Online Protractor & Angle Finder
Last updated: October 4, 2026
Measure angles on screen, over a photo or live through your camera - this online protractor works offline and uploads nothing.
Angle Properties & Complementary
Keyboard Shortcuts:
Was this reading right? Help us make this the best angle finder.
Measurement History
Saved in this browser only - never uploaded.
| Time | Mode | Degrees | Radians | Slope |
|---|---|---|---|---|
| Every angle you lock is logged here. | ||||
What is an online protractor?
An online protractor - also called an angle finder or a virtual protractor - measures the angle between two lines directly in your browser. You place a reference ray along one edge, drag a second ray onto the intersecting edge, and the tool reads out the angle in degrees, radians and slope percentage, together with the complementary and supplementary angles. No app to install and no drawing to print: the measuring instrument is the page itself.
Because the protractor is rendered on a canvas, you can use it on top of a photograph as well as on a blank canvas. Take or upload a picture of a roofline, a miter joint or a staircase stringer, set the scale in the calibration mode with a known length, and read the angle off the image. Photos, camera frames and everything else are processed locally in the browser - nothing is uploaded to a server - and the FAQ explains how the tool is used in practice.
Once you have a measurement, convert it with the calculators below: turn a pitch into a ratio with the roof pitch calculator, halve a corner for a miter with the miter angle finder, or translate a grade into degrees with the slope percentage converter.
Five angle types, one real example each
Every corner you measure falls into one of these five ranges. Match the readout against the range below and you know straight away what kind of angle is in front of you.
| Angle type | Degree range | Real-world example |
|---|---|---|
| Acute | 0° < θ < 90° | A 45° miter cut on a picture-frame corner. |
| Right | θ = 90° | A door frame checked with a carpenter's square. |
| Obtuse | 90° < θ < 180° | A laptop lid left open at about 120°. |
| Straight | θ = 180° | A level windowsill running along a wall. |
| Reflex | 180° < θ < 360° | A parking-garage ramp doubling back 270°. |
Who uses it
Four kinds of visitor open this page most days, each with a different job to finish.
Students and teachers
Geometry homework becomes checking instead of guessing: drag the rays onto the diagram and read the angle. The same tool goes on a classroom projector to work through examples together.
Carpenters
Photograph the corner before the saw is set and know the miter angle in seconds. Framing, trim and bench work get verified against a real reading instead of a marked line.
Photographers
Straighten a leaning horizon or a skewed door frame by measuring how far it sits off level. Architectural and interior shots are corrected against a true 90° reference.
Solar installers
Rest the phone on the mounting rail and read the tilt of the installed array. The as-built degrees can be reported without climbing back onto the roof.
How this online protractor is different
Plenty of pages draw a semicircle on the screen. This one measures a real corner, keeps the reading accurate and hands the number straight to a calculator.
-
Live camera measurement
Point the phone at the corner and measure it where it stands, then freeze the frame to take your time over the reading.
-
Gyroscope tilt
Device Tilt mode turns the motion sensors into a level that reads to a tenth of a degree, and the digital protractor page does the same from the edge of the phone.
-
Works offline
After the first visit the app is cached, so it keeps measuring in a basement, a loft or a site where the signal drops.
-
Nothing is uploaded
Photos, camera frames and saved measurements are processed in the browser and stay there - no account, no server, no upload.
-
Six calculators included
Roof pitch, miter angle, degrees to radians, slope to degrees, solar tilt and the triangle solver all sit one scroll below, ready to take the angle you just measured.
-
Free, with no sign-up
Every mode is unlimited and costs nothing, whether the angle comes from a drawing, a photograph, the camera or the tilt of the device.
How to Use Angle Finder Protractor
Learn how to use angle finder protractor on screen, over uploaded images, or live through your mobile camera stream with sub-pixel vector accuracy.
1. On-Screen Virtual Protractor
Overlay a semi-transparent 180° or 360° digital protractor directly over any web page, document, or diagram. Drag the center vertex V and rotate rays A and B to measure any corner or angle.
2. Upload Photo or Blueprint
Upload architectural blueprints, geometric homework diagrams, or construction photos (PNG/JPG). Pan and pinch-to-zoom images under the protractor to calculate precise structural angles.
3. Live Mobile Camera (WebRTC)
Stream live video directly from your smartphone or laptop camera. Superimpose digital measuring lines over physical objects (woodwork, roof pitches, tiles, pipes) and freeze frame for easy reading.
Tools & Guides
-
Camera Angle Finder
Point your phone camera at a corner, roofline or joint and read the angle live. Free browser-based camera angle finder with a protractor overlay - no app, no upload.
-
Photo Angle Finder
Upload a photo and overlay a digital protractor to measure rooflines, corners and cut angles. Free, private photo angle finder that runs entirely in your browser.
