Online Ruler — Actual Size in cm, mm & inches
A free, accurate online ruler to measure objects directly on your screen in centimeters, millimeters, and inches. Calibrate this virtual ruler once to your display for true, life-size, real ruler measurements.
Default is 102.5. Saved automatically.
Subdivision ticks per inch on the imperial scale.
Calibrate with a real object
Pick an object, hold it against the bar below, and use −/+ until the on-screen width matches exactly. Your PPI is saved automatically.
Screen resolution: detecting…
Frequently asked questions
How accurate is the online ruler?+
Accuracy depends on calibration. Set your browser zoom to 100% and use the calibration helper with a credit card or sheet of paper so the on-screen ruler matches real-world sizes.
Does my calibration get saved?+
Yes. Your PPI and inch graduation are stored in your browser's localStorage, so the ruler stays calibrated the next time you visit on the same device and browser.
Can I use the ruler on a phone or tablet?+
Yes. The layout adapts to small screens, the ruler scrolls horizontally, and the controls stack vertically with touch-friendly buttons.
How do I switch between 1/8, 1/16, and 1/32 inch marks?+
Use the inch graduation selector under the ruler to choose how many subdivision ticks appear per inch on the imperial scale.
How many millimeters are in a centimeter and an inch?+
There are 10 millimeters (mm) in 1 centimeter (cm), and 25.4 millimeters in 1 inch. One inch equals 2.54 cm. This online ruler shows all three units at once so you can read mm, cm, and inch measurements together.
Is this a real, life-size ruler?+
Yes — once calibrated, it is a real, actual-size ruler on your screen. Because every display has a different pixel density, calibrate it to your monitor or phone so 1 cm on screen equals 1 cm in real life.
Can I use it as a scale ruler or virtual ruler?+
Yes. It works as a virtual, on-screen ruler for measuring small objects in real size, and you can change the PPI to read measurements accurately on any device.
Free online rulers in actual size
These online rulers let you measure objects on your screen in real, actual size. Whether you need a cm ruler actual size online, a millimeter (mm) ruler, or an inch ruler, all three scales are shown together so you can read measurements at a glance. It works as a digital, virtual ruler on desktop, laptop, tablet, and phone — no app or download required.
Accurate online rulers to scale
To get an accurate online scale for measurement, the tool must match your display's pixel density. Use the calibration helper above with a credit card or sheet of paper, and adjust until the on-screen size matches the real object. After that, the ruler shows true life-size, to-scale measurements — including a precise mm ruler actual size online — every time you visit.
Ruler measurements explained
Each centimeter on the metric scale is divided into 10 millimeters — so 1 centimeter actual size equals 10 mm marks, with a taller mark every 5 mm and a numbered mark every centimeter. The inch scale shows scale in inches online divided into 1/8, 1/16, or 1/32 subdivisions, with ¼, ½, and ¾ fraction marks for easy reading. Remember: 1 inch = 2.54 cm = 25.4 mm. New to reading a ruler? See our full guide to ruler measurements explained.
More measuring tools
Need a paper ruler? Download a true-to-size printable ruler in inches and cm or a protractor. Not sure of your screen's pixel density? The PPI calculator works it out from your resolution and screen size so you can calibrate the ruler precisely.