A pixel is not the fixed unit it pretends to be
Set a heading to 32px and you might assume you've fixed its size for good. You haven't. A pixel in CSS is an absolute unit only in the sense that it never changes relative to anything else on the page, but it says nothing about what happens when a reader changes their own settings. Someone with low vision who bumps their browser's default text size from 16px to 24px will find that your 32px heading stays exactly 32px, because pixel values don't listen to that setting at all. That's the practical reason most style guides now push type, spacing and line-height towards relative units, and it's worth understanding before you reach for the CSS Unit Converter to translate a design file's pixel values into something a stylesheet can use properly.
None of this makes px useless. Border widths, box-shadow offsets and other details that should stay visually consistent regardless of a reader's font preferences are often better left in pixels, because you want a 1px hairline to stay a hairline. The judgement call is deciding which properties should scale with the reader's preferences and which should hold still, and that judgement is exactly what a converter can't make for you. It can tell you that 16px is 1rem; it can't tell you whether this particular value should be a rem in the first place.
Rem and em solve different problems, and mixing them up causes real bugs
Both units describe a size relative to something else rather than an absolute measurement, which is the whole point of using them, but they anchor to different things and that difference trips people up constantly.
Rem: relative to the root, wherever the root sits
A rem (root em) is always relative to the font-size set on the html element, full stop, regardless of how deeply nested the element using it happens to be. Most browsers default that root to 16px, so 1rem equals 16px until someone overrides it, either through their own browser settings or through a deliberate choice in your stylesheet. Some teams set the root to 62.5% specifically so that 1rem equals 10px, which makes mental maths easier when reading a design handoff in pixels, but it also means a converter needs to know your project's actual root value to give you a number that means anything. This is the main limitation worth flagging: a tool that assumes 16px will quietly mislead you on a project that doesn't.
Em: relative to the parent, which is why it compounds
An em is relative to the font-size of the element it's applied to, or its parent's font-size when used for something other than font-size itself, and that local relationship is both its strength and its most common source of confusion. Nest a few elements that each set their font-size in em and the sizes multiply through each level, so a button inside a card inside a sidebar can end up rendering at a size nobody typed anywhere. This compounding is a genuine feature when you want padding or spacing to scale with a component's own type size, which is why em still earns a place in well-built design systems, particularly for things like icon sizing or button padding that should track the text sitting next to them. It's a liability the moment you use it for global type scales, where rem's fixed anchor point behaves far more predictably.
Viewport units make layout follow the screen.
vw and vh describe a percentage of the browser's viewport, the visible area of the window. 1vw is one hundredth of the viewport's width and 1vh is one hundredth of its height, so a value of 100vh should, in principle, fill the screen exactly regardless of how much content sits inside it. That independence from the document's own layout is what makes viewport units useful for full-bleed hero sections, fluid type scales that grow with screen width, or a fixed-height panel that needs to match the window.
The catch, and it's a well-documented one, is mobile browser chrome. On many phones, the address bar and toolbar expand and contract as you scroll, which changes the actual visible viewport height without the page reloading, so an element set to 100vh can visibly jump or leave a gap at the bottom. Newer units, dvh, svh and lvh, were introduced specifically to address this by describing the dynamic, small, and large viewport respectively, and they're worth reaching for on any layout where that jump would be noticeable. A converter that only handles the classic vw/vh pair won't warn you about this behaviour, because the conversion maths is identical; the problem lives in how mobile browsers render the viewport.
What the converter can tell you, and what it can't
Feed the tool a pixel value and a base font-size and it will return the equivalent rem, em, or viewport figure correctly, because that part is fixed arithmetic. What it can't do is know your project's actual root font-size unless you tell it, whether your design uses the 62.5% convention, or whether the property you're converting is one that should be relative at all. Treat the output as a starting figure to check against your stylesheet's real root value.
There's a related accessibility point worth knowing if you're setting type sizes. The Web Content Accessibility Guidelines (WCAG), the standard most UK accessibility audits are measured against, include a success criterion requiring that text can be resized up to 200% without losing content or functionality. Pixel-based type sizes fight against that requirement because they resist the browser's zoom and font-size controls; rem-based type sizes work with it, because they scale in step with the root value the reader has set. That's a concrete reason to default to rem for type and reserve px for the details that genuinely need to stay fixed.
If your team is standardising these values across a project, it's usually worth storing the base sizes as CSS custom properties so the whole scale updates from one place. The CSS Variable Architect is built for exactly that step, and pairing it with the unit converter gives you a workflow: convert the individual value here, then bank it as a variable there so nobody has to redo the maths by hand on the next project.