Online Protractor

Last updated: 2026-10-01

An online protractor is a digital version of the half-circle tool from a geometry set. It puts a 0–360° scale on your screen so you can draw an angle, read its size in degrees and radians, and — with photo mode — measure the angle between two lines in a picture, all inside your browser.

How to Use the Online Protractor

1. Type or slide. Enter a value from 0 to 360 in the Angle box, or move the slider. The arm on the dial jumps to that angle.
2. Drag the arm. Press anywhere on the dial and drag. The round handle follows your finger or mouse, and the label shows the angle in whole degrees.
3. Read the cards. Below the dial you get the angle type (acute, right, obtuse, straight or reflex), the radian value, and the complementary and supplementary angles.

The dial uses standard position: the fixed baseline points right and the angle opens counterclockwise, so 90° points straight up and 270° points straight down. Pair the online protractor with our Unit Circle Calculator when you need exact trig values for the same angle.

Measure an Angle on a Photo

Photo mode turns the online protractor into an angle finder online for real objects — a roof pitch, a chair leg, a ramp, a knee bend in a physio photo, or an angle in a printed worksheet. Tap Take photo to use your phone camera, or Choose image to open a picture you already have. A transparent protractor appears over the image. Drag the hollow point onto the corner (the vertex), then drag each solid point along one of the two lines. The readout shows the angle between the lines to 0.1°, plus the reflex angle on the other side (360° minus the angle).

Because this protractor online runs entirely on your device, the image is read with your browser's own file API and drawn on the page; it is never sent to a server, so it works for private photos and even offline once the page has loaded. For the most accurate result, hold the camera square to the surface. A photo taken at a slant squashes angles through perspective, the same reason a bubble level or an AR measuring tape is better for angles you cannot photograph straight on.

Using the Online Protractor for Geometry Homework

Students and teachers can check homework answers with the online protractor in seconds. Type the angle from your worksheet into the degree box, then read off the type, the radian equivalent, and both the complementary and supplementary angle. To check a problem like "what is the supplement of 47°?", set the input to 47 and the Supplementary card shows 133°. If the worksheet only gives you a drawing, photograph it and use photo mode to read the angle. The scale follows the vertex-at-origin convention used by Khan Academy's angle measurement guide. When you finish, open the Coordinate Plane Plotter to plot the rays, or the Graph Plotter to graph sine or cosine across the same angle range.

Acute, Obtuse, Right, and Reflex Angles Explained

An acute angle measures more than 0° but less than 90° — think of the sharp tip of a slice of pizza. A right angle is exactly 90° and is marked with a small square at the vertex; it is the foundation of rectangle area calculations. An obtuse angle is wider than 90° but less than 180°, like an open laptop lid. A straight angle is exactly 180° — a flat line. A reflex angle measures more than 180° and less than 360°, and a full turn is 360°. These categories follow the standard definitions in Britannica's geometry reference. For statistics worksheets that also need angles (think pie chart slices), our Pie Chart Maker converts percentages straight to degrees. Drag the arm on the online protractor above and the Type card changes as you cross each boundary.

Understanding Angles

An angle is formed when two rays share a common endpoint called the vertex. Angles are measured in degrees (a full rotation is 360 degrees) or radians (a full rotation is 2 pi radians). Understanding angles is fundamental to geometry, trigonometry, engineering, architecture, and everyday tasks like reading a clock or setting up a camera angle. An online protractor makes those ideas visible: you can watch one ray turn away from the other and see the number change.

Types of Angles

Acute angles measure between 0 and 90 degrees. They are sharp and narrow. Right angles are exactly 90 degrees, forming an L-shape. Obtuse angles are between 90 and 180 degrees — wider than a right angle. Straight angles are exactly 180 degrees, forming a straight line. Reflex angles measure between 180 and 360 degrees.

Complementary and Supplementary Angles

Two angles are complementary if they add up to 90 degrees. For example, 30 and 60 degrees are complementary. Two angles are supplementary if they add up to 180 degrees. For example, 110 and 70 degrees are supplementary. These relationships appear constantly in geometry proofs and real-world design.

Measuring Angles with a Protractor

Place the protractor's center point on the angle's vertex. Align the baseline with one ray. Read the degree marking where the second ray crosses the scale. Photo mode on this online protractor does exactly that for you: the overlay's baseline snaps to the first line and the scale is read where the second line crosses it. For fraction-based angle problems (such as "1/4 of a turn"), the Fraction Visualizer helps you see the part-of-a-whole relationship before converting to degrees.

Degrees, Radians, and Unit Conversion

To convert degrees to radians, multiply by pi/180. To convert radians to degrees, multiply by 180/pi. For example, 90 degrees equals pi/2 radians, and 180 degrees equals pi radians. Radians are the standard unit in calculus and most scientific applications. One full revolution equals 360 degrees or 2pi radians. Knowing both measurement systems is essential for students moving from geometry to trigonometry and for anyone working in engineering, physics, or computer graphics where radian-based calculations are the norm. The online protractor shows both units side by side, so you never have to convert by hand.

Online Protractor for Classroom & Remote Learning

This online protractor is a free classroom-ready tool for grade 4-10 geometry, GCSE maths, and high-school trigonometry units. Teachers can project it on a whiteboard, share the URL in Google Classroom, or link it from a worksheet — no install and no sign-up. Common classroom uses: demonstrating the difference between acute and obtuse angles during a Common Core 4.MD.5 lesson, walking through complementary and supplementary pairs for Common Core grade 7 geometry, or showing reflex angles when introducing the unit circle. Students can also photograph angles around the classroom — a door opening, a book cover, a set square — and measure them in photo mode. Drag-to-measure works on touchscreens, so tablets and Chromebooks behave the same as a laptop. For straight-line measurements, the Online Ruler is the companion tool.

Common Angle Cheat Sheet

Quick reference for the most-searched angle values: 30° — acute, π/6 rad, complement 60°, supplement 150° (used in 30-60-90 triangles). 45° — acute, π/4 rad, complement 45°, supplement 135° (the isosceles right-triangle angle). 60° — acute, π/3 rad, complement 30°, supplement 120° (equilateral triangle interior angle). 90° — right angle, π/2 rad, complement 0°, supplement 90°. 120° — obtuse, 2π/3 rad, no complement, supplement 60° (regular hexagon interior angle). 180° — straight, π rad. 270° — reflex, 3π/2 rad. 360° — full rotation, 2π rad. These values match the reference table on Wikipedia: Angle and the angle taxonomy in Britannica's geometry entry. Type each into the online protractor above to see it drawn live.

Standard Position, Sign and Why Two Angles Can Look Identical

An angle drawn on its own is ambiguous until you fix a convention, and mathematics fixes it the same way every time. In standard position the vertex sits at the origin and the initial side lies along the positive x-axis; rotation counterclockwise is positive and clockwise is negative. That single rule explains most confusion about an online protractor drawing: a negative angle and its positive partner can land the terminal side in exactly the same place, and so can any angle reached by adding whole extra turns. Angles that share a terminal side are coterminal, and they have identical sine, cosine and tangent values even though the numbers you typed differ. The reference angle is the related idea used in calculation: the acute angle between the terminal side and the x-axis, which is what turns a value in any quadrant into one you can look up, with the quadrant supplying the sign. Degrees and radians simply measure the same rotation on two scales, and the radian is the unit used in calculus because it makes the derivatives of the trigonometric functions come out clean. Unit definitions are maintained by NIST.

📅 Last updated: 2026-10-01 · Sources: Britannica — Angle, Khan Academy — Angle measurement, Wikipedia — Angle.