How To Measure Exhaust Diameters

Exhaust pipe sizes can be a little tricky to pin down. The measurement process seems simple enough: identify the exhaust inlets and outlets, then measure them using the appropriate tool. 

However, note that there’s a lot to consider to get accurate measurements, like the outside or internal diameters as well as exhaust inlets and outlets. Here’s what you need to know if you need to measure your vehicle’s exhaust pipes and other components:

Outside vs. Inside Diameters

With exhaust pipes and connected components, there are two diameters to take note of when measuring from the open ends: the external and internal diameters. 

The external or outside diameter is the distance from one edge of the pipe to the other, including the thickness of the tube’s walls. 

The inside diameter is the distance from one edge of the pipe to the other without the thickness of the tube’s walls.

But when checking the exhaust pipe diameter, is the inside or outside diameter the right measurement to take? What about other components?

Mufflers and exhaust tips use internal diameter measurements. They slip over the exhaust pipes. Meanwhile, the pipes use external diameter measurements because they fit inside the exhaust tips and mufflers.

What Are Exhaust Inlets and Outlets?

When taking measurements, check both the inlet sizes and outlet sizes. 

Exhaust fumes flow in one direction from the engine to the tailpipe. The inlet size is the section of the component that points toward the engine, while the outlet size is the part that points toward the tailpipe.

The exhaust inlet and outlet diameters can differ in the same pipes or components. Exhaust pipe diameters sometimes differ at various points along the exhaust line. 

For example, the pipes between the exhaust manifold and muffler might start at 3 inches, but they switch to 3.5 inches at the tailpipe.

Identifying Exhaust Inlets and Outlets

While exhaust inlets and outlets can be a little tricky to identify, sometimes, a visual cue is all you need. Exhaust parts have design hints that show their flow direction. 

Mufflers have a wider inlet diameter to accommodate incoming exhaust gases. The muffler then tapers toward the outlet. If the pipe or component has an arrow, it’ll show the direction from the inlet to the outlet.

The inlets and outlets on pipes and resonators are a little trickier. If the side of the pipe or resonator attaches to broader sections of the system, it’s likely the inlet side. The outlet, on the other hand, typically narrows and leads to the tailpipe.

For catalytic converters, the inlet is the side closer to the engine.

Rust and soot buildup often occur more on the inlet side. The inlet side generally shows more wear because it receives a lot of hot engine gases exiting the system. The outlet side typically looks cleaner and has a different patina.

These visual cues are a good place to start when identifying the exhaust sections, but remember they aren’t 100% accurate.

Why Are Accurate Measurements Important?

Having correctly fitted parts will help keep your vehicle’s exhaust system functioning at peak capacity. If you buy an inlet pipe that’s too small, it can restrict the exhaust gas flow. On the other hand, an outlet pipe that’s too large will hurt your vehicle’s efficiency and power output.

That’s why learning how to measure the exhaust pipe diameter is so important. The diameters of other exhaust components also matter for getting the right parts of course.

Exhaust Diameter Measurement Tips

So how do you measure an exhaust pipe and other exhaust components? Here are some tips for how to measure exhaust sizes:

Clean Both Sides of the Component Well

Wipe away dust, dirt, rust, and grime from both the inlet and outlet. Debris can throw off your measurements.

Measure Across the Opening Using the Right Tool

We recommend a caliper. Remember to keep in mind whether the component you need to replace is sized on the inside or outside diameter.

Double-Check the Measurements

Take two or three measurements of the diameter at different angles. Bent or banged-up pipes might not be perfectly round anymore.

Compare Your Measurements to the Part’s Listed Dimensions

You can also check the connecting pipes to ensure you have the right measurements.

If the part is still attached to the exhaust system, it’ll be more difficult to take the inlet and outlet diameters. Try checking your vehicle’s service manual or online forums for your vehicle’s make and model and the part you’re trying to measure.

Tools for Measuring Exhaust Diameters

There are a number of tools you can use to measure an exhaust pipe or component diameter. 

Use a digital caliper for smaller components like muffler inlets, so you can see readings down to the hundredth of an inch. For larger pipes, try using a tailor’s tape measure or a pi tape. A ruler will work in a pinch too.

Remember to jot down all the measurements you take and label everything correctly. It’s easy to mix the numbers up in the middle of all your measurements.

Exhaust Diameter Measurement Mistakes To Avoid

There are a couple of mistakes you should avoid when taking exhaust diameter measurements. 

First, never assume all the parts in your vehicle’s exhaust system are the same size. Factory exhausts will often taper and shift in diameter between one point and another.

Another mistake is measuring the outside diameter when the part requires the inside diameter and vice versa. Double-check what you need. If you want to be extra safe, take both measurements and label them accordingly.
Lastly, it’s tempting to measure once and move on, but double-checking is a must. Even small dents in a pipe can lead to the wrong exhaust component parts. Check your numbers at least twice.

How To Choose the Right Exhaust Pipe Size

Keep your engine size, performance goals, and intended use in mind when choosing your vehicle’s replacement exhaust pipe. 

Larger engines require larger exhaust pipes, but smaller engines will often benefit from smaller-diameter pipes. 

If you want to boost horsepower and improve performance, pick a larger exhaust pipe to reduce back pressure and increase the exhaust gas flow. Smaller pipes, on the other hand, can help you prioritize torque and low-end power.

Next, consider your intended use for your vehicle. If you use it for street driving and daily commutes, it might be best to stick to a moderate exhaust pipe size increase. If you’re going for a performance upgrade though, larger diameter pipes might be better.

If you want to be sure, consult a trusted professional and tell them what you want out of your vehicle’s exhaust system. They can steer you in the right direction.