Check out example codes for "responsive font sizing css". It will help you in understanding the concepts better.

Code Example 1

/* Uses vh and vm with calc */
@media screen and (min-width: 25em){
  html { font-size: calc( 16px + (24 - 16) * (100vw - 400px) / (800 - 400) ); }
}

/* Safari <8 and IE <11 */
@media screen and (min-width: 25em){
  html { font-size: calc( 16px + (24 - 16) * (100vw - 400px) / (800 - 400) ); }
}

@media screen and (min-width: 50em){
html { font-size: calc( 16px + (24 - 16) * (100vw - 400px) / (800 - 400) ); }
}

Code Example 2

html { font-size: calc(1em + 1vw); }

Learn ReactJs, React Native from akashmittal.com