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Air Filter Calculator – Calculate air filter size for BHP/Power

3 min read

Featured image of an air filter on a page that contains a calculator that gives the required air filter area for a certain horspower

This air filter calculator will tell you the approximate filter size you need for your engine’s power.

What we are doing

This calculator has two steps

  • Find out the filter area you need
  • Enter the dimensions of your current air filter to see if it is bigger or smaller than the area you need (it will also help you choose a filter of the correct size)

This air filter calculator needs some basic information about your engine

  • RPM of Peak Power
  • Redline RPM
  • Horsepower of the engine
  • Engine Capacity in CC

Now we know the air filter size the engine needs we can see if the filter you are using is big enough and if it isn’t, what size of air filter you need.

Cone Filter

If you have a cone filter, enter it’s dimensions here

Panel Filter

If you have a flat air filter eg you are using the standard air box, enter the filter dimensions here

About air filter sizing

You always want to use the biggest air filter possible.

The benefits of a bigger air filter are legion

  • More airflow
  • Longer service life ie does not need replacing so often.
  • Better filtration. The bigger the surface area, the slower the dirt hits the filter helping filtration

There are no negatives to fitting a huge air filter except maybe weight

If you are using the car’s standard airbox, there is nothing you can do about filter size but if you are choosing an aftermarket cone filter you can choose the biggest filter that will fit in your engine bay.

About this air filter calculator

This calculator takes into consideration the service life of the filter, when it gives the filter size.

When an air filter is new it flows more air than when the filter has been used for some months.

We want to be sure that even if the filter has been used (ie it has been slightly clogged) the area is still large enough to flow the air the engine needs.

It is less than ideal to have an airbox that is big enough for the air flow requirements but only when we are using a clean, brand new filter.

This air filter size calculator builds in some headroom to take into consideration the reduced flow capacity of an air filter that has been used for some time.

The numbers are approximate.

Different manufacturers will use different paper filter material, they will use pleats of different thicknesses and different manufacturers will use different pleat densities.

Ultimately, the figures this calculator puts out are approximate.

Why use this calculator?

You have increased the power of your engine and you are still using the car’s standard air box. Find out if the standard air filter is big enough.

You are looking for a performance cone filter for your engine and you want to know what size you need for your engine power.


To calculate the size of the filter required we have used data from The State of Louisiana, Department of Natural Resources. They have given a number of 2.5 cfm per square inch of filter for a HVAC application.

To calculate and engine’s volumetric efficiency, we have used this website

A fantastic air filter test was carried out by a Duramax enthusiast under lab conditions. You see read the full test results here.

This page was last modified Mar 24, 2021 @ 2:32 pm

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