Detecting device…

PPI / DPI

96
Pixels per inch

Screen Width

—
Physical width

Screen Height

—
Physical height

Resolution

—
px × px
Enter your screen diagonal for instant calibration.
📐 How to Use This Ruler
01

Auto Detection

The ruler auto-detects many devices and applies the correct PPI instantly.

02

Quick Calibrate

Enter your screen diagonal (in inches) for immediate accurate scaling.

03

Credit Card

Use the calibration modal to align a physical credit card (85.6 × 54 mm).

04

Hover to Measure

Move your cursor over the ruler to read exact measurements at any point.

❓ Frequently Asked Questions

How accurate is this online ruler?

After calibration using your screen diagonal, device selection, or physical credit card, the ruler is highly accurate. Without calibration it uses a default of 96 PPI which may vary by device.

Why should I keep browser zoom at 100%?

Any zoom other than 100% scales the canvas and produces inaccurate measurements. Always ensure your browser zoom is exactly 100% (Ctrl/Cmd + 0 to reset).

Does this work on mobile?

Yes! The ruler is fully responsive and fills your entire screen on smartphones, tablets, laptops, and desktops.

How do I use the credit card calibration?

Click Calibrate → Method 2 – Credit Card. Place a physical card next to the blue rectangle and drag the slider until they match, then click Save.

Is this tool free?

Yes, completely free. No downloads or sign-ups required — works directly in your browser.