Bootstrap’s font sizes are calculated off of the body font size by using rem values. If you change the body font size all styles will be increased/decreased automatically. Rem stands for “root em” because it calculates the size based on the size of the root of the document or body tag.
So Bootstrap’s global default font-size is 16px.
1rem = 16px
Default styles applied to the body of the document
Body | Styles |
---|---|
body | font-size: 16px;
line-height: 1.5; font-family: “Helvetica Neue”, Helvetica, Arial, sans-serif; |
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Bootstrap 4 Text Sizes
Tag / Class | Font Size |
---|---|
p | 1rem / 16px |
h1 | 2.5rem / 40px |
h1 small | 80% / 32px |
h2 | 2rem / 32px |
h2 small | 80% / 25.6px |
h3 | 1.75rem / 28px |
h3 small | 80% / 22.4px |
h4 | 1.5rem / 24px |
h4 small | 80% / 24px |
h5 | 1.25rem / 20px |
h5 small | 80% / 16px |
h6 | 1rem / 16px |
h6 small | 80% / 12.8px |
.display-1 | 6rem / 90px |
.display-2 | 5.5rem / 82.5px |
.display-3 | 4.5rem / 67.5px |
.display-4 | 3.5rem / 52.5px |
What is the default size of h1 Bootstrap heading?
2.5rem / 40px
What is the default size of h2 Bootstrap heading?
2rem / 32px
What is the default size of h3 Bootstrap heading?
1.75rem / 28px
What is the default size of h4 Bootstrap heading?
1.5rem / 24px
What is the default size of h5 Bootstrap heading?
1.25rem / 20px
How to Increase the Default Bootstrap Font Size
The default font-size for Bootstrap is 1rem. This converted to the pixel equivalent is dependent on the default font-size of the browser. The majority of browsers today use 16px, but this can vary for older browsers.
To change the default font size, add this CSS rule to your theme stylesheet after the Bootstrap stylesheet.
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html { /* default font-size is 16px - this is set in the default browser stylesheet */ font-size: 20px; font-weight: 400; line-height: 1.5; } /* Increase all font sizes on mobile */ @media (max-width: 767px) { html { /* default is 1rem or 16px */ font-size: 25px; } }
How to Change the Bootstrap Font Family for All Headlines
Add a google font stylesheet below the Bootstrap CSS stylesheet.
https://fonts.googleapis.com/css?family=Oswald<link href="
" rel=“stylesheet”>
Add this to your theme stylesheet to override Bootstrap’s default values.
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h1, h2, h3, h4, h5, h6, .h1, .h2, .h3, .h4, .h5, .h6 { margin-bottom: 0.5rem; font-family: ‘Oswald’, sans-serif; font-weight: 500; line-height: 1.2; color: inherit; }
How to make Bootstrap 4 Font Sizes Responsive
In Bootstrap version 4.3 they introduced responsive font sizes. This featured is turned off by default but can be enabled when you compile your own version of the Sass source files. You can learn more about this feature in the GitHub repo documentation.
Summary
So as you can see, changing the document font size and font family is pretty easy because Bootstrap uses rems for typography font sizes. If you change the default body font size of 16px all headings and paragraphs will adjust off of that size. If you need more variation, you will need to write new CSS classes to specify new rem values to override what Bootstrap is using. A good way to do this is to look at the typography styles and write your overrides in a separate document.