Air Filter Calculator – Calculate air filter size for BHP/Power
This air filter calculator will tell you the approximate filter size you need for your engine’s power.
How It Works
1 – Find the filter volume requirements for the current power level by entering your engine specs in the first calculator below.
2 – Enter the size of the filters you are interested in and see if they give you the area you need from the previous step. Using either the “Cone Filter” calculator and/or “Panel Filter” calculator below.
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, enter the dimensions of the filter you have and/or are interested in to see if the filter is big enough.
A Word About Foam Filters
These calculators assume we are using pleated filters, like we see in the stock application. Foam filters are generally not pleated, so my understanding their surface area will be considerably less. This does not necessarily mean a restriction and it does not necessarily mean foam filters are more porous to enable the same flow rate with less area. But it may do. In short, the results of these calculators may or may not be in the ball park for foam air filters.
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
Examples
How do the numbers from the calculator compare to the actual filter sizes as supplied by the car manufacturer from the factory?
| Car | Engine CC | RPM (Peak Power) | BHP | Actual Filter Size cm2 | Dimensions (mm) | Stock Filter Volume cm3 | Calculated Size cm2 | Calculated Volume cm3 @ 4cm Thick Filter |
| 911 Turbo 3.8 2016 | 3800 | 6400 | 533 | 931 | 613x152x50 | 4658 | 1517 | 6068 |
| Audi S3 2018 | 2000 | 5500 | 310 | 516 | 292x177x70 | 3617 | 1026 | 4104 |
| Mini JCW 2011 | 1600 | 6000 | 208 | 516 | 356x145x46 | 2374 | 607 | 2428 |
How Accurate?
For the Audi and the Porsche the calculated required filter area size is around double the actual filter size. But the Mini filter calculation is close.
However if we look at filter volume, we can see the S3 and Mini filter predicted optimum size is around the size of the stock filter. The Porsche predicted filter size is around 30% larger than the actual filter.
Factory Tolerances
From the three examples above it seems that the tolerances between manufacturers can vary by a lot. Audi/Mini fit what is expected, the Porsche seems undersized.
The size of the factory filter could even come down to filter placement and the ducting. If the factory airbox receives a lot of dirty air, then the filter size maybe larger to cope with the increased dirt it will be exposed to. For a filter with average or restrictive ducting the filter will be exposed to less dust so can be smaller. Perhaps the engine bay itself is a limiting factor.
Peak VE – Peak RPM
Another reason for a difference between the calculated required filter size for a stock engine and the actual filter size is that the formula is assuming we are getting peak VE (Volumetric Efficiency) at redline.
We do this to build in some headroom for modified engines.
If you do not want the calculator to do this, set the RPM of peak power and the redline RPM as the same.
Air Filter Sizing – In General
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.
Sources
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.
