Online Ruler — Actual Size
Calibrate once for accurate on-screen measurements. Switch between cm, mm, and inches, or download a printable ruler PDF at true scale.
Calibrate
Saved per screen. Current display: init
Method 1: Match this rectangle to your credit/debit card’s width, then press Save.
The rectangle’s left and top edges are anchored. It never leaves the area.
Method 2: Enter your screen diagonal in inches. Fine-tune with nudges afterward.
Current scale: 3.780 px/mm
Free Online Ruler — Measure Anything on Screen
This online ruler works directly in your browser — no downloads, no plugins. Use it to measure objects in centimeters, millimeters, or inches on any device. One quick calibration step ensures accuracy tailored to your specific screen.
How to Calibrate the Online Ruler
Accurate measurement starts with calibration. There are two methods:
- Credit card method — A standard credit card is exactly 85.6 mm wide. Hold it against the ruler and drag the calibration handle until the ruler matches.
- Diagonal method — Enter your screen's diagonal size (e.g., 15.6") and the tool calculates the correct pixel density automatically.
Calibration is saved per device, so you only need to do it once.
Printable Ruler PDF
Need a physical ruler? Click Download PDF to get a print-ready ruler in your chosen units. Print at 100% scale (no "fit to page") for true-to-size output. Works great for classrooms, workshops, and field measurements.
Who Uses This Tool?
- Students — Quick measurements for homework or lab reports without a physical ruler nearby
- Teachers — Print classroom ruler sets in any unit
- Engineers & makers — Verify dimensions of components or materials on screen
- Designers — Check real-world dimensions of UI or print layouts
- Crafters — Measure fabric, paper, or materials at the desk