How to print an image at its actual size
Printing something at an exact physical size sounds like it should be the default. It is not — and the reason is a single setting that almost every print dialog turns on for you.
Why your prints come out the wrong size
An image file has no physical size. It has pixels, and it may carry a DPI number in its metadata, but nothing in the file says "this is 8 centimetres wide". The physical size is decided at print time, and if you do not decide it, your printer will.
What it decides is almost always "Fit to page" — scale the image up or down until it fills the printable area of the sheet. That is a sensible default for a holiday photo and a destructive one for anything measured. Worse, most dialogs apply an extra reduction to keep clear of the printer's unprintable margin, so even an image that was already the right size comes out slightly small.
Getting it right in three steps
The Real size tool exists to take the guesswork out of the first two, and to make the third the only thing you have to remember.
- Add your image and type the size you want — width or height in centimetres, inches or millimetres. The other dimension follows automatically so nothing is distorted.
- Pick your paper (A4 through A0, Letter, Legal or Tabloid) and your margins. The tool arranges the images to use as few sheets as possible and tells you how many you will need.
- Download the PDF and print it with scaling set to 100% — labelled "Actual size", "Original size" or "None" depending on your dialog. Not "Fit to page", and not "Shrink to fit".
Check it with a ruler once
Print a single test sheet and measure the result. If you asked for 4 cm and got 4 cm, your printer and its settings are trustworthy and you can print the rest without thinking about it again.
If it comes out consistently small — 96% or so is the classic symptom — your dialog is still applying a fit-to-page reduction somewhere. Turning on cut lines in the tool helps here: they give you a printed edge to measure to, rather than guessing where a pale image actually ends.
How much resolution you need
Physical size and resolution are linked by DPI: dots per inch. The pixels you need are the printed size in inches multiplied by the DPI you want. For something held in the hand, 300 DPI is the traditional target and 150 DPI is usually acceptable; below about 100 DPI the softness becomes visible at reading distance.
A worked example: to print 10 cm wide at 300 DPI you need roughly 1180 pixels of width, because 10 cm is 3.94 inches and 3.94 × 300 ≈ 1180. If your file is smaller than that, you have two honest options — print it smaller, or upscale it first.
What people use this for
Stickers and labels that have to fit a specific container. Photos at standard sizes without a lab. Sewing and craft patterns, which are only usable at 1:1. Woodworking and model templates traced directly off the sheet. Cards, tags and packaging mock-ups where the cut size is the specification.
The common thread is that the measurement is the point. Anywhere a ruler comes out afterwards, this is the tool.