Measuring then excavating to find the top and closest corner of a septic tank (C) Daniel FriedmanActual Septic Tank Depth
How to estimate & then measure the actual depth of a septic tank below ground surface

  • POST a QUESTION or READ FAQs about how deep the septic tank should be placed and how deep in the ground it may be found at various building sites

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Septic Tank Depth:

This article describes how to find the depth of a buried septic tank. Sometimes we can get the septic tank depth immediately by simple visual observation. But if the septic tank lid or top cover has been buried and we've not dug it up recently it may take some measurements and estimating to have a good idea just how deep the septic tank top is below ground.

A companion article linked at the end of this one gives a separate description of the design depth for septic tanks - how deep should a septic tank be?

Our page top photo shows us measuring the distance from house to septic tank nearest-corner (blue arrow) and also a septic tank cleanout-riser (red arrow). Looking through the septic tank riser top told us that we'd not have to remove much soil to expose the top of this septic tank.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How To Find the Actual Depth of a Septic Tank Below the Ground Surface

Photograph of  a conventional septic tank during installation.

[Click to enlarge any image]

Article Series Contents

Reader Question: septic tank depth must permit adequate drain line slope

(Feb 12, 2014) Anonymous said: ©

If my sewer discharge pipe is 6 ft+ below the ground surface does the septic tank inlet not have to be slightly lower in order for the waste to flow into the tank. Can a person use an effluent pump to discharge the waste to the field or do you use the effluent pump before the septic tank.


A waste line drains by gravity and slopes between 1/8" and 1/4" per linear foot. If your drain line had to run 20 feet from house outlet to tank inlet, the tank inlet has to be at least (20 x .125) " lower than the elevation of the waste line where it exits the building (unless your septic system is using a sewage pump or ejector pump).

At 1/8" per foot minimum slope that's (20 ft x 0.125 " per foot = just 2.5 inches below the house main waste line depth - the fat green arrow #9 in our sketch above. At 1/4" per foot that would place your septic tank inlet depth at 5 inches lower than the green arrow #9.

Our schematic above shows the measurements involved in finding the depth of a septic tank and should assist in understanding and picturing the septic tank depth location steps that we give just below.

How to Estimate, then Find the Actual Depth to the Septic Tank Lid in 10 Steps or Less

  1. Find the septic tank: First, if we don't know where the septic tank is buried, we have to find the septic tank location. If there are septic tank risers, or clean-out ports poking up above ground then we know the tank location and we can go on to subsequent steps in this list.

    But if there are no visual clues (pipes above ground) then we can still make some intelligent guesses at where the tank is by inspecting the building piping and the site conditions to see where a septic tank could reasonably be expected to have been buried.

    Details of these procedures are
    at SEPTIC TANK, HOW TO FIND where you'll see several approaches.
  2. Inspect Septic Tank Cleanouts or Risers: if there are one or more septic tank cleanout ports or risers or access ports (arrow #4 in our septic tank depth sketch above) that are above ground or close to ground level and perhaps marked by a stone, it's pretty easy: open the access port top or cover and measure with a tape the distance down to the top of the sewage effluent. See the orange arrow #6 in our septic tank depth sketch above.

    If the septic tank is in use and is full to its normal level, the top of waste in the tank will be a few inches under the under-side of the septic tank lid. If the tank is steel, plastic or fiberglass, then that's about the depth to the surface of the septic tank lid, with some variation if the upper surface of the septic tank is round rather than flat. If the septic tank is made of concrete then the dept from ground top to septic tank top is about 4-12" less than the depth of the septic sewage level from ground top.
  3. Use the building main drain exit point + distance x sewer line slope: If there are no septic tank risers or access ports that we can find, there are still some clues about possible septic tank depth, provided that the system works by gravity. A typical septic system into which sewage or wastewater flows by gravity must be lower than building's main drain exit point, and sewer piping between the building main drain and the entry port of the septic tank must slope between 1/8" and 1/4" (or more) per foot for wastewater to flow into the septic tank.

    Our septic tank depth sketch above shows all of the measurements and measurement points that might be involved in finding the septic tank depth using this approach.

