
Air Filter Flow Calculations 
Chuck Leeper with permission from K&N

Introduction 
Below is a series of formulas to determine air filter size for a given air flow capacity. Chuck was trouble shooting a problem when he found that air flow he needed was more than his 9" filter could provide. K&N shared this information with him to help trouble shoot his problem. 
Formulas 
 Square inches of filter required for a motor = CID x Maximum RPM / 20839
 CFM required for a 4 stroke motor = CID x Maximum RPM / 3456
 CFM required for a 2 stroke motor = CID x Maximum RPM / 1728
 CFM required for a turbocharged or supercharged motor = Pounds of Boost / 14.7
Add 1.0 to this answer. This relates to #2 and #3
Example: (CID x maximum RPM x #4 ) / 3456 or 1728
 To find circumference of a circle multiply diameter by 3.1416
 To find diameter of a circle multiply circumference by .31831
CFM Formulas for Filters
 Round: Diameter x Height x 6 x 3.14
 Flat Panel: Length x Width x 6
 Universal Round Taper: (Base + Top) / 2 = A
Take A x Length x 6 x 3.14
 Universal Round Straight: Diameter x Length x 6 x 3.14
