While Bootstrap uses ems or rems for defining most sizes, pxs are used for grid breakpoints and container widths. This is because the viewport width is in pixels and does not change with the font size.
                                    
See how aspects of the Bootstrap grid system work across multiple devices with a handy table.
| 
                                                    Extra small <576px  | 
                                                
                                                    Small ≥576px  | 
                                                
                                                    Medium ≥768px  | 
                                                
                                                    Large ≥992px  | 
                                                
                                                    Extra large ≥1200px  | 
                                            |
|---|---|---|---|---|---|
| Max container width | None (auto) | 540px | 720px | 960px | 1140px | 
| Class prefix | 
                                                    .col-
                                                 | 
                                                
                                                    .col-sm-
                                                 | 
                                                
                                                    .col-md-
                                                 | 
                                                
                                                    .col-lg-
                                                 | 
                                                
                                                    .col-xl-
                                                 | 
                                            
| # of columns | 12 | ||||
| Gutter width | 30px (15px on each side of a column) | ||||
| Nestable | Yes | ||||
| Column ordering | Yes | ||||
