Free CSS unit converter. Convert px to rem, rem to px, pt to px, and any CSS length unit including em, in, cm, mm, vw, vh and percent. Configurable root font size and viewport. Runs in your browser.
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Enter a value and pick units.
How css unit converter works
CSS length units fall into three families. Absolute units (px, pt, pc, in, cm, mm) map to a fixed number of pixels using the CSS reference pixel where 1 inch equals 96 pixels. Font-relative units (rem, em) are multiples of the root font size, which defaults to 16px but is configurable here. Viewport units (vw, vh) are a percentage of the viewport width or height.
The converter first converts your input value to pixels using its source unit, then converts those pixels to the target unit. For example, 16px becomes 1rem when the root font size is 16px, and 12pt becomes 16px because 12 times 96 over 72 equals 16. Percent is handled relative to a configurable base.
Because rem, em, vw, vh, and percent depend on a context (root font size, viewport dimensions, or percent base), the converter exposes those as inputs you can change. The defaults of a 16px root font and a 1920x1080 viewport match the most common web design assumptions.
Frequently asked questions
What is the difference between rem and em?
rem is relative to the root element's font size, so 1rem is the same everywhere on the page. em is relative to the current element's font size, which can change as you nest. This converter treats em like rem against the root font size, which is the most common conversion people need (px to em at the root).
Why does 1 inch equal 96 pixels?
CSS defines the reference pixel as 1/96th of an inch, so 1in = 96px regardless of the physical screen. This keeps layouts consistent across devices with different pixel densities.
How are vw and vh computed?
vw is 1% of the viewport width and vh is 1% of the viewport height. The converter uses your viewport width and height inputs, so 50vw at a 1920px viewport equals 960px.
What is the percent base?
Percent in CSS is relative to a parent value that depends on the property (width, font-size, padding). The converter uses a single configurable base, defaulting to the root font size, so you can convert things like 150% font-size to px.
