Free tool
Online Ruler: Set It to True Size With a Bank Card
A screen has no built-in idea of a real centimetre, so an honest on-screen ruler needs one quick setup step. Match the box on the page to a bank card, or type your screen's diagonal, and the ruler is then true to size.
Marker:
Set it up: match a bank card
Or type your screen size
Printable ruler
Bank cards share one size: 85.60 x 53.98 mm (ID-1). The setting is for this screen and browser zoom.
Your setting is kept in this browser so the ruler stays true next time. Nothing is sent anywhere, and the PDF is built on your device.
Why a screen ruler needs setting up
People open a ruler to measure a small object, such as a ring, a screw, a label or a product they plan to ship. The browser, though, only knows pixels. It assumes a fixed number of them per inch, and real screens rarely match that figure. Until you set the page up, the ruler uses the browser's guess and says plainly that lengths are only a rough idea.
The setup uses something you already own with a known size. A bank card has the same dimensions everywhere, and the page draws a box of 85.60 by 53.98 mm. Hold a card to the screen and drag the slider, or press the minus and plus buttons for half-step nudges, until the box and the card are the same size. From then on, a centimetre on the ruler is a centimetre in the real world.
If you know your screen diagonal, there is a shortcut. Type it in inches, such as 13.6, and press Use this size. The page works out the density from your screen's pixel count.
Taking a measurement
- Pick Centimetres or Inches. The centimetre scale shows millimetre ticks and labelled whole centimetres. The inch scale divides each inch into sixteenths.
- Press Turn upright if you want to hold the object vertically against the screen. The button then changes to Lay it flat.
- Drag the purple marker to the end of your object, or click the ruler to place it. The reading appears above in both a decimal form and a small unit, with inches also shown as a fraction such as 3 and 5/8.
- For fine control, focus the ruler and use the arrow keys for one millimetre, or one sixteenth of an inch. Hold Shift for a whole centimetre or inch, and press Home to go back to zero.
The line under the ruler tells you the state of the setup: not set up yet, set up for this screen, or set up on a different screen.
What keeps it accurate, and what breaks it
| Situation | What happens |
|---|---|
| You zoom the page in or out | The ruler stays true, because the setting is stored in device pixels |
| You open the page on another screen | The page notices the different screen and asks you to match the card again |
| You press Forget the setting | The ruler goes back to the browser's guess and says it is not set up |
| The object is longer than the screen | Measure it in parts, or use the printable ruler instead |
A screen ruler also has a natural limit. Your eye sits at an angle to the glass, and a thick protective layer can add a small gap, so read objects from straight on. For anything that matters, such as a purchase of a custom part, check with a physical ruler as well.
The printable ruler
Choose A4 or US Letter and press Download a printable ruler. You get a one-page landscape PDF with a centimetre ruler, an inch ruler and a square that should measure 5 cm on each side. The file is drawn in exact points, so it prints true if you print at 100 percent, which some printers call Actual size. Never choose Fit to page. After printing, lay a known ruler on the square to confirm.
Cut along the outer lines and you have a cheap ruler for a drawer or a travel bag. It works well for checking the print size of an image you plan to use in a brochure. If you are sizing images for the web instead, our website optimization page explains how we handle image weight and display sizes.
Frequently asked questions
Why does the page ask me for a bank card?
Cards share one standard size, so they are a reliable reference. The tool draws a box of 85.60 by 53.98 mm and you resize it until it matches the card in your hand.
Is the ruler accurate without setup?
Not reliably. Before you set it up, it uses the browser's assumed pixel density and says lengths are only a guess.
Does it stay accurate if I zoom the browser?
Yes. The setting is stored in screen pixels, so zooming in or out keeps lengths true.
How do I print a ruler at the right size?
Download the PDF, then print at 100 percent or Actual size and avoid Fit to page. The 5 cm square on the sheet is there so you can confirm the scale.
Can I measure in inches?
Yes. Switch Units to Inches and the scale shows sixteenths, with the marker reading given as a decimal and as a fraction.
Does the tool save my setting?
It keeps it in this browser so the ruler is ready next time. Use Forget the setting to remove it, and nothing leaves your device.
More free tools
- PPI calculator: Work out pixels per inch, dot pitch and physical size from a screen's resolution and diagonal.
- screen resolution checker: See your screen size in real pixels, the pixel ratio, window and page area, and an aspect ratio.
- Aspect ratio calculator: Find a ratio, resize without distortion, and work out crops or bars for a new shape.
- image resizer: Resize pictures by pixels or percent with fit, crop or stretch, presets and file type choices.
- Change image DPI: Write a new DPI into JPG and PNG files, or resample a picture to a print size.