How to change the font size with Sass in Bootstrap 4?

With all the confusion and so many different answers. I approached the authors of bootstrap with the question to clear it up once and for all.

It is now crystal that we will need to change $font-size-base which will directly change the root font-size.

I was also advised by their contributor to not control the font-size with html element rather change the bootstrap's variable instead.

REF: https://github.com/twbs/bootstrap/pull/24060

Caution about using just CSS override

using the supplied css example will not work for all aspects of bootstrap sizing.

The compiled result of scss uses the $font-size-base variable to size many things and may be used in more areas in the future.

List of sized elements

• dropdown font
• button font
• popover font
• input-group font
• forms font
• reboot font

-------------- Example SCSS --------------

This is an example for your scss files, note that you need to define the $font-size-base BEFORE including bootstrap in your main scss file. In this case, the app.scss file is the one being compiled.

app.scss

// Fonts
@import url("https://fonts.googleapis.com/css?family=Raleway:300,400,600");

// Variables -- this is where you defined the variables 
//              BEFORE importing bootstrap which will use it's  
//              defaults if the variable is not predefined.
@import "variables";
// Bootstrap
@import '~bootstrap/scss/bootstrap';
// DataTables https://datatables.net/download/npm#DataTables-core
// @import "datatables";
@import "datatables.min";

_variables.scss

// Body
$body-bg: #ffffff;

// Typography
$font-size-base: 12px; // this changes the font-size throughout bootstrap
$font-family-sans-serif: "Raleway", sans-serif;
$font-size-base: 0.9rem;
$line-height-base: 1.6;

I'd suggest you to update directly the html {font-size: 16px} property instead of changing Bootstrap's $font-size-base, for several reasons:

1. html's font size will be directly used in calculating final font size. There's no benefit to use an "intermediary", that is, body that is being adjusted by Bootstrap's $font-size-base.

2. Bootstrap's SCSS system calculate heading size etc using $font-size-base, so all Bootstrap affected CSS rule will be affected, which includes body (set into $font-size-base$).
   However, browser calculate any CSS font size rule in rem relative to html {font-size}, not from $font-size-base affected elements (like body).

3. Setting html {font-size} using px will affect any rem value, both inside and outside Bootstrap affected elements, if this is what you want to achieve.

Conversely, if you only want to set sizes for Bootstrap-affected-elements, do set the $font-size-base relative to html {font-size}, so CSS elements outside of Bootstrap will not be affected. But I'd say this is more of an edge case, instead of the norm.

Note:
If you're setting $font-size-base or any other Bootstrap variables, you don't have to modify variables.scss file. You should define variables before importing node_modules/bootstrap/scss/variables and other Bootstrap files, so your values got used instead of !default values set in Bootstrap. The advantages: no need to edit the whole variables.scss file.

Bootstrap defines base font-size in it's _reboot.scss as

body {
    font-family: -apple-system,system-ui,BlinkMacSystemFont,"Segoe UI",Roboto,"Helvetica Neue",Arial,sans-serif;
    font-size: 1rem;
    font-weight: 400;
    line-height: 1.5;
    color: #292b2c;
    background-color: #fff;
}

NOTE:- Never change default values into frameworks files always modify values using custom css/js files.

so in order to change your font-size to 14px use this in your own style.css(custom css file)

body {
    font-size: 0.875rem;
 }

Here is the working fiddle

use !important you are not able to see your changes to override your change on default.

In your custom.scss file do like this:

$custom-variable:0.875rem;

Then use it like this:

@import "./src/scss/_modules/custom"; 
@import "./bower_components/bootstrap/scss/bootstrap";

body {
font-size:$custom-variable;
}