Actual Size Ruler
A ruler drawn at true size on your screen, with inches on top and centimeters below. Match a bank card to the outline once, and the ruler is exact for your screen.
Loading the ruler.
Match a card to make it exact
- Hold a bank card or ID card flat against the screen.
- Move the slider until the outline matches the card exactly.
- Press Save. The ruler above now matches your screen.
Why is an on-screen ruler not actual size?
Browsers draw an inch as 96 pixels, whatever the screen. Real screens differ, so 96 browser pixels can measure well under or well over an inch.
A bank card is the same size everywhere, so matching one tells the ruler how big your pixels really are.
What can I measure with it?
Anything that fits across your screen: screws, coins, cables, small parts and printed photos. Lay the object flat against the glass, with one end at the zero mark.
For longer items, use a real tape measure. A screen is only as long as the ruler on it.
Our guide on how to measure on screen shows how to check rings, screws, photos and paper sizes step by step.
How do I read the ruler in inches and centimeters?
The top edge is in inches. The longest marks are whole inches, then come half, quarter, eighth and sixteenth inch marks, each a little shorter. The bottom edge is in centimeters, with a mark for every millimeter.
One inch is exactly 2.54 cm, or 25.4 mm. So an object that reaches 2 in on the top edge reaches 5.08 cm on the bottom.
Can I use it as an online measuring tape?
For short lengths, yes. The ruler is as long as the page is wide, which covers most of a phone or laptop screen. The status line under the ruler tells you whether it is matched to your screen yet.
To measure something longer, mark where the ruler ends, slide the object along and add the two readings. For a room or a piece of furniture, a real tape measure is quicker and more accurate.
Is there a ruler on my phone?
This page works as one. Open it on the phone, match the card and lay small objects on the glass. For larger things, such as a table or a doorway, iPhones come with the Measure app, which uses the camera.
How accurate is the ruler?
As accurate as your card match. Line the card edges up with care, and look straight at the screen, since an angle makes the edges harder to judge. Check the match again after you change the screen resolution.
Why is the ruler wrong on my other monitor?
Each screen has its own pixel size, so a match made on one does not carry over to another. Drag the window to the new screen and match the card again.
Questions
Does browser zoom change the ruler?
Yes. Zooming makes everything on the page bigger, the ruler included. Keep zoom at 100%, or match the card again after you change it.
Does it work on a phone?
Yes. Hold the phone upright and match the short edge of the card, since the long edge does not fit across most phone screens.
Is anything saved or sent?
The match is kept in your browser for this visit only. Close the tab and it is gone, and nothing is sent to us.
How we measure
An ID-1 card is 85.60 × 53.98 mm. When the outline matches it, the outline width in pixels divided by the card edge gives pixels per mm. The ruler is drawn at that scale.