The "root" document font-size is 16px

### 160px square

Fixed size

### 10rem square

10 x root font size
= 160px by default (10rem x 16px)

### 10em square

10 x this square's font size so not reliably 160px

The most common default font size is 16px

10rem is ten times the 16px root font-size, giving 160px:

`10rem x 16px = 160px`

So to figure out the rem equivalent to 160px do the calculation in reverse:

`160px / 16px = 10rem`

To get the rem equivalent for any px value just
divide it by the root font size for example:

`32px / 16px = 2rem`
`50px / 16px = 3.125rem`
`1px / 16px = 0.0625rem`