# How to Calculate Septic Drain Field Size? (Answered)

“Septic Drain Field” is a term that many of us are familiar with. However, for those who are hearing this for the first time, allow us to explain what a septic drain field is.

Septic drains, often known as septic drain fields, are wastewater treatment solutions popular in rural areas where consolidated sewage lines are not easily available. It is a drainage system that is used in various facilities to remove waste, debris, and contaminants from water.

Although you might have heard this term at some point in your life, there is a lot more to it and its mechanism. Now, let’s look at how to calculate the size of a septic drain field and some key information about it.

## How to Calculate Septic Drain Field Size?

Firstly, to calculate the size of a septic drain field, determine the septic absorption field size. Moreover, you must also consider the size of your drain, specifically focusing on how large you want it to be. Lastly, these two items should be aligned with the drain’s trench line placement.

The utilization of a sewage system was not feasible in every part of the world. As a result, there had to be some alternate solution to the drainage system that could be utilized in those places in the world.

Therefore, the concept of a septic drain field came into light in the year 1860, which was given by Jean-Louis Mouras. It was first officially utilized in France.

As time passed by, septic drain fields gained immense popularity all over the world. There are currently over 26 million septic drains in use by 20% of the total population of the United States of America. Now, let us know the steps to calculating the septic drain field size.

### Determine the septic absorption field size:

Firstly, in order to calculate the size of a septic drain field, it is very important to determine the septic absorption field size. Because, a lot of factors are significantly dependent on it in calculating the size of a septic drain field.

### Estimate the size of the drain:

Secondly, depending on the soil percolation rate per hour or per minute, estimate the size of the drain in order to calculate the size of the septic drain field.

### Find the ideal measurements for the trench line:

Lastly, find the ideal measurements of the trench line of the septic drain field, which would mainly constitute the length, width, diameter, slope, and spacing.

Note them down carefully, and it is highly recommended to get the help of a professional in terms of taking the measurements and estimating the ideal measurements while calculating the size of your septic drain field.

## How big should a septic drain field be?

The size of a septic drain field is completely dependent on the rate of percolation of the soil. Now, as many might get quite puzzled about what soil percolation rate is, let us clear that out for you.

The term “percolate” means ‘to filter’ or ‘to strain or drain’. A soil percolation rate is defined as the rate at which water percolates into the soil.

How big a septic drain field should be is quite dependent on the soil percolation rate. Again, this size would also differ depending on how many bedrooms are there in a house and what amount of work the septic drain field needs to perform in order to cover the whole area.

In the instance of a three-bedroom house, the drain should be 400-450 square feet in size if the soil percolation rate is good. However, if the soil percolation rate is inadequate, a 900-950 square foot septic drain field will be required to address the poor drainage in the soil.

These figures might significantly vary based on the condition of the location, soil condition, the number of bedrooms in a house, and the rate of soil percolation as well.

## How deep is a septic drain field?

A septic drain field is generally about 1.5 to 2.5 feet, i.e., 18 to 30 inches deep.

However, the U.S. Department of Agriculture, i.e. the USDA, has estimated a fixed range of depth for the placement of septic drain fields, which is 2 feet and 5 inches, to be exact, which is nearly 29 inches.

However, the soil layer covering and the gravel layer should also be taken into consideration when determining the final depth of a septic drain field.

With 1 foot of gravel layering below each trench of the septic drain field and 3 feet, i.e. 36 inches of soil covering at the very top in order to finish the bedding, the overall final depth of a septic drain field is going to be approximately 3 feet to 4 feet.

Again, these figures might vary depending on the size of the establishment, local codes, the condition of the soil and weather, and how many bedrooms an establishment has altogether.

## What are the septic drain field trench specifications?

Septic drain field trench specifications are discussed below –

### The length of the trench:

There are several opinions from different experts about the ideal length of trench in a septic drain field. Some experts recommend keeping the length of the trench line at about 150 feet long.

On the other hand, other professionals suggest keeping the trench lines about 100 feet long. Other than these numerical figures, the length of the trench line can be three to four times longer as well, depending on the establishment, location, and soil condition.

The length of the septic drain field trench would significantly vary depending on a variety of other factors.

### Width of the trench:

The width of the septic drain field trench mostly ranges from either 8 to 12 inches or 18 to 36 inches. This width is applicable for both conventional and traditional designs of septic drain field trenches.

Incase of specifying the width in the design of your septic drain field trench, it is also important to keep in mind that 1 square foot of drainage area is provided by each linear foot of drain field line.

### Spacing between the trench pipes:

It is recommended to leave a 6-foot gap between the centers of each excavated trench in order to maintain its smooth functionality. Besides, due to this spacing between the positioning of trench pipes, replacing them becomes easier if necessary.

### The diameter of the trench pipes:

The estimated diameter of the trench pipes is taken from 4 inches to 5 inches. It might vary depending on how big the pipe is and the soil percolation rate as well.

Besides, getting a professional point of view from an expert is highly recommended in terms of deciding the size of the trench pipe and its diameter.

### Slope measurements for placing the trench pipes:

The placement of the trench pipes is done in a slightly slanted manner. It helps to maintain better positioning of the trench pipes.

Usually, these slopes are 1/8 inch or less per foot, to be precise.

## How many feet of septic chamber do I need?

Considering all the factors and placement of your septic drain field, it is highly recommended to keep the septic chambers no more than 45 to 50 feet apart.

Keeping your septic chamber at this range would ensure the best outcome of your septic drain field in terms of mechanism.

But, the measurements of every septic chamber differ depending on the establishment. Thus, in the case of large establishments, the requirement for a chamber as well as its size would also differ.

If the chambers are placed side by side, it is recommended to keep 6 to 8 feet of distance between each of the chambers in order to maintain their smooth functionality.

## How deep is a septic leach field?

A septic leach field should be 1 to 3 feet in depth, i.e. around 12-36 inches, and 1.3 to 1.5 feet wide, i.e. 16 to 18 inches. This is considered the ideal range for the depth of any septic leach field.

But, even after a few specifications, these figures would vary depending on the respective local codes on a country or state basis.

Following these local codes is very important in order to determine the depth and any other measurements of your septic drain field. Because, each country and locality do have different kinds of soil conditions with distinct soil components depending on the weather and climate.

As a result, the measurement that would be applicable in California would not be the same in Florida due to the conditions of the soil, its layering and components, weather conditions, and certain other sorts of limitations.

Thus, the depth of the septic leach field would vary as per the respective local codes.

## Final Thoughts

Several criteria must be considered for calculating the size of a septic drain field. Some of them are the size of the septic absorption field, trench measurements, and the size of the septic drain field. Additional aspects should be considered while calculating the size of a septic drain field.