How to Use the Online Ruler

Step 1 — Calibrate Your Screen

Click the Calibrate button in the toolbar. You have three options: Auto-Detect matches your device against a database of 200+ known screens; Screen Diagonal calculates PPI from the size you enter in inches; and Credit Card lets you resize an on-screen outline to match a standard card for a physical reference. Calibration is a one-time step — your settings are saved in your browser.

Step 2 — Choose Your Unit

Use the unit buttons to switch between cm, in (inches), and px (pixels). The ruler updates instantly. In centimeter mode you'll see millimeter subdivisions; in inch mode you'll see 1/16-inch marks.

Step 3 — Place a Ruler

Click T, B, L, or R (or press those keys) to display a ruler along the top, bottom, left, or right edge. You can activate several edges at once to measure both width and height.

Step 4 — Measure Your Object

Hold a physical object against the screen and read the markings directly, or use the Measure tool to click a start and end point on screen and get the exact distance between them.

Bonus — Convert Units

Open the Convert widget to instantly translate a value between mm, cm, inches, and pixels without leaving the page.

Keyboard Shortcuts

• T / B / L / R — toggle the top, bottom, left, or right ruler.
• M — measure distance tool.
• U — open the unit converter.
• F — fullscreen. D — toggle dark/light theme.