Choose the measurement method before placing points
Three-point angle mode uses the movable protractor center as the vertex and two draggable endpoints as rays. Multiple rays mode keeps 2 to 12 numbered endpoints around that same vertex and labels every angle between adjacent ray directions. Two-line intersection mode uses four endpoints and compares the direction of the two complete lines, so the crossing point can sit outside the visible crop. Transparent overlay mode reports the rotation of the protractor baseline while leaving the tick scale visible for a manual reading.
The Geometry, Roof, Ramp, Blueprint, and Screenshot buttons draw small Canvas examples instead of downloading demonstration files. Roof and Ramp reveal slope conversions because those contexts have a meaningful horizontal baseline. Geometry, Blueprint, and Screenshot keep the result focused on degrees, radians, complement, supplement, and reflex angle.
Measure an angle on an image or PDF
- Choose an example, upload a PNG/JPG/WEBP image, paste a screenshot, import a PDF page, or begin on the blank canvas. PDF.js renders only the page you select in this browser.
- Select Three-point angle, Multiple rays, Two-line intersection, or Transparent overlay. The instruction immediately below the mode buttons states how many points or handles the method uses.
- Zoom with the wheel or trackpad and place points on long, clear portions of each edge. For a three-point reading, drag the overlay center onto the vertex; for two lines, mark both ends of each line.
- Check whether the smaller, supplementary, or reflex value answers the actual task. Slope and roof-pitch conversions appear only for a roof, ramp, or slope-specific context.
- Save useful readings, give each one a descriptive name, hide or reorder comparisons, and export only after the visible annotation matches the source.
What the browser calculates
- For points A–V–B, the three-point angle is arccos(((A−V)·(B−V))/(|A−V||B−V|)). The displayed interior result is between 0° and 180°.
- Multiple rays uses the same vector calculation after sorting numbered endpoints around the shared center; the canvas labels each adjacent pair and saves the complete ray set.
- For lines P1–P2 and Q1–Q2, the direction difference is reduced to the smaller intersection angle between 0° and 90°; its supplement describes the other angle at the crossing.
- Complement is 90°−θ only when θ is no greater than 90°. Supplement is 180°−θ. Reflex is 360°−θ for a nonzero interior angle.
- Percent grade is 100×tan(θ), slope ratio is 1/tan(θ), and roof pitch is 12×tan(θ). Those conversions require θ to describe rise from a level baseline.
- Point spacing improves repeatability but cannot correct perspective, lens distortion, scan skew, or an edge that is not in the same plane as its reference.
Worked measurement examples
- Geometry example: place the vertex at (560, 420), one ray at (835, 420), and the other at (690, 195). The vectors point right and up-right; the calculated smaller angle is about 60°. Moving the far endpoint by a few pixels changes the answer less than moving a point close to the vertex.
- Two-line example: mark endpoints on each of two blueprint diagonals. The algorithm compares line direction, not the apparent crossing location. If the smaller direction difference is 72°, the adjacent angle at the same intersection is its 108° supplement.
- Roof example: a side-on profile measures 26.57° above a verified horizontal baseline. tan(26.57°) is approximately 0.5, so the contextual outputs are 50% grade and about 6 in 12 pitch.
- Screenshot example: enlarge a thin diagram before placing points and share an annotated PNG. The export documents point placement; it does not make the source more accurate.
Canvas controls that keep the measurement adjustable
A practical tool should feel closer to a working measuring surface than a static ruler image. This browser workspace keeps the drawing, photo, PDF page, or blank canvas visible while you adjust the protractor center, size, rotation, opacity, and color preset.
Use the canvas controls when the source is not perfectly aligned. You can zoom the canvas, move the overlay, rotate the protractor, add several measurement lines, compare saved angles, and export the result when the setup is clear enough to share.
- Use grid and guide settings when the overlay needs a horizontal, vertical, or repeated spacing reference.
- Use color presets when the tick marks need stronger contrast against a dark drawing, blueprint, or light worksheet.
- Use PNG, PDF, CSV, Excel, JSON, or SVG export when a measurement needs a reviewable record.
When the browser tool fits better than an angle meter
A phone angle meter is useful when the device can physically rest on a surface. A workspace is better when the angle already exists in a screenshot, classroom diagram, scanned PDF, CAD export, map crop, roof photo, or blueprint image.
This browser protractor is also useful when you need to explain how the angle was read. The visible vertex, side points, protractor overlay, angle label, and export table make the result easier to audit than a single number typed into notes.
- Choose the workflow for remote review, homework, diagrams, and files that cannot be measured with a physical tool.
- Choose a calibrated physical tool when fabrication, safety, or compliance depends on the final angle.
- Use both methods when the browser result is a first pass and the real object still needs a field check.
Accuracy, local files, and appropriate use
This is browser-based visual measurement for planning, learning, and documentation. It measures the rendered pixels and marks you provide, not an unseen three-dimensional object. Perspective can make parallel or perpendicular real-world edges appear to converge; scans can be rotated; compressed screenshots can blur the exact edge. Safety-critical or regulated work requires suitable calibrated instruments and qualified review.
Images and selected PDF pages are decoded and drawn locally. The workspace does not send file bytes, canvas pixels, point coordinates, notes, or sensor readings to analytics. Generated PNG, SVG, CSV, JSON, spreadsheet, and PDF files are created on the device; temporary object URLs are revoked when a source changes.