-
Digital Protractor
Turn your phone into a digital protractor with its built-in gyroscope: live tilt in degrees, pitch and roll, zero/tare and a bubble level, entirely in your browser.
-
How to Use a Protractor
Learn how to use a protractor in five steps: line up the centre, read the right scale, avoid the supplement mistake. Practise on the interactive protractor below.
How to Read a Protractor Angle Finder
Understanding how to read a protractor angle finder requires aligning the center origin (vertex V) with the vertex point of the angle you wish to measure.
- • Align Base Ray A: Drag Ray A along the baseline of your geometric shape or horizontal reference.
- • Rotate Angle Ray B: Drag Ray B to line up with the intersecting edge or slope line.
- • Read Multi-Unit Readout: The tool instantly displays the measured angle in Degrees (°), Radians (rad), and Slope Percentage (%).
Trigonometric Conversion Formulas
Example: 45° × (3.14159 / 180) = 0.7854 rad
Example: tan(45°) × 100 = 100% slope (1:1 rise over run)
Where to Use an Online Angle Finder
Carpentry & Woodworking
Check miter cut angles, bevel joint angles, and frame squareness by overlaying the digital protractor on wood blueprints or live camera feeds.
Roof Pitch & Architecture
Calculate roof slope percentage (%) and pitch angles from elevation photos or PDF site plans without climbing onto roofs.
Solar Panel Alignment
Determine optimal tilt angles for solar photovoltaic panel mounting racks based on latitude and seasonal sun positions.
Geometry & Engineering
Verify triangle interior angles, vector angles, and CAD drawing dimensions for physics homework or engineering validation.
Angle Reference & Conversion Table
Quick conversion between standard geometric angles, radians, and slope percentages.
| Degrees (°) | Radians (rad) | Slope Percentage (%) | Classification |
|---|---|---|---|
| 0° | 0.000 rad | 0.0% | Zero Angle |
| 30° | 0.524 rad (π/6) | 57.7% | Acute |
| 45° | 0.785 rad (π/4) | 100.0% (1:1) | Acute |
| 60° | 1.047 rad (π/3) | 173.2% | Acute |
| 90° | 1.571 rad (π/2) | Undefined (Vertical) | Right Angle |
| 120° | 2.094 rad (2π/3) | -173.2% | Obtuse |
| 180° | 3.142 rad (π) | 0.0% | Straight Angle |
| 270° | 4.712 rad (3π/2) | Undefined | Reflex Angle |
| 360° | 6.283 rad (2π) | 0.0% | Full Rotation |
Calculators for pitches, miters and conversions
Each calculator takes one measurement and returns the numbers you actually need - degrees, ratios and percentages - with worked examples. Everything runs in your browser.
Roof Pitch Calculator
Free roof pitch calculator. Enter rise and run to get the pitch angle in degrees, slope percentage and x:12 ratio, with worked 4/12 and 6/12 examples.
pitch angle = arctan(rise / run) × 180 / π · slope % = rise / run × 100Miter Angle Finder
Find the miter angle for any corner. Enter the corner angle to get the miter saw cut angle, with a 90° corner table and framing tips for trim and crown molding.
miter angle = corner angle / 2Degrees to Radians Converter
Convert degrees to radians instantly. Formula rad = deg × π/180, worked examples for 30°, 45°, 60°, 90°, 180° and 360°, plus a full conversion table.
radians = degrees × π / 180Slope Percentage to Degrees
Convert slope percentage to degrees and back. Formula, examples for 4.76%, 8.33%, 10%, 50% and 100%, plus a grade-to-angle table for ramps, roads and roofs.
degrees = arctan(slope% / 100) × 180 / π · slope% = tan(angle) × 100Solar Panel Tilt Angle Calculator
Estimate the best fixed solar panel tilt angle from your latitude, with winter and summer adjustments. Rule-of-thumb formulas, worked examples and orientation tips.
year-round tilt ≈ |latitude| · winter ≈ |latitude| + 15° · summer ≈ |latitude| - 15°Triangle Solver
Free triangle solver. Enter three values as SSS, SAS or ASA to get every missing side and angle, plus area and perimeter, with worked examples.
law of cosines: a² = b² + c² − 2bc·cos A · law of sines: a / sin A = b / sin B · area = ½·a·b·sin C · Heron: √(s(s−a)(s−b)(s−c)) with s = (a+b+c) / 2Measure an angle on screen
Use the virtual protractor above to measure an angle directly on the canvas, over an uploaded photo or through your camera, then convert it with the calculators on this page.
angle = measured on screen (no upload)Frequently Asked Questions
Everything you need to know about measuring angles online with high precision.