Text that looks perfect on screen can print far too big or far too small, and the reason is that screens and printers measure type in different units. Web design uses pixels, print design uses points, and most people think in millimetres or inches when they hold a printed page. A 16-pixel heading on your laptop is not 16 of anything on paper, and guessing the conversion is how posters, labels, and invitations come out wrong. This converter translates one size into all four units, so the size you see is the size you print.
The conversions rest on two fixed definitions. One inch equals 96 CSS pixels on a standard screen, and one inch equals 72 points in print. That means one point equals 96 divided by 72, or 1.333 pixels, and one pixel equals 0.75 points. Millimetres follow from the inch: one inch is exactly 25.4 millimetres, so one pixel is 25.4 divided by 96, or about 0.265 millimetres. These are definitions, not measurements, so the conversions are exact rather than approximate, as long as you use the standard 96-pixel inch.
Here is a worked example. Enter 16 pixels and the tool returns 12 points, about 4.23 millimetres, and about 0.167 inches. That tells you that ordinary web body text, usually 16 pixels, prints at 12 points, which is a normal book size and easy to read on paper. Enter 72 points instead, the classic one-inch headline, and you get 96 pixels, 25.4 millimetres, and exactly 1 inch. Designers who know these two anchor points, 16 pixels equals 12 points, and 72 points equals 1 inch, can sanity-check any conversion in their head.
Points are the right unit for print because they are physical. A 12-point letter is 12 seventy-seconds of an inch tall, on any printer, in any country, on any paper. Pixels are not physical: a CSS pixel is a reference unit that screens scale to their own density, so 16 CSS pixels can be 16 physical pixels on an old monitor, 32 on a retina laptop, or 48 on a high-density phone. If you design a label in pixels and send it to a printer, the printer must guess what you meant, and different printers guess differently.
Common print sizes are worth memorising. Body text in books is usually 10 to 12 points, business cards use 7 to 9 points for small details and 12 to 16 for names, posters use 48 points and up for headlines, and anything below 6 points is hard to read for most people. If a client asks for text that is 5 millimetres tall, that is about 14 points, and if they ask for 3 millimetres, that is about 8.5 points, which is small but readable for a footnote. Converting first, before you design, stops you discovering the size problem at the printer.
Screen design has its own trap: relative units. Web pages often set text in em or rem units, which scale with the base font size, so a heading of 2 rem is twice the base size, whatever the base is. That is good for accessibility, because readers can enlarge the base size, but it means the pixel value changes with the settings. When you need a print size from a web page, find the computed pixel size in the browser developer tools first, then convert it here, rather than converting the em value directly.
Use this converter whenever text crosses from screen to paper: invitations, labels, posters, worksheets, or a document that must match a printed form. Enter the size you have, in the unit you have, and read off the size you need, in the unit the printer needs. Type sizes are small numbers with big consequences for readability, and getting them right before printing saves paper, ink, and the embarrassment of a poster nobody can read from two metres away.