1Shortcut: P · Movable Center
Place Vertex Directly Over Any Corner
Drag the protractor hub ring over the vertex of an on-screen diagram, CAD PDF, or pasted photo. Press P anytime to show or hide the protractor.
0° – 360° Geometry · Dual Movable Rays · Reflex & Interior Readout
Interactive 360° Online Protractor & Angle Meter
Drag the vertex center anywhere on screen or over a photo, sweep Ray A and Ray B independently across 360°, and read exact interior and reflex angles.
Interactive Online Screen Ruler Workspace
Embed Ad-Free Online Ruler (<iframe>)
cm
Origin (0, 0) · Top-Left Dashed Datum
Align your object with the 0 tick mark. Click any ruler or grid to drop draggable measurement guides.
100% Client-Side Photo & Clipboard Ruler
Press Ctrl+V to paste a screenshot, drop an image here, or choose a file. Nothing is ever uploaded.
Live Unit Converter@ 96.0 PPI
3 3/8 in (3 3/8 in)
Display Density Calibration · Saved Locally Per Screen
Calibrate Your Screen for True Physical Scale
Browsers default to 96 CSS pixels per inch, which rarely matches physical display panels. Choose one of the three calibration methods below so every millimeter and fractional inch tick corresponds to actual physical length.
Active ScaleEstimated calibration
96.0 PPI·3.780 px/mm
CSS reference scale (96 px per inch)
Display Profiles:
ISO/IEC 7810 ID-1 Standard · 85.60 × 53.98 mm
Align a Physical Bank, Credit, or ID Card
Hold any standard payment card, driver’s license, or national ID card flat against the outline below and match its physical width.
Target:
Drag the blue corner handle or slider to match your object
Top-left card alignment datum · 1:1 physical outline
ISO/IEC 7810 ID-1
85.60 mmPlace Physical Card Here
Align left & top edges · Match outer border
3.370 × 2.125 in53.98 mm
Optical Alignment Checklist
- Hold card flat: Place a real credit or ID card flush against the top-left corner of the box.
- Match outer edge: Drag the slider or press ← → keys until the right edge aligns exactly.
- Avoid parallax: Keep your eyes perpendicular to the right edge of the card.
Computed Scale: 96.0 CSS PPI · 3.780 px/mm
Keyboard fine-tuning: click the card stage and press ← → (Shift for 5px steps).
Geometric Display Calculation · √(W² + H²) / Diagonal
Calibrate by Visible Display Diagonal
Enter the diagonal measurement of your visible screen panel in inches (corner to corner inside the bezel, not the outer laptop chassis). Ensure browser page zoom is at 100%.
Common Display Diagonals (Inches)
Accuracy notes for diagonal calibration:
- Display vs. Device Body: Enter the active panel diagonal (e.g., 13.6″ or 15.6″), not the external case dimensions.
- Logical CSS Pixels vs. Physical Pixels: Operating systems often scale HiDPI panels (such as 125% or 150% on Windows, or “Looks like” Retina scaling on macOS). The browser reports logical CSS pixels at 100% browser zoom, which this calculation uses directly.
Browser Display Telemetry & Hardware Database
Automatic Device & Screen Density Detection
Online Ruler Plus inspects your browser’s reported logical screen dimensions, device pixel ratio, orientation, and platform to match known hardware profiles or compute a sensible initial estimate.
- Detected Profile
- Checking display…
- Detection Status
- Estimated calibration
- Estimated Density
- 96.0 CSS px/in
- Screen CSS Size
- —
- Device Pixel Ratio
- —
- Orientation
- —
Why verify manually? Web browsers do not expose physical monitor sizes over HDMI/DisplayPort or custom OS scaling factors. Always verify with a physical card or known diagonal before critical measurements.
Physical Verification Gauge
Verify With Everyday Objects
Hold a real coin against these live-scaled reference outlines to confirm your calibration immediately.
24.26 mm
US Quarter23.25 mm
€1 Euro19.05 mm
US Penny50.0 mm Reference Bar (5.0 cm)1.969 in
Local Session Log
Measurement History
Place two parallel guide lines in the ruler workspace and click Save to History to record point-to-point spans (ΔX / ΔY) locally on your device.
