Online Ruler - Measure in cm, in & px
Use a free online ruler to measure objects in centimeters, inches, or pixels. Calibrate your screen for accurate real-world measurements.
Unit
Ruler edges
Keyboard shortcuts1 cm2 in3 pxG GuidesC CrosshairF FullscreenEsc Clear guides
Before measuring: Set browser zoom to 100% (Ctrl/Cmd + 0). Display scaling is accounted for where supported; recalibrate if you change display settings or move the ruler to another screen.
Place an object along the ruler edge or click anywhere to add a measurement guide.
Width—
Height—
Size—
How to Use the Online Ruler
- Click Calibrate in the toolbar and enter your screen's diagonal size, pick your device, or match a credit card. This sets the correct PPI so the ruler matches real-world sizes. For the most reliable physical-size measurement, use the credit-card calibration method when available.
- Choose a unit — cm, in, or px — from the unit toggle. Tick marks automatically scale to your calibrated screen.
- Activate one or more ruler edges (Top, Bottom, Left, Right) so the ruler sits exactly where you need it.
- Place your object against any ruler edge and read the measurement, or click the stage to drop guide lines for precise readouts.
- Enable Crosshair mode to follow your cursor with live X/Y coordinates, or press Fullscreen for edge-to-edge measuring.
About the Online Ruler
An online ruler turns your screen into a measuring tool. Every display has a unique pixel density (PPI), so without calibration an "inch" on screen is only an estimate. Calibrating once makes this a true actual-size ruler.
The ruler supports three units: centimeters and millimeters (the international metric standard), inches (used mainly in the US), and pixels — useful for designers and developers measuring UI elements or screenshots at their native pixel size, bypassing physical calibration.
Because measurements happen entirely in your browser, nothing you measure ever leaves your device. Calibration is saved locally and remembered on your next visit.
Frequently Asked Questions
How do I make the online ruler accurate?
Click Calibrate, then enter your screen's diagonal in inches, pick your device from the list, or match a credit card with the slider. This sets the correct PPI so 1 cm on screen equals exactly 1 physical cm.
How do I calibrate an online ruler?
To calibrate an online ruler, open the calibration tool and either enter your screen size, select your device, or use a known-size reference such as a credit card. Adjust the reference until it matches the physical object on your screen. Once calibrated, the ruler can display measurements closer to their real-world size.
Does browser zoom affect accuracy?
Yes — browser zoom is the most common source of error. If your browser is zoomed in or out (Ctrl +/-), every measurement will be proportionally wrong. Make sure your browser zoom is at 100% before measuring. OS display scaling (125%, 150% on Windows) is handled automatically via
devicePixelRatio, so you don't need to change that.Can I use my phone as a ruler?
Yes. Open this page in your phone's browser, tap Calibrate and choose your device (or use the credit card method). Once calibrated, your phone screen becomes a ruler accurate to a fraction of a millimeter.
What does PPI mean?
PPI stands for pixels per inch — how many screen pixels fit in one physical inch. A 27-inch 1080p monitor is about 82 PPI, while a modern phone can exceed 400 PPI. Calibration sets the right value for your display.
What happens if I rotate my phone?
The ruler redraws automatically when the screen resizes or rotates. Tick marks and edges re-orient to match the new layout, so you can rotate mid-measurement without losing accuracy.
How do I read centimeters on the ruler?
Numbered tick marks are centimeters; the smallest ticks are millimeters (1/10 cm). Align one end of your object with the 0 mark and read where the other end falls.
How do I read inches on the ruler?
Numbered ticks are full inches. Each inch is divided into halves, quarters, eighths, and sixteenths. The longest small tick between two numbers is the half-inch mark.
Is my data private?
Yes. All measuring and calibration happens in your browser. Your calibration is stored only in your browser's local storage and is never uploaded.