How to Calculate Exhaust Size

Written by colette mccormick
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How to Calculate Exhaust Size
Calculating exhaust size need not be as daunting as it first appears. (Auto Exhaust image by Digital Photique from

When you see the formula involved in doing it, calculating the size of exhaust that you need may seem daunting for anyone but a math wiz. However, as with a lot of difficult things, there is an easy way round it. There are some facts you will need to know, like the rpm of your engine and its displacement. You should be able to find both of these in your owner's manual.

Skill level:


  1. 1

    Use a mathematical formula to work out the exhaust size that you will need. The formula is as follows.

    Multiply the rpm by the engine displacement and divide by two to get the intake volume. Use this figure to work out the rest. Assume that a pipe allows a flow of 115 cubic feet per minute (cfm) per square inch of area and that your exhaust needs to flow at 2.2 cfm per unit of horsepower to work out the size.

  2. 2

    Use an online exhaust pipe calculator to work it out. You will still need to know the rpm, displacement and horsepower of your engine, but once you input those figures, the rest is done for you.

  3. 3

    Read as many articles as you can either online or in magazines that advise on the best size/length of exhaust that is suitable for your vehicle. Use what you read to work out which exhaust you need.

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