Storage: localStorage onlyZero network sync
Screen Angle Metrology · Dual-Ray Precision
How to Measure Angles on Screen & Inside Images
Built for geometry homework, woodworking miter cuts, slope roof pitches, CAD verification, and X-ray or anatomical angle checks.
2Independent Ray A & Ray B
Align Zero Reference & Measure Sweep
Unlike fixed 180° templates, both Ray A and Ray B rotate independently around 360° so you never have to tilt slanted drawings upright first.
3Linear + Angular Combined
Combine With Floating Arbitrary-Angle Rulers
Add angled floating rulers alongside the protractor to measure mitered board lengths and angles in one workspace, saved automatically in local storage.
Protractor & Angle Measurement
FIG. 07 · ANGLES
Interactive Motion Guide
See Every Measurement Mechanics in Motion
Motion playing
Sweep the second ray around the center vertex to read primary, supplementary, and reflex angles simultaneously.
θ = atan2(ΔY, ΔX)Angle Classification & Geometry Guide
Acute, Right, Obtuse & Reflex Angle Reference
Angle measurement depends strictly on XY pixel coordinates, so circular angle readings stay exact even before running screen PPI calibration.
Interior vs. Reflex angle readout
The subtle arc highlights the interior sweep (0°–180°) while letting you inspect supplementary 180°−θ complements and full 360° bearings.
Keyboard fine adjustments
Focus either draggable ray handle and use arrow keys for precise single-step rotation tuning without mouse slip.
Woodworking miter & bevel help
For a 90° corner joint, split your measured vertex angle in half to find the exact chop-saw miter setting.
Standard Reference Angles
Common Carpentry, Drafting & Compass Angles
| Angle Type | Degrees (°) | Complement / Slope | Common Application |
|---|---|---|---|
| Right Angle (Square) | 90.0° | 90° corner | Cabinetry, framing & PCB orthogonals |
| Standard Miter Cut | 45.0° | 1:1 slope (100%) | Picture frames & 90° baseboard corners |
| Hexagon Interior | 120.0° | 60° exterior | Hex bolts, honeycomb & isometric grid |
| Isometric Axis | 30.0° | 57.7% pitch | 3D technical axonometric drafting |
| Octagon Miter | 22.5° | 135° interior | Bay windows & octagonal trim |
| Roof Pitch 4/12 | 18.4° | 33.3% slope | Residential gable rafters |
| Pentagon Interior | 108.0° | 72° exterior | Polygon star & tile geometry |
| Straight Angle | 180.0° | Flat line | Edge collinearity alignment check |
Protractor accuracy requires isotropic 1:1 square monitor pixels (standard on modern LCD/OLED panels).
Switch Ruler Axis or Unit Scale
Explore Other Calibrated Online Ruler Modes
Metric · cm
Centimeter Ruler
1 cm & 0.5 cm scale + cm-to-inch chart
Imperial · 1/16″
Inch Ruler (12″)
1/2, 1/4, 1/8, 1/16″ fractions
Fine Pitch · mm
Millimeter Ruler
1 mm & 5 mm hardware scale
Top & Bottom
Horizontal Ruler
Wide spans & CSS pixel (px) scale
Left & Right
Vertical Ruler
Upright height & mobile portrait
Related Editorial Guides
Deep-dive walkthroughs and conversion charts for this ruler mode.
Protractor FAQ
Common Questions About Measuring Angles Online
Do I need to calibrate PPI to measure accurate angles?
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No. Because modern displays use square pixels (1:1 aspect ratio), atan2(ΔY, ΔX) angular measurements remain accurate regardless of monitor diagonal size or OS scaling.
Can I place an image behind the protractor?
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Yes! Paste an image with Ctrl+V or click Photo inside the toolbar, then keep the Protractor active to measure joints, roof slopes, or CAD parts.
How is this different from basic 180° screen protractors?
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Both rays move freely through all 360° and the vertex can be dragged anywhere over the stage, removing the need to rotate tilted objects horizontally.
Can I measure distance and angle simultaneously?
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Yes. Combine the 360° Protractor with draggable ΔX/ΔY guides or floating angled rulers inside the same calibrated workspace.