    More about typical sewer line drain piping depths is

    With these concepts if we know the distance of the septic tank from the building we can calculate the approximate septic tank minimum depth below ground. We'll need to take into consideration the terrain shape. When ground slopes away from the building foundation wall (which we hope it does) then the septic tank depth will of course reflect that slope as well (as we show in our sketch).
  4. Main drain exit point: Find the point at which the building main drain exits the foundation wall - Green Arrow #9 in our sketch.
  5. Distance to septic tank: Measure the distance to the septic tank's nearest corner - Gray Arrow #10 in the septic tank sketch
  6. Main drain exit depth: Measure or estimate the depth below ground at which the building main drain exits the structure. This is the outdoor sewer line starting depth - Green Arrow #9 in our sketch.
  7. Compute minimum depth of the septic tank wastewater entry: Multiply the exit point to septic tank distance by 1/8" per foot to get the minimum depth of the septic tank entry piping connection. This will be the depth of the bottom of the septic tank wastewater entry pipe. This depth will be the same as the top of the septic tank wastewater level - Orange Arrow #6 in our sketch above
  8. Measurements give the minimum or the actual depth to the top of the septic tank - Red Arrow #5 in our sketch - will be about a 12" to 24" above this entry point in step 4, allowing for the additional thickness or dimension of:
    • the diameter of the entering septic or sewage wastewater pipe, typically 4" to 6" for residential properties
    • the air space between the top of the wastewater in the septic tank and the under-side of the septic tank lid, typically this distance is 6-8" in a septic tank.
      Watch out: if the sewage or septic wastewater is right up to the under-side of the septic tank then something is seriously wrong. See SEPTIC TANK LEVELS of SEWAGE

      and (plus)
    • the thickness of the septic tank lid itself, ranging from about 1/8" to about 4" depending on septic tank material and tank lid structural requirements discussed
      and (plus)
    • the height of any septic tank cleanout or access risers and their lids - Blue Arrow #4 in our septic tank depth sketch above
      and (plus)
    • the depth of soil backfill over the septic tank lid or septic tank riser lid, ranging from 0" (which means you should see it) to just a few inches (which means grass may be dead in this area) to 6-12" or even more.
  9. Consider (add or subtract) the effects of ground slope - Gray Horizontal line #7 in our sketch above - on possible septic tank depth below a horizontal line projected from the house sewer line exit point to the septic tank.

    If the terrain slopes away from the building by more than 1/8" to 1/4" per foot, then the septic tank will of course be further below a horizontal line projected from the building and it may be closer to the ground surface since the sewer line in this case will already be sloping more than the standard slope range of 1/8-1/4" per foot.
  10. Dig: ultimately the precise depth to the septic tank top from the ground surface - Red Arrow #5 in our septic tank sketch - will be confirmed by excavating down to the top.

    Naturally site conditions and installer discoveries during original septic tank installation can cause considerable variation in septic tank depth, from septic tanks that are actually above ground to those that are deeper than the minimum depth for proper sewer line slope.

Septic tank cover found just a few inches below ground (C) Daniel FriedmanBut it's reasonable to consider that an excavator who is digging the hole to install a septic tank would have little reason to dig deeper than necessary to bury the tank deeper than the depths we described

Our photo (left) is a close-up of the excavation to expose a septic tank cover as shown at the top of this page. This septic tank is located in Northern Minnesota. The page top photo includes a red arrow pointing to a permanently-installed (though a bit small) septic tank riser used for periodic septic tank pumping.

Also see DRAIN FREEZE PROTECTION where we describe the depth of sewer and septic system piping.

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. Links to related septic system testing and design information are given at More Reading just below.


Continue reading at SEPTIC TANK DESIGN DEPTH to read about the recommended depths for septic tanks, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SEPTIC DRAINFIELD SIZE for specifications for disposal field trenches, layout, size, setbacks, trench depth, etc.


Or see PUMPER TRUCK VACUUM PUMPS for a discussion of the ability to pump out septic tanks at depth or distance from the pumper truck

Or see U.S. SEPTIC AUTHORITIES & DESIGN SPECIFICATIONS - sources of septic system design, installation, inspection, testing, codes & regulations

Suggested citation for this web page

SEPTIC TANK DEPTH at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman