Calibrated100% client-side. No sensor or calibration data leaves your device.

Online Ruler

Measure objects in centimeters, millimeters, and inches directly on your screen. Calibrate once for real-size physical measurement.

Draggable Caliper0.00 cm

Inches0.00 in

Drag the measurement pointer along the ruler to mark an exact length.

PX/MM:—

Method:—

CM / MM (Top Edge)

The ruler is not calibrated yet

Enter your display size to start measuring.

Place an object against the screen or drag the red caliper line

0.00 cm

INCHES / 1/16" (Bottom Edge)

How to Use the Online Ruler

Place an object directly against the screen, aligned with the zero mark on the top metric edge. The ruler shows centimeters and millimeters along the top, and inches along the bottom. You can also drag the red caliper line to measure — the readout updates in real time in centimeters, millimeters, and inches.

For accurate real-size measurement, calibrate the ruler first. Open the calibration dialog and choose a method: select your device model, or enter your screen's diagonal size and aspect ratio. The calibration is saved in your browser and persists across sessions. You can use the ruler in fullscreen mode for a longer measuring surface, and switch between landscape and portrait orientation.

Frequently Asked Questions

How do I use the online ruler?▼

Calibrate the ruler by selecting your device or entering your screen size. Then place an object against the screen edge, or drag the red caliper line to measure. The readout shows the length in centimeters, millimeters, and inches.

Can I measure in centimeters and millimeters?▼

Yes. The top edge shows a metric scale in centimeters and millimeters. The bottom edge shows inches with 1/16" subdivisions. The caliper readout displays all three units simultaneously.

How do I calibrate the ruler?▼

Click 'Calibrate Scale' and choose a method. The device method applies the factory PPI for a known phone, laptop, or monitor. The display-size method lets you enter your screen's diagonal in inches and aspect ratio — the ruler calculates the physical width from geometry. Calibration is stored in your browser's localStorage.

Can I use the ruler vertically or horizontally?▼

Yes. The ruler works in both landscape and portrait orientation. In fullscreen mode, it provides the longest possible measuring edge. Resize and orientation changes do not affect calibration.

Can I use it on a phone?▼

Yes. The ruler works on any phone, tablet, or computer with a browser. For real-size measurement on a phone, calibrate using the display-size method or select your phone model from the device list.

Can an online ruler show real-size measurements?▼

After calibration, the ruler renders at the correct physical scale on your specific display. Accuracy is typically within ±1–2 mm, limited by parallax, screen scaling, and browser zoom. Keep browser zoom at 100% (Ctrl+0 / Cmd+0) for the best results.

100% client-side. No sensor or calibration data leaves your device.

No tracking pixels. No analytics. No account. Sensor data and your calibration live only in this browser tab.