How Do You Measure an Exhaust Pipe?

Print Friendly, PDF & Email

Vehicle owners usually know horsepower off the top of their heads, but most owners scratch their heads when it comes to exhaust pipe sizing. Thankfully, measuring the exhaust is straightforward; all you need is a soft tape measurer or a ruler.

External and Internal Diameter 

Exhaust pipes typically focus on two measurement types: the external and internal diameters. The outer diameter is the measurement across the pipe opening end-to-end, including the pipe thickness. The inner diameter is the distance across the opening without including the thickness. 

You will often use the internal measurement when measuring for an exhaust pipe adapter, tips, or a muffler because they fit over the exhaust pipe. Most exhaust measurements are external because they fit inside the muffler.

Measuring the External or Outside Diameter

Open or exposed exhaust systems are the easiest to measure because nothing is blocking access to the end of the pipe. For an open or exposed pipe, you can use a measuring tape or ruler to find the diameter of the exhaust. Measure from one outside edge to the opposite edge across the center point.

If the end of the exhaust is not exposed, you can use a pair of calipers or a tailor’s measuring tape. The calipers make the job easy. You simply adjust the device over the pipe; a digital readout should give you the measurement. Unfortunately, if access to the pipe is restricted, you will need to use some arithmetic. Wrap the tailor’s tape measurer around the pipe to get the circumference. Then, divide that number by 3.14 to get the external diameter.

You can use a string if you don’t have a tailor’s tape measurer. Wrap the string around the exhaust pipe and mark where the two ends meet. Take the string and lay it out in front of a ruler or tape measure. Measure the string from one end to the mark. This will give you your circumference, which you can then divide by 3.14.

Measuring the Internal Diameter

You can only measure the internal diameter with an exposed exhaust. While this makes measuring easier, it can also mean you have to cut into your exhaust to find the correct measurement. Thankfully, if you need to cut into your system, you can use Nickson 2in x 6ft exhaust flex pipe to make repairs. 

Remembering Inlet Versus Outlet Measurements

Exhaust fumes flow from the inlet to the outlet of the exhaust pipe. Some exhaust systems contain different diameter tubing from the inlet to the outlet. When replacing your exhaust, you want to measure the inlet and outlet diameters to ensure you purchase an adequate replacement. An auto parts professional or mechanic can help you make the correct purchasing decision.

Are you getting ready to replace your vehicle’s exhaust system? If so, you must measure the pipe diameter correctly. Buying the incorrect size can result in installation problems and failed emissions tests. Contact a local auto parts retailer or mechanic to learn more about exhaust measurements and replacement parts for your car.