How to Measure Angles with an Online Protractor

4-step tutorial: position vertex, align baseline, rotate arm, and correctly read inner vs outer scales. Works for photos, blueprints and screenshots.

Interactive Tutorial

Follow these four steps to measure any printed worksheet angle, digital diagram, screen shape, or technical blueprint with zero guesswork.

Place the point where the two angle lines intersect (the vertex) directly under the protractor center crosshair origin.

Tip: Click and drag the center crosshair or use the 3x Magnifier Loupe for pixel-level alignment.

Line up the flat bottom baseline of the protractor with one arm of your angle. The line should pass straight through the 0° mark.

Tip: If your drawn line is tilted, use the "Protractor Rotation" slider to tilt the entire tool to match.

Grab the red indicator handle and rotate the measurement arm until it aligns precisely over the second ray of the angle.

Tip: Use the keyboard Left / Right arrow keys to micro-nudge the angle by 0.1° increments.

If your angle opens counterclockwise from the right, read the outer scale (0° to 180°). If it opens clockwise from the left, read the inner scale.

Tip: The live HUD automatically calculates and displays the exact angle, complement, supplement, and radians.

Scale Guide

Traditional 180° semicircular protractors have two sets of numbers on the curved rim:

Outer Scale (0° on Right → 180° on Left)

Use the outer scale when the baseline of your angle points to the right side of the protractor and opens in a counter-clockwise direction.

Inner Scale (180° on Right → 0° on Left)

Use the inner scale when the baseline of your angle points to the left side of the protractor and opens in a clockwise direction.

Golden Rule of Protractors

“Always start counting from 0°”

Whichever side your baseline ray is resting on, find the scale that starts with 0° on that side, then follow those numbers around to the second ray!