Free CSS Unit Converter calculator with clear step-by-step results.
Every CSS length resolves to a pixel value at render time, so conversion goes through px as the pivot. rem depends on the root font size, em on the parent's computed size, pt on the fixed 96-per-inch CSS reference, and vw on the viewport - which is why the same declaration can mean four different things.
rem to px
px = rem x root font size
px to pt
pt = px x 72 / 96
rem for font sizes, because it stays predictable regardless of nesting. em is useful for spacing that should track the element's own text size, such as margin below a heading.
In CSS, yes - the specification fixes 1 inch at 96 px and 1 pt at 1/72 inch. That is a reference unit, not necessarily the physical density of the display.