Tool
Online ruler, actual size
A ruler on your screen in centimetres, millimetres and inches — made life size by matching a bank card, because a screen’s own idea of an inch is almost never an inch. Tap the ruler to read a length, turn it upright for the long side of a phone, and measure a ring’s size on the screen.
Not calibrated yet: this is your browser’s idea of size, not real size. Match a card below first.
Tap the ruler to place a marker.
Make it life size
Hold any bank, credit or ID card flat against the screen, its corner in the box’s top-left corner. Make the box bigger or smaller until its edges meet the card’s edges exactly.
Until you do, the ruler above is drawn at 96 pixels to the inch, the size browsers assume for every screen.
Keeps this screen's scale and the marks you chose, nothing else. Bookmark it on this device; on another screen, calibrate again.
Ring size on screen
Calibrate first, then lay a ring that fits flat on the circle. Step the size until the circle’s outer edge meets the inside of the ring all the way round. No ring? Print a ring sizer with a strip for the finger.
Inside diameter 17.2 mm (0.677 in), circumference 54.0 mm. The same ring is US 6.75, UK N, JP 14.
Not calibrated yet, so this circle is not true size.
Why an on-screen ruler needs calibrating
Web pages are laid out in CSS pixels, and the web’s rule is that 96 of them make an inch. Screens do not follow it: the number of pixels in a real inch depends on the screen’s size and resolution and on the scaling the operating system applies, and a web page has no way to ask the screen how big it is. So an “inch” in a browser is whatever 96 pixels happen to measure — about 85% of an inch on a common 15.6-inch laptop, more on some monitors, and different again on every phone. Anything of known size fixes that; a bank card is the one nearly everyone carries.
When to print a ruler instead
A screen is fine for small flat things — a screw, a button, a ring, a photo print. For anything longer than the screen, or anything that will not lie flat against glass, a printable ruler goes to 560 mm and carries its own check line. It has to be printed at 100%: how to print at actual size.
Frequently asked questions
Is an online ruler accurate?
Only once it is calibrated. Browsers draw a "CSS inch" as 96 pixels on every screen, so an uncalibrated on-screen ruler is almost never life size — on a 15.6-inch 1920 × 1080 laptop at 125% scaling it shows about 85% of real size. Matched to a bank card, this ruler is accurate to about a pixel, a quarter of a millimetre on most screens, which is as good as the marks can be read.
Why calibrate with a credit card?
Because every bank, credit, debit and ID card is made to one size, ISO/IEC 7810 ID-1: 85.60 × 53.98 mm (3.370 × 2.125 in), and nearly everyone has one to hand. Hold it against the screen in the box and adjust the box until its edges meet the card. Any card of that format works — a library card or a hotel key card usually does too, but check it against a bank card once.
I do not have a card. Can I use the screen size instead?
Yes: enter the screen’s diagonal in inches (the size it was sold as) and the ruler works out its scale from the screen’s pixel size. It is right at 100% browser zoom, but a laptop or monitor sold as "15.6-inch" is not always exactly that, so it can be off by a percent or two. A card is more accurate.
Why is the ruler wrong after I zoomed the page?
In Chrome, Edge and Firefox, browser zoom (Ctrl + or Ctrl −) is allowed for: the page reads the new zoom and redraws the ruler at the same real size. Safari does not tell a page its zoom, so in Safari keep the page at 100% (⌘0), or calibrate again at the zoom you use. Pinch-zoom on a phone or trackpad magnifies the page without changing it, so the page shows a warning and the ruler is not life size until you zoom back out. On another screen, calibrate again: every screen has its own scale.
How do I keep the calibration?
Use "Copy a link that keeps this calibration" and bookmark the link on this device. The link carries the scale and the marks you chose, nothing else, and nothing is stored by the site. It opens the ruler life size on this screen; on any other screen, calibrate again.
Can I find my ring size on screen?
Yes. Calibrate with a card, then lay a ring that fits flat on the circle below and step the size up or down until the circle’s outer edge meets the inside of the ring all the way round. It reads the size in US, UK, European and Japanese sizes. For a finger with no ring, the printable ring sizer has a strip to wrap round it.
What is the longest thing I can measure?
The screen’s width, less a margin — on a phone turned upright, choose Vertical for the longer side, and use Full screen where the browser offers it. For anything longer, a printed ruler goes to 560 mm, or measure in steps and add them up.
Measuring and scale tools
- Printable rulerLife size in mm or inches, or any drawing scale
- Scale ruler guide1:50, 1:100, 1/4" = 1'-0" and how to read them
- Scale converterDrawing to real size, or find a drawing’s scale
- Printable protractor180° or 360°, any size, with a check square
- Ring sizerRing size circles and a finger strip, card-checked
- Print scale calibrationThe exact percentage your printer prints at
- Printer test pageInks, fine detail and how close to the edge it prints