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How to Test a Septic System

Sarah Terry

Whether you’re a home inspector or a homeowner, testing your septic system is essential to determining the health of your septic system and whether you have a system failure.

Perform a thorough test of your septic system by studying the interior plumbing, inspecting any visible parts of the septic system and performing a simple dye test. Purchase a septic-system dye test kit at most home improvement or plumbing retailers.

Inspect the Interior Plumbing and Visible Septic Components

  1. Run the water in all the sinks, bathtubs, washing machines and dishwashers to note the flow of the draining water. Slow-draining water can indicate a full septic tank.

  2. Flush all the toilets to look for any sewage backup or slow emptying of the toilet bowl, which can also indicate a full tank or clogged lines.

  3. Inspect the basement or crawl space of the house, if it’s accessible. Look for any leaking sewage or wastewater from sewage backups.

  4. Locate the area in the yard where the septic tank and drain field resides. Check the area for any signs of a clogged drain field or full septic tank, such as sewage smells, wet areas in the soil, noticeably greener grass over the drain field or black sewage seepage on the yard area.

  5. Inspect any storm-water drainage pipes around your property, especially those adjacent to the drain field area. Look for any wastewater or sewage seepage in these drainage pipes.

Perform a Dye Test

  1. Calculate the necessary water volume that you’ll need to run after pouring the dye at a rate between 3 gallons per minute (GPM) and 5 GPM. Multiply the number of bedrooms in the house by 75 gallons to determine the needed water volume and add 50 gallons for every whirlpool tub or spa. For example, a three-bedroom house with one hot tub would need a volume of 275 gallons of water for the test (75 gallons multiplied by 3 bedrooms equals 225 gallons plus 50 gallons equals 275 gallons of water).

  2. Pour the dye into one or more of the sinks, bathtubs or toilets. Run the water immediately after pouring the dye.

  3. Run water from one or several faucets to achieve the desired flow rate of between 3 GPM and 5 GPM. Run the water until you’ve reached the needed water volume for the test. For example, if your needed water volume is 275 gallons and you’re using a flow rate of 5 GPM, run the water from the faucets for about 55 minutes (275 gallons divided by 5 GPM equals 55 minutes).

  4. Inspect the drain field area in the yard for any visible traces of the dye, including any nearby ditches or storm-water drainage pipes. Inspect the basement or crawl space, if accessible, for dyed wastewater.

  5. Flush the toilet in the lowest part of the house or closest to the main septic line to look for any sewage backup or backup of the dyed wastewater.

  6. Tip

    To determine the rate of flow from a single faucet, use a flow meter device or time how long it takes to fill a 1-gallon milk jug with water from the faucet.


    If you have multiple homes or structures with pipes running to the same septic system, include all these structures in your inspection. Also, use different-color dyes for the separate homes or structures to pinpoint potential problems with the piping running from the structure to the septic tank.

    Remember that a dye test alone won’t always reveal a septic system failure. Perform a thorough inspection in addition to the dye test to find potential septic problems.