, place them at the 5 and 7 o'clock positions. That means you won't end up storing or collecting sludge in your pipes or the effluent at the bottom of the pipe. This saves some headaches in the long run.

If you don't put that drain line with its perforations and one row of holes pointing straight down, it basically means you'll have to put off clogging the pipe and wastewater drain holes. This is the "up or down" argument in a nutshell.

Drain field trench specifications

For the depth of your drain field trench, a good rule of thumb is that it should be somewhere in the 18 to 30 inch range.The maximum ground cover over your field should be 36-inch tops. The USDA will recommend 2-5 feet in total, so the 36 inches would fall in between that range.

In colder climates, it's a good idea to make sure the drain field trenches are deeper than normal. This also means that while the effluent runoff can be protected from freezing, the treatment may ultimately be marginal.

(Video) What size septic system do I need? | #AskThePumperdude

Width discharge field

In most cases, the width of septic slots will be in the range of 8 to 12 inches, although there are also 18-inch to 36-inch wide slots. These are all conventional designs when it comes to your septic drain field.

Whatever the width, there is something to consider for design purposes.That is, each linear foot of your drain field line should provide approximately one square foot for the drainage area. This is according to the health department or building code septic approving authorities.

Proper spacing and length of drains

When you excavate or have the trenches excavated, make sure they are about six feet from the center of the pipe to the next center of the pipe. This is so replacement slots can be fitted if the first set of tubes fails.

As for the length of the trenches, they are usually in the 150 foot range. Of course, depending on bottom drainage and the size of the tank, it can be up to three times as long. While it would be easy to recommend a length, it all comes down to the location involved and any soil conditions.

Ultimately, creating the right trench has some ground rules, but there are some variations that can lead to differences from what you might expect. It's best to have a professional come over to investigate the site and at the very least make recommendations if you plan on doing the installation yourself. It's better to be safe than sorry when it comes to the measurements required for a drain field.

Related Questions

How deep is a septic leach field?

The depth of your septic leach field depends on your local regulations. Typically, each of your trenches should be between 1 and 3 feet deep and about 18 inches wide. The gravel layer should be at the bottom of each trench and be about a foot deep. Also, the perforated pipes must be connected to the distribution box that leads to the septic tank.

How many feet of septic chamber do I need?

You should limit the length of your trenches to about 50 feet to achieve full saturation and the best treatment. However, you should always vent the ends of your rooms to ensure that healthy air flows into the system.

What can I do about a saturated septic field?

When you experience a "septic problem," the septic tank is usually not the problem, but the saturated solid surrounding the tank is. If this happens to you, you should stop using the septic until the water goes away and the soil dries out completely. Otherwise, you run the risk of flooding your septic tank and your garden.

Do you need a septic system installation or replacement?

Get free, no-obligation quotes from professional contractors near you.


How to Calculate Septic Drain Field Size | Upgraded home (3)

In summary

If your septic tank doesn't have the right amount of space to drain, then you're going to experience some horrible problems that you don't want to deal with. By making sure your septic drain field is the right size, you can be proactive in preventing septic or drain field problems. That way you don't have to deal with the annoying problems after they happen.

Related Guides

  • Septic tank alternatives for land that will not benefit
  • Can a homeowner install a septic system?
  • How many years does a septic system last?
(Video) DIY Septic System Install: Passed Inspection! Infiltrator Chamber Install Pt. 2

Ryan Wommeldorf

Ryan Womeldorf has over a decade of writing experience. He enjoys blogging about construction, plumbing and other home topics. Ryan also enjoys hockey and is a lifelong Buffalo sports fan.

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(Video) Overview of the Math, Design, and Build of a Septic System


How to Calculate Septic Drain Field Size | Upgraded home? ›

Required Septic Absorption Field Area = Flow Rate (GPD) / Application Rate (GPD / Sq. Ft.) Note that this table considers only the dimensions of the bottom of the drainfield

Septic drain fields, also called leach fields or leach drains, are subsurface wastewater disposal facilities used to remove contaminants and impurities from the liquid that emerges after anaerobic digestion in a septic tank. Organic materials in the liquid are catabolized by a microbial ecosystem. › wiki › Septic_drain_field
trench in considering the effective soil absorption area.

How big is a leach field for a 3 bedroom house? ›

If the soil has good percolation conditions—it's comparatively sandy and waste water seeps down with little resistance—figure a seepage field of 4,500 square feet (say 100 feet long and 45 feet wide) for a three-bedroom house with normal waste output.

How big is a leach field for a 4 bedroom house? ›

Generally speaking, the size of the septic field should be between 1500 and 2000 square feet for a typical residential home.

How many square feet should a drain field be? ›

Sewage System Sizing
Number of BedroomsTotal Septic Tank CapacityDrainfield Size (Clean Medium Sand)
2 or less1500 gallons1000 square feet
32000 gallons1200 square feet
42500 gallons1600 square feet
53000 gallons1800 square feet
1 more row

How do you calculate septic tank size in a house? ›

How to Calculate the Septic Tank Capacity in Gallons. 3.14 x radius squared x depth (all in feet) = cubic capacity. Cubic capacity x 7.5 = gallons capacity. Length x width in inches / 231 = gallons per inch of septic tank depth.

How big is the average leach field? ›

Leach fields are usually in a large, flat and open area of the yard. They are composed of multiple trenches that are usually placed about six feet apart from each other. These trenches can be anywhere from 18 to 36 inches deep, are typically between one to three feet wide and can be up to 100 feet long.

How big is a typical drainfield? ›

If an absorption bed drainfield is used the minimum drainfield area shall be 100 square feet with an additional 50 square feet for each additional bedroom over two bedrooms.

What is a standard leach field? ›

A standard leach line is considered to be three (3) feet wide and three (3) feet deep with a length as required. A non-standard leach line is wider, narrower, and/or deeper than three (3) feet with a length as required.

How many bedrooms is a 900 gallon septic tank? ›

The current rule sets a minimum septic tank size of 900 gallons for a 1-3 bedroom residence.

How many pipes are in a leach field? ›

Between four and nine lateral pipes run out of the distribution box and radiate outward in trenches to form the leach field.

How deep is a typical septic drain field? ›

A typical septic drain field (see Figure 1), also known as a leach field, is a series of perforated pipes that are set in trenches and buried with aggregates (½- to 2½-inch gravel or ½- to 4-inch rubber chips) and soil. These drain lines are at a minimum depth of 6 inches and are typically 18 to 36 inches wide.

Can a drain field be higher than the septic tank? ›

Uphill areas, areas that are higher than the elevation of the septic tank are not a first choice to contain the drainfield or leaching beds. Unless a septic pump or effluent pump system are installed (you'd find wiring, and perhaps alarms) the drain field is going to be at or below the elevation of the septic tank.

What makes a good drain field? ›

Leveled and Stable Soil

One of the apparent signs of a healthy leach field is level and stable soil near the drainfield. When your home's drainfield cannot expel all of the wastewater from the septic tank, it will eventually collect into the ground. This will cause your soil to be “squishy” and even condense on itself.

How many gallons is a 3 bedroom septic tank? ›

A three-bedroom house will need a minimum of a 1,000-gallon water tank, which regularly handles about 360 gallons of water per day.

How big are most home septic tanks? ›

Common residential septic tanks range in size from 750 gallons to 1,250 gallons. A septic tank is a self-contained unit designed to hold residential wastewater. The system is comprised of two main components: the tank and the drain, or soil absorption field.

How many gallons a day for septic system? ›

How Many People Can A Septic Tank Handle? It comes down to daily water usage. Most residential tanks have a capacity ranging from 750 gallons to 1,250 gallons and the average person uses 60 gallons to 70 gallons of water a day.

How do I calculate the size of my drain field? ›

Required Septic Absorption Field Area = Flow Rate (GPD) / Application Rate (GPD / Sq. Ft.) Note that this table considers only the dimensions of the bottom of the drainfield trench in considering the effective soil absorption area.

What happens when leach field is full? ›

If the drainfield is overloaded with too much liquid, it can flood, causing sewage to flow to the ground surface or create backups in toilets and sinks. Finally, the wastewater percolates into the soil, naturally removing harmful coliform bacteria, viruses and nutrients.

What are the 3 types of septic systems? ›

The common types of septic systems are gravity, pressure distribution, sand filter, and mound systems.

How big is a 2000 gallon septic tank? ›

Additional Information
Store IDX9803535
Size Info
Capacity (Gallons)2000
Dimensions126" Length x 98" Width x 51" Height
Weight (lbs)704
25 more rows

What is the slope of a leach field pipe? ›

Slope of the watertight pipe should be a minimum of 1/4-inch per foot toward the field to prevent clogging. Holes in the perforated pipe should be 1/2- to 3/4-inch in diameter.

Can a leach field be on a slope? ›

The maximum slope allowed for leach line trenches is 40 percent (2-1/2:1 slope). 2. All leach lines on steep slopes shall be installed in five-foot deep trenches with 12 inches of leach rock below the leach pipe or with approved chambers or other gravel-less system.

What is the difference between a leach field and a drain field? ›

This is probably why many homeowners are confused about the difference between a drain field and a leach field. Well, it may be surprising to learn that there's no difference at all! In the argument of drain fields vs. leach fields, they are simply different words for the same septic device.

How long does it take for a leach field to dry out? ›

The owner switches the system to use the other drain field for the next two years, allowing the first field to dry out completely and “rest.” At any time, one of the septic fields is on a two-year rest. That's enough time for the biomat to die off and allow the drain field to recover its absorbency.

Does shower water go into septic tank? ›

You can put wastewaters from your sinks, baths, showers, toilets, washing machine and dishwasher into your septic tank system.

How often should a 900 gallon septic tank be pumped? ›

Under current Pennsylvania law a 900 gallon septic tank is the minimum size that may be used for a home with three bedrooms or less. If six people reside in a three-bedroom house, the tank should be pumped every 1.3 years. If the same system serves a family of two, the tank would need pumping every 5.2 years.

How many gallons is the average septic? ›

The average here is 1,250 gallons, though many tanks hold 1,750 gallons or more. In decades past, 750-gallon tanks were popular, though these are now considered inadequate for basic residential tasks. Nowadays, the average range is 1,000 gallons to 1,500 gallons, depending on the demands and needs of the homeowners.

Is a 1000 gallon septic tank big enough for 4 people? ›

Septic tanks are designed to accommodate the number of bedrooms in a home and potential in flows. Therefore, an average home with one to three bedrooms is intended to house four or five residents and would have a tank that would accommodate 1000 gallons. This is large enough to handle normal inputs.

What is the best pipe for a drain field? ›

Corrugated pipe is typically used for drain fields. Septic systems use drain fields to treat the septic tank effluent for the removal of impurities and contaminants.

How do I know if my septic drain field is bad? ›

Signs of Septic System Failure
  1. Water and sewage from toilets, drains, and sinks are backing up into the home.
  2. Bathtubs, showers, and sinks drain very slowly.
  3. Gurgling sounds in the plumbing system.
  4. Standing water or damp spots near the septic tank or drainfield.
  5. Bad odors around the septic tank or drainfield.

Does a leach field have a vent pipe? ›

With nowhere to go, it's important to create an outlet where gas can leave your septic system. This is where your septic vent comes in. Found near the leachfield in your yard, the septic vent pipe brings out the air trapped in your tank.

What is the best rock for a leach field? ›

Granite gravel is the best option for a leach field bed. The gravel amount depends on soil composition and septic system size. Leach field depth should be around 3 to 4 feet, depending on the size of your space.

Can you plant a garden over a septic field? ›

The most important reason you should not install a vegetable garden on top of, or right next to, a septic system disposal field is because the plants can become contaminated by wastewater that has not yet been renovated by the field. Plants on disposal fields can absorb wastewater pathogens.

Can you put pavers over a septic drain field? ›

Paving Over Your Septic Tank

You should never pave over your septic tank. Although soil compaction is not a major issue for septic tanks, there are other dangers presented by placing an insecure septic tank underneath concrete and heavy vehicles.

What ruins a drain field? ›

Oversaturation. Dripping faucets, running commodes, and malfunctioning water softeners put extra water into the septic system, which can flood out your drainfield. Higher-than-normal usage on the system from occupants or visitors can overload the system as well.

What determines the size of a septic field? ›

The size of the drainfield is based on the number of bedrooms and soil characteristics, and is given as square feet.

Why do septic drain fields fail? ›

Why septic systems malfunction. Most septic systems malfunction because of inappropriate design or poor maintenance. Some soil-based systems (those with a drain field) are installed at sites with inadequate or inappropriate soils, excessive slopes, or high ground water tables.

Should there be water in my drain field? ›

The water level should always be at the level of the drain line connecting the septic tank to the leaching field. If the water level is higher than the drain line, it means that the leaching field cannot hold any more water and it is backing up into the septic tank.

What can you put on top of a septic field? ›

Put plastic sheets, bark, gravel or other fill over the drainfield. Reshape or fill the ground surface over the drainfield and reserve area. However, just adding topsoil is generally OK if it isn't more than a couple of inches.

Why do drain fields go bad? ›

If the buildup of organic material is not filtered by the septic tank, this can cause the drain-field to fail. If your first step of treatment is not done properly in the septic tank, causing low quality effluent to be discharged to the soil, the build up of organic waste can lead to drain-field failure.

How many bedrooms does a 2 000 gallon septic tank support? ›

You may also be interested in these popular precast concrete septic tanks.
2000 Gallon Septic Tank Details
Tank dimensions12' x 6'6” x 5'8”
Number of bedrooms supported6 or 7
Ideal for high water tableNo, but our 2000 gallon monolithic tank is.
13 more rows

How often should you pump your septic tank? ›

For a typical household, septic tanks are usually pumped every three to five years. Routine pumping can prevent expensive failures such as a clogged drainfield or sewage backing up into the home. Using a garbage disposal will increase the amount of solids entering the septic tank, requiring more frequent pumping.

How many bedrooms will a 1250 gallon septic tank hold? ›

1250 gallon septic tank is used in up to 4 bedroom systems.

How do you calculate septic tank design? ›

Measurements taken on the internal dimensions of the septic tank are in feet. 3.14 x radius squared x depth (all in feet) = cubic capacity. Cubic capacity x 7.5 = gallons capacity. Length x width in inches / 231 = gallons per inch of septic tank depth.

How long do septic tanks last? ›

The more proactive you are in maintaining your system, the longer it will last. In fact, septic tanks can last as long as 30 years or more. The main goals of a septic tank maintenance program are to prevent the accumulation of solids, as well as any groundwater contamination.

How deep is a 2000 gallon septic tank? ›

Bottom of Tank to Bottom of: Inlet (Inches) - 45.75. Outlet (Inches) - 42.75.


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