Is your printer scaling your pages?
Almost certainly yes, at least some of the time — and it is why printable graph paper so often comes out the wrong size. This page tells you whether it is happening and by how much.
Opens the generator; use the "Printer calibration page" button to download it.
Why this happens
Every printer has a small unprintable border around the edge of the sheet. To avoid clipping anything, print dialogs default to options like "Fit to printable area" or "Shrink oversized pages", which quietly reduce the whole page by two or three per cent. You never notice on a letter or a photo. On graph paper it means your 5 mm squares print at 4.85 mm, and any measurement you take off the sheet is wrong.
How to use the page
Print it with Scale: 100% selected. Then take a real ruler and measure the 100 mm line, the 6 inch line and the 50 mm square. If they measure what they claim, your printer is faithful and you can trust everything else on this site. If they come up short, the page has been scaled.
Working out your correction
Divide 100 by what you actually measured, then multiply by 100 — that is the scale percentage to reprint at. Measure 96 mm and the sum is 100 ÷ 96 = 1.042, so reprint at 104%. Measure 97 mm and you need 103%. It is worth writing the number on a sticky note and leaving it on the printer, because the same printer will do the same thing every time.
When it matters most
For dot grid you are journalling on, a two per cent error is invisible and irrelevant. It matters a great deal for perfboard layouts, where components physically have to line up with printed pads; for knitting and cross-stitch charts used as 1:1 templates; and for any scale drawing you intend to measure with a ruler.