Online Ruler in CM: Measure in Centimeters
An accurate centimeter ruler that works right in your browser, scaled to your screen's true size.
OnScreenRuler's cm ruler lets you measure small objects directly from your phone, tablet, or computer screen. It's calibrated to your exact device so centimeters shown on screen match centimeters in real life, useful for crafts, packaging checks, schoolwork, and anything else where you need a quick, accurate measurement.
If auto-detection doesn't recognize your screen, calibrate manually using your screen's diagonal size or a standard credit card, which is always 8.56 cm wide.
How to Use the Online Ruler
- Choose an edge โ click Left, Right, Top, or Bottom above to pin the ruler to that edge of your screen.
- Switch units โ use the "Also measure in" links to open the cm, inch, mm, or px version of this ruler.
- Check the status banner โ green means your device was auto-detected and the ruler is accurate. A yellow banner means calibration is needed.
- Calibrate if needed โ click Calibrate and pick your device, enter your screen's diagonal size, or use a standard credit card.
- Measure โ place any object against the ruler edge. Hover for a live coordinate readout. Use Fullscreen for maximum measuring area.
How to Calibrate for Accurate Measurements
Accuracy depends on your screen's PPI being correctly set. There are three reliable ways to calibrate:
- Pick your device โ select your exact phone, tablet, or laptop model for the factory-precise PPI.
- Screen diagonal โ find your screen's diagonal size (printed on the box or searchable by model name) and enter it in the Calibrate panel. Works for any screen.
- Credit card calibration โ place any standard bank or credit card flat against the blue guide. All ISO 7810 cards are exactly 85.6 mm wide. Drag the slider until the guide matches the card edge.
Your calibration is saved in your browser and restored on your next visit.
Frequently Asked Questions
How do I measure in cm online?
Open onscreenruler.com, let it auto-detect your screen or calibrate manually, then place your object against the ruler edge and read the centimeter value.
How to read a ruler online?
Numbered tick marks are centimeters. The smaller ticks between them are millimeters, ten to a centimeter, exactly like a physical ruler.
Is the online cm ruler accurate?
Yes, typically within 1 to 2 percent once calibrated, which is reliable for most everyday measurements.
Can I switch to inches or mm?
Yes, use the unit links on this page to open the inch or mm version without losing your calibration.