Free tool
Online ruler, actual size
This ruler shows real centimetres and inches on your screen once you set it with a card. Hold a standard bank card against the outline below and drag the slider until the outline is exactly as wide as the card. Websites can’t tell how big your screen is, so this one-time step is what makes the ruler true to size.
Set the ruler with a card
- Hold a bank card upright and flat against your screen, its left edge on the outline’s left edge.
- Drag the slider until the outline’s right edge lines up with the card’s right edge. The outline’s top and bottom don’t matter.
- Tap Save. The ruler above is now set for this screen.
53.98 mm
Why it needs a card
Web pages draw in “CSS pixels”, and the web standard ties one CSS inch to 96 of them. It also warns that on a screen, these units might not match their physical measurements (W3C). A phone, a laptop and a monitor all draw a CSS inch at different real sizes, and a page has no way to ask how big the glass is.
A card fixes that. Cards in the ID-1 size, which most wallet cards use, measure 85.60 mm by 53.98 mm (ICAO). Once you match the outline to one, the page knows how many pixels make a real millimetre on your screen, and saves it in your browser on this device only.
Tips for a good reading
- Set it at the zoom level you’ll use. Zooming the page changes the size of everything.
- Lay small objects flat on the glass and look straight down, not at an angle.
- One inch is exactly 2.54 cm (NIST); the two scales use that.
Questions
Is an online ruler actual size?
Only once it has been set for your screen. Web pages can’t tell how big a screen physically is, and the web’s own standard says on-screen inches and centimetres might not match real ones (W3C). Setting this ruler with a card fixes that for your screen.
What card can I use?
Any card made in the ID-1 size, which is 85.60 mm × 53.98 mm (ICAO). Most wallet cards are this size, but if you have a real ruler, check the one you use.
Do I need to set it again?
It remembers the setting on this device and browser. Set it again if you zoom the page, change the display size, or use another device.
Can I measure something longer than my screen?
This web ruler is as long as your screen. The Ruler in The Measure App scrolls: slide the phone along and the reading keeps counting.