PrintProtractor
Guide

Protractor PDF versus a printout

If you search for a protractor you will find PDFs, PNG images, and on-screen tools. They are not equally trustworthy when the size has to be exact. A PDF carries real physical dimensions, so it can print to scale; a web image is just pixels and will print at whatever size the page happens to choose. This guide explains the difference and what to verify before you measure with any of them.

A protractor on a PDF sheet that carries real millimetres and prints to exact scale, beside the same protractor as a PNG image that is just pixels and prints at the wrong size.

A PDF knows its real size. An image does not.

The difference comes down to what each format stores. A PDF measures everything in points, where one point is exactly one seventy-second of an inch. So a PDF page can say, with certainty, that it is 150 millimetres across, and a viewer prints it that way.

In PDF, the default user-space unit is one point, which is 1/72 of an inch. That fixed relationship to a physical inch is what lets a PDF print to a known size.

Source: Datalogics, PDF size in inches (ISO 32000): https://kb.datalogics.com/article/how-large-can-a-pdf-document-be-in-inches-101.html

A PNG or a JPG is just a grid of pixels. A pixel has no fixed physical size, so when you print an image the app has to guess how many pixels make an inch. Change the guess and the printed protractor changes size. That is why a saved web image of a protractor so often prints a little too big or too small.

SVG can be exact too

An SVG sits between the two. It is made of shapes, not pixels, and it can declare its size in real units. An SVG that says it is 150 millimetres wide prints at 150 millimetres, just like a PDF. The catch is that it has to declare those units; the web standard for lengths spells out the fixed relationship that makes this work.

The CSS and SVG length standard fixes the units precisely: one inch is 2.54 centimetres is 96 pixels, and one point is 1/72 of an inch. A file authored in real units therefore prints to a real size.

Source: W3C, CSS Values and Units: https://www.w3.org/TR/css-values-3/

What to check before you trust any of them

Whatever the format, the safe move is the same: print at 100 percent and measure a known feature. A PDF or a real-unit SVG should land exactly. An image might, if it was saved at the right resolution, but you will not know until you measure. A built-in calibration bar removes the guesswork.

The protractor tool gives you both a PDF and a real-unit SVG, each with a 100 millimetre calibration bar, so the file is exact and you can prove it. The print step itself is covered in how to print a protractor to scale.

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Frequently asked questions

Why is a PDF better than a PNG for a protractor?

A PDF stores real physical dimensions in points, where a point is one seventy-second of an inch, so the page knows it is, say, 150 millimetres across. A PNG is just a grid of pixels with no fixed real-world size, so it prints at whatever size the page or app decides.

Does an SVG print to scale?

Yes, if it declares its width and height in real units like millimetres. An SVG that says it is 150 mm wide will print at 150 mm at 100 percent, the same as a PDF. PrintProtractor exports SVGs with real millimetre dimensions for this reason.

I have a protractor image. Can I still use it?

You can, but you must verify it. Print at 100 percent, then measure a known feature against a ruler. Without a calibration mark you are trusting that the image happened to be saved at the right resolution, which it often is not.