Free Online Protractor — Measure Angles on Any Screen

How to Use the Online Protractor

1. Open the protractor tool on your device (phone, tablet, or desktop).
2. You will see two colored rays extending from the center — a blue ray and a red ray.
3. Drag either ray handle to rotate it. The angle between the two rays is displayed in the center.
4. Enable 'Snap' to lock the angle to common values like 15°, 45°, or 90°.
5. Use the reset button to return both rays to their default positions (0° and 90°).

When Do You Need an Online Protractor?

• Students: Measure angles for geometry, trigonometry, and math assignments when a physical protractor is not available.
• Designers: Check angles of design elements, gradients, and rotations in mockups and layouts.
• Engineers: Quick angle verification for diagrams, schematics, and technical drawings.
• DIY & Crafts: Measure cut angles for woodworking, sewing patterns, or paper crafts.
• Photography: Determine tilt angles for composition adjustments and leveling.

Features

• Full 0° to 360° range with 0.1° precision.
• Touch-friendly — works on phones and tablets with drag gestures.
• Angle snapping at 1°, 5°, 15°, or 45° intervals.
• Dark mode support for comfortable viewing.
• No installation, no sign-up — just open and measure.

Accuracy Tips

The online protractor measures angles between two rays on screen. It is ideal for measuring angles in images, diagrams, or between objects placed on your screen. For physical-world accuracy, place the object flat on the screen and align the rays with its edges. Note that screen curvature and viewing angle can introduce minor errors on very large or curved displays.

Types of Protractors

Protractors come in several common forms, and the best one depends on the measurement you need. Our online tool works like a full-circle digital protractor so you can read any angle from 0° to 360° without flipping the ruler.

• Half-circle (180°) The classic classroom protractor — a flat semicircle with a straight edge. It measures angles up to 180° and is the most common style taught in schools. Good for basic geometry, but you have to flip it to measure reflex angles above 180°.
• Full-circle (360°) A complete circle with degree markings all the way around. It reads reflex angles directly and is preferred by drafters, machinists, and engineers. Our tool follows this style so obtuse and reflex angles are one motion away.
• Digital protractor A hardware or software protractor that reports the angle numerically. Hardware versions are common in carpentry and metalworking. The screen ruler online tool is a free software equivalent — no battery, no purchase, just a browser.

Frequently Asked Questions

How accurate is an online protractor?

An online protractor is accurate to the precision the screen can display — typically within 0.5°. The tool itself calculates angles mathematically from the positions of the two rays, so the only source of error is the anti-aliasing of pixel positions, which is negligible for everyday measurement.

Can I use this protractor on my phone?

Yes. The protractor is fully touch-optimized — drag either ray with your finger to rotate it. The angle readout updates in real time as you move the ray. It works on phones, tablets, laptops, and desktops without installing anything.

What is the difference between a half-circle and a full-circle protractor?

A half-circle protractor (180°) is the flat semicircle taught in schools — it measures angles up to 180° and you have to flip it for reflex angles. A full-circle protractor (360°) reads any angle directly, including reflex angles between 180° and 360°. Our online tool is full-circle so you never have to flip it.

How do I measure an angle between two lines in an image?

Open the image in a viewer next to this protractor page and drag each ray on top of one of the lines. Align the rays visually, then read the angle displayed at the center. For more precise work, use the Snap toggle to lock to common angles (15°, 30°, 45°, 60°, 90°).

Does the protractor save my measurements?

The tool does not save measurements between sessions — reload the page and it resets to 90°. If you need to record a measurement, take a screenshot or note the angle value from the readout. All calculations happen locally in your browser, so no data leaves your device.

Why does the protractor start at 90°?

The default position shows a right angle because it is the most recognizable starting point — one ray horizontal, one ray vertical. This makes it easy to visualize the tool and start adjusting from a familiar reference.