I suggest using X-Powder software, it will help you with this matter

I'd agree with Dr. Liss. I would just add that if you're unsure which model is best I would find FWHM manually, i.e. literally look at what x-axis values I=Imax/2. After all, a good model would fit the data perfectly and give the same value, while a bad model would give a different and wrong value.

Origin software or Fullprof can be used to calculate FWHM. Hopefully you removed instrumental broadening using Si standard before taking the XRD. 

You can use Origin software to calculate the FWHM by using lorentian or gaussien functions.

In principle, it is the width of the x-ray peak at half maxima. So, naturally, you need to find out Imax first i.e. (maximum intensity value - background intensity). Then calculate 1/2 I max value. Add the background intensity. Find out the two positions on the peak with this intensity value. Measure the distance between these two points in 2 theta and then convert it to radian. That's the FWHM of the peak.

I guess u wana extract the crystallite/domain size and micro strain.

First u need to take the pattern from the standard sample (NIST SRM 660B, SRM 640d, CeO2, etc) to extract ur instrumental profile. The relative broadening of ur samples is the effect of the crystallite/domain size and micro strain.

There a software from Professor Matteo Leoni (PM2K) that directly model the physical meaning of the boroadening.

An easy way is to use an appropriate curve fitting software. 

Free version of Fityk may help you.

http://fityk.nieto.pl/

Download Orgin Pro 8 from www.originlab.com .Origin is a proprietary computer program for interactive scientific graphing and data analysis. It is produced by Origin-Lab Corporation, and runs on Microsoft Windows. Convert your XRD .raw file to .dat file .( for this conversion use link www.csrri.iit.edu/cgi-bin/reflex.cgi). open converted .dat file with orgin pro 8. select both columns, click below any  options gives xrd-gram , select required peak >analysis>fitting>fit single peak> use Gauss function to fit, you will get required results in a window. 

  Model orgin plot and result window attached 

I have a question about FWHM in Fityk software. The value of FWHM is in degrees or radians

FWHM is easy to determine using Origin software. You can refer the link below:

https://www.youtube.com/watch?v=2Nv7bJjm2Yk

Dear Ahmed Mounir, value of FWHM is in Degree and you should convert it into Radian

You can determine the FWHM by using the software tool as origin but during the XRD from the raw data file you can automatically achieve the FWHM from the instrument by which you did XRD. Some times this is hidden feature during the extracting the data from raw file.So in the instrument setting select the field of FWHM and directly you can get the FWHM for the corresponding peaks. 

You can look this link, pleace. in this video you have a good explication about this.

https://youtu.be/8r1W4kmIF84

Ahmed Mounir ,

value of FWHM is in Degree and you have to convert it into Radian.

"value of FWHM is in Degree and you have to convert it into Radian", in most formulae using Bragg Peak Broadening. In most such application one would need to convert the FWHM measured into INTEGRATED Breadth by assuming a shape for the profile. However, if the Bragg peak is well resolved then it may be easier to compute the integrated breadth (=integrated intensity/peak value) from the numerical relative intensity data after identifying the "shoulder" and "tail" of the Bragg profile correctly.