A printable ruler that shows you whether it printed at actual size

Print it at 100%, then check it

1. Choose the length, the scales and the paper above. The readout says how much ruler the sheet holds; ask for more and it is clamped with both numbers printed, never drawn short under a longer label.
2. Press Print this sheet. The sheet is redrawn from the controls at that moment, so what prints is what the readout says.
3. In the print dialog set Scale to 100% — not Fit to page, not Shrink to fit — and set Margins to None. Turn off any "print headers and footers" option, which steals a strip of the page.
4. Check the paper in the dialog is the paper named in the readout. This one matters more than it looks: see the paragraph below.
5. Print, then do the two checks on the sheet: a bank card must fill the outline, and the solid bar must measure 100 mm.
6. If either check is out, the print was scaled. Change the dialog and print again — do not adjust the ruler to compensate, because that moves every graduation by a factor nothing on the paper records.
7. Cut along the dashed line for a strip you can carry.

The check block is on every sheet rather than being an option, because a printed ruler nobody checked is probably a few per cent short and says nothing about it. That is the failure this site refuses to ship on a screen, and paper is not exempt from it.

Why the paper in the dialog has to match. This page asks the browser for a page of the size the sheet is drawn at, and browsers do not all treat that ask the same way — which was measured here rather than assumed. In one engine the request sets the page size outright and a Letter sheet prints on Letter. In another it sets only the orientation, and the paper comes from the print dialog: a sheet drawn for Letter and sent to a printer holding A4 came out scaled by the ratio between them, about 2.7% small, with the 100 mm bar measuring 97.4 mm and nothing on screen saying so. There is no way for a web page to read which paper is loaded, so this cannot be fixed from here — which is exactly why the bar and the outline are on the sheet, and why they catch this in one look.

Lengths people print, and what each one needs

Each of these sets the controls for you. The paper named beside it is the first sheet in the list that actually holds that length once the printer's edge allowance is taken off both sides — computed from the same figures the tool draws with, not typed in by hand.

• 10 cm ruler100.00 mm — fits US Letter portrait, 203.90 mm across
• 15 cm ruler150.00 mm — fits US Letter portrait, 203.90 mm across
• 20 cm ruler200.00 mm — fits US Letter portrait, 203.90 mm across
• 25 cm ruler250.00 mm — needs US Letter landscape, 267.40 mm across
• 30 cm ruler300.00 mm — needs US Legal landscape, 343.60 mm across
• 6 inch ruler152.40 mm — fits US Letter portrait, 203.90 mm across
• 8 inch ruler203.20 mm — fits US Letter portrait, 203.90 mm across
• 12 inch ruler304.80 mm — needs US Legal landscape, 343.60 mm across

Nothing here is tiled across two sheets. A ruler joined at a seam is only as true as the join, and the join has not been measured, so it is not offered — the tool clamps to what one sheet holds and prints the number it drew.

Why paper needs no calibration and a screen does

A browser is never told how large your display is. The CSS specification fixes an inch at 96 pixels by definition, so an on-screen ruler drawn in mm is drawn against an assumption rather than a measurement, and on a modern laptop or phone that assumption is typically 25–40% out. That is why the ruler on the front page asks you to match a bank card before it will print a millimetre figure, and why it states an error band once you have.

A sheet of paper is a physical object, and the SVG that becomes it is authored in millimetres: the drawing says 100 mm and the printer lays down 100 mm. No property of your screen is involved, so there is nothing to calibrate first and no error band to earn — the ink is as true as the printer's own placement.

What replaces the calibration problem is a settings problem, and it is a smaller one because you can see it. A print dialog set to fit the page shrinks everything by a few per cent; a printer driver set to a different paper size does the same. Both are visible the moment you lay a card on the outline. There is also one failure a screen ruler has that paper simply does not: a stored calibration cannot notice a swap to a different monitor of the same resolution, so the figure it prints stays confident while the glass underneath it changed. A printed strip has no state to go stale — it is the same object tomorrow as it was when it came out of the tray.

How the graduations are drawn

Every claim a ruler makes is a claim about where a line is, so the conventions are worth stating rather than assuming. The sheet is an SVG whose width, height and coordinate system are all in millimetres, which is why the ink does not depend on the resolution the printer happens to run at, and on top of that:

• A mark is centred on the position it names. A line straddles its own path, so a 0.20 mm line at 37 mm lays 0.10 mm of ink each side of 37.00 mm. The line weight is a control and is printed on the sheet, so the convention can be checked rather than taken on trust.
• The check bars are filled shapes, not lines. Their ink is exactly the length they claim — 100.00 mm, and 101.60 mm for the four-inch bar — with nothing straddling the ends to argue about.
• The card outline is sized on the inside. The rectangle is drawn 0.30 mm wider and taller than the card with a 0.30 mm line, so the clear space between the printed lines is 85.60 × 53.98 mm and "it must fill this outline" means what it says.
• Nothing is snapped to a grid. Positions carry the engine's own precision and are never rounded to whole units. A rounded coordinate is a different coordinate, and over a short reading that difference is the entire error budget.
• A level that cannot be drawn honestly is dropped and named. If a heavier line would leave less white between two marks than the line is itself wide, that level is not drawn, and the page says which level went and what the numbers were.

The graduation levels are the same ones the screen instruments use — centimetre, 5 mm, millimetre, half-millimetre; inch, half, quarter, eighth, sixteenth, thirty-second — so one vocabulary covers the whole site. The floor that decides whether a level survives is different, though, and deliberately so: a screen drops a level when the marks land closer than three device pixels apart, and paper has no pixel grid this page could know about. On paper the limit that bites is ink width, so that is what is tested.

What is measured, how it is kept, and what it cannot tell you

What is measured. Nothing on your machine is measured, and that is the point of this route. The sheet is a drawing whose coordinates are millimetres: the ruler's length, the position of every graduation, the 100 mm bar, the 85.60 × 53.98 mm card outline and the printer-edge allowance are all figures in the drawing, and the readout above prints the same figures the drawing used. The one number that comes from your browser is the paper size instruction, and the readout says which mechanism set it, or says that none did.

How it is kept. It is not kept. This page writes nothing to your browser's storage, reads no calibration from it, sets no cookie and sends nothing anywhere; the settings live in the address bar if you want to keep one, and in nothing else. The sheet leaves through the print dialog only, and the stamp along the bottom of it is composed at the moment you press print, from the same values that drew the ink — so a sheet cannot carry a stamp describing settings you changed after it was drawn.

What it cannot tell you. It cannot tell you whether your printer scaled the page — no web page can see that — which is why the check block is printed rather than described. It cannot tell you how far your paper has moved with humidity, which is real but small beside a scaled print. And it cannot measure anything on your screen: the preview above is a picture of the sheet reduced to fit the column, and a screen measurement needs the calibrated ruler instead.

Where to go next

If you would rather measure something against the screen than print anything, the calibrated ruler is the instrument for that, and it prints the error band it earns. The millimetre ruler and the inch ruler are the same instrument with the graduations this sheet prints, and the calibration page shows every reference object the site will match against, with the sources and dates behind each one.

Guides for this sheet

This page has no sibling sheet to sit beside, so what it points at is the reading behind it. The card outline printed on every sheet is the same object the first guide is about, and the second is why the sheet needs no calibration while the screen it was designed on does.

• Credit card size, and the other objects you can calibrate from
• Why an online ruler is wrong: a CSS inch is not an inch