Hi, It depends on which method are you willing to use, each one has different calculations.  There is the asphalt institute method which uses the following equations for fatigue and rutting receptively  :

N_f=0.0796*(ε_t )^(-3.291)*〖(E_1)〗^(-0.854)

N_f=1.37*〖10〗^(-9)*(ε_v )^(-4.477)  

Also the attached excel sheet is a useful tool if you are willing to use the AASHTO 1993 design method.

If you are following AASHTO there is an equation 

which described in a graph

this site is summarizing this method

http://www.pavementinteractive.org/article/1993-aashto-flexible-pavement-structural-design/  

there are many methods, like what Mr. Omar and Mr. Muath told, the common one is AASHTO 1993. another method is method that developed by ministry of transport in every country. maybe you can refer to the method from your country.

You can follow AASTHO or highway pavement manual by your country highway department . But the primary inputs are Traffic data, growth rate , CBR values, existing layer composition, modulus values of bituminous and granular layers.

As indicated before you can use AASHTO 1993 design guide, which is still adopted worldwide. You can find a summary on the following website: 

http://www.pavementinteractive.org/article/1993-aashto-flexible-pavement-structural-design/

At the bottom of the page, there is a calculator that may help you.

Good luck!

designing Flexible pavements have many methods, First of all, you have to know that there are pavement design equations (for both new and overlaid pavements), based mainly on data currently available(Profile Data, Rut-Depth Data and Layer Thicknesses).

As noted in the AASTHO Road Test Report 5,

the current PSI for a given section of flexible highway can be calculated from the average slope variance, average rut depth, and cracking and patching.

for more detailed information you can check the attached Flexible Pavement Design Manual, Also you can check the attached  AASHTO Flexible Design Procedure slides. 

Dear Ifeoma

You can use the following excel file to perform the design ...you can see the solved example

It is very useful DR Amjad. Thanks

Availability of Data dictates on appropriate method for Pavement Design

Please open this link and see book Pavement Analysis and Design by Huang. Open chapter 11 titled Flexible Pavement Design for further information. Use can use KENPAVE software for design too. There are various methods of design so you have to study them for choosing optimal one for your use. Hope this helps

https://drive.google.com/file/d/1w_ECSkTedIxt1NpaBHNRYkX_OvVJ2Igv/view