Physical Mode
Edge Protractor
A true-size protractor that docks to any screen edge. Calibrate once, hold real objects against the glass — or drop in a photo and watch the rays find its edges automatically.
Updated Online Protractor
True Size
Calibrate Your Screen
Browsers assume 96 pixels per inch, but real displays vary. Calibrate once and the edge protractor below renders at real-world size — hold a physical object against the screen and measure directly.
Method 1 · Auto-Detect Device
One-click estimate from your reported resolution and pixel ratio. Fast, but honest: browsers cannot read physical panel size, so treat this as a starting point and verify with a credit card for exact work.
- Resolution
- —
- Pixel Ratio
- —
- Estimate
- Not detected yet
Method 2 · Screen Diagonal
Enter your screen’s advertised diagonal (inches). We combine it with your detected pixel resolution: PPI = √(w² + h²) / diagonal.
Method 3 · Credit Card Calibration
Most accurate. Hold a standard credit card (85.60 × 53.98 mm) against the outline below and drag the slider until the widths match exactly. Each step re-computes PPI = outline width / 3.370 in.
- Hold a real card to the screen.
- Align left edges.
- Drag until right edges line up.
Stored only in this browser (localStorage). No upload, no account.
Browser zoom must stay at 100% while calibrating — zoom changes CSS pixel size.
Physical Mode
Edge Protractor & Image Stage
Dock a true-size protractor to any edge, hold a real object against the screen, and place your photo anywhere in the stage. Drag the vertex and the ray — the readout follows live.
A quiet stage, ready to measure
Drop a photo here, or drag the vertex & ray to measure against the true-size edge scale.
True size needs calibration — calibrate above, keep browser zoom at 100%, and re-check with a ruler for exact work. Angle math is exact for the points you place; physical accuracy depends on calibration and holding the object flat to the screen.
New here? Read the calibration chapter of the handbook first.
How the edge stage works
Browsers assume 96 pixels per inch; your display differs. Auto-detect guesses in one click, diagonal math uses your advertised screen size, and credit-card matching is exact. The live HUD shows your active PPI; keep browser zoom at 100% while calibrated.
Top, bottom, left, right or free-floating. Docked tools slide along their edge like a physical instrument, with a vertex-tracking origin marker on the true-size inch/cm ruler. The arc is auto-sized to never clip.
Upload, drag & drop or paste. On-device line detection finds the two strongest straight edges, seats the vertex at their intersection and aligns baseline + ray. Then drag the photo by mouse or finger, scale, rotate, fade or lock it while you refine.
The HUD shows the interior angle against your movable baseline. Jump to 30°–120° with one tap, fine-tune with Ray/Base sliders or snap, then copy. Grid and fullscreen viewing are one click away.
Honest limits of auto-detect
- Needs two clear straight edges — brackets, miters, rooflines, diagrams. Curves, faces and clutter fall back to manual with a spoken hint.
- Parallel lines, single edges and blur below threshold are refused rather than guessed.
- Detection runs on a 220px downscale: fast (<300ms) but blind to hairline detail — zoom and refine by hand for exact work.
- It measures the photo's 2D projection. Tilted shots and wide-angle distortion still skew results; shoot straight-on.
- Physical accuracy is only as good as your calibration — verify against a ruler for exact work.
Edge protractor questions
What is the edge protractor?
A true-size on-screen protractor that docks to the top, bottom, left or right edge of a stage — or floats free. After a one-time screen calibration it renders at real physical inches or centimeters, so you can hold objects against the screen or measure photos placed anywhere in the stage.
How does photo auto-detect work?
On upload, the stage downscales the photo and runs Sobel edge detection plus a Hough line transform entirely in your browser. The two strongest straight edges set the baseline and ray, and the vertex goes to their intersection. It needs two clear straight edges and announces when to measure manually instead.
Which calibration method should I use?
Auto-detect for a 10-second estimate, screen diagonal for a solid value from your advertised display size, or credit-card matching — holding a real 85.60mm card against the on-screen outline — for exact true size.
Can I move the photo with touch?
Yes. Drag the photo directly with mouse or finger, position it on the 9-point grid, scale, rotate and fade it, or lock it while you drag the vertex and rays over it.
Is my photo uploaded?
No. Everything runs locally via blob URLs. Removing or replacing the photo revokes its URL immediately.