title: Get real size of someone's face
body: <p>I am developing an iOS app where I want to get the size of user's real face so that I can suggest him/her suitable (matched) sized glasses.</p>
  
  <p>I have detected user's face using <strong>OpenCV</strong> and got various dimensions of <em>eyes, nose, face,</em> etc. </p>
  
  <p>But I want the real size from that dimensions (i.e in <strong>millimetres</strong> that I am getting in pixels).</p>
  
  <p>I have searched a lot but could not get any solution matching with my requirement.</p>
  
  <p>Has anyone idea how to calculate real size (i.e in millimetres) of the someone's face?</p>
  
  <p>Thank you.</p>