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.
A free online ruler in actual size
This online ruler lets you measure objects on your screen in real, actual size. Whether you need a centimeter ruler, 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, life-size measurements
To get a real, accurate ruler on screen, 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 every time you visit.
Ruler measurements explained
Each centimeter on the metric scale is divided into 10 millimeters, with a taller mark every 5 mm and a numbered mark every centimeter. The inch scale is 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.
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.