How do I measure an angle on a photo?

Press Open a photo, or drop the file anywhere on the dark stage. Three points appear: the vertex, and the two ray handles marked A and B. Press or tap anywhere on the stage and the nearest of the three jumps to that spot, which beats dragging across a large picture, and from there you can drag it to fine tune. Put the vertex on the corner you are measuring, lay handle A along one edge and handle B along the other, and the angle, its reflex and the vertex position update as you move.

How accurate is a protractor on a screen?

The tool uses a stable floating-point calculation and reports the angle to a tenth of a degree (0.1°). It takes the arctangent of the cross product against the dot product of the two rays, which avoids the loss of precision the cosine formula can have near zero and 180 degrees. That reporting interval does not promise the same accuracy for your placement or the object in a photo. A ray only a few pixels long turns a one pixel slip into a degree or more, so drag both handles well out from the vertex. Zooming in does not change the angle, because the angle between two rays does not depend on their scale, but it does help you place a handle on the intended pixel.

Why does the angle not match the real corner in my photo?

Because a photograph is a flat projection of a solid scene. If the camera was not square to the surface, that surface is foreshortened, and foreshortening changes angles: a corner that is a true right angle can measure 53 degrees in a photo taken from the side. This tool measures the angle as it appears in the image plane and never claims otherwise. For the reading to be the angle of the real object, the surface needs to be flat and the camera roughly square to it, which is the ordinary case for a drawing, a screenshot, a plan or a photo taken straight on.

What is the reflex angle it shows?

The angle measured the other way round the vertex. The two always add up to 360 degrees, so an opening of 62.4 degrees has a reflex of 297.6 degrees. It sits beside the main reading, and the Show and Hide buttons in the Reflex cell control it. That choice is remembered in this browser for next time.

Can I measure an angle without a photo?

Yes. The tool opens on a blank grid with a 60 degree angle already set, and everything works the same way. It is the quick way to check an angle you have on paper against the screen, or to see what a given number of degrees actually looks like: set snap to 15 degrees and swing a ray around the dial. Pressing Blank grid after opening a photo goes back to it.

Does it work with a keyboard, or on a phone?

Both. Tab to any of the three points and the arrow keys take over: left and right turn that ray one degree around the vertex, shift with an arrow turns it ten, and up and down make the ray longer or shorter. On the vertex the arrows move it a pixel at a time, ten with shift. On a narrow screen each handle is a 44 pixel target, and a tap brings the nearest of the three points to your finger.

Does my picture get uploaded anywhere?

No. The file is decoded by your own browser from a local reference and drawn on the page. There is no server in this tool, no account, and nothing about the picture, the file name or the angle goes to analytics. It is also why no size limit is imposed: the only ceiling is the memory your device can give the tab, and a very large file gets a note saying so rather than a refusal. The one thing kept is your snap and reflex choice, in this browser's storage, and the Start over button above the tool clears it.

Can I save or copy the measurement?

Two ways. Copy puts a single line on the clipboard: the angle, the reflex, the same angle in degrees, minutes and seconds, the vertex position and the note that it was measured in the image plane. Save PNG writes the picture back out at its own resolution with both rays, the arc and the figure drawn over it, named after the picture and the angle, for example beach-photo-62.4-degrees.png. On the blank grid the PNG is the grid with the angle on it.

What does the snap setting do?

It holds each ray to a whole multiple of 1, 5 or 15 degrees, which makes the angle between them a multiple of that step as well. A pair of rays sitting near 58 degrees reads exactly 60.0 with a 15 degree snap, and the handles move to show where that is. Snap is off by default, so the rays stay exactly where you put them, and the Snap cell shows which step is on.

How can I practise using this interactive protractor?

Choose Blank grid to explore an angle without a photo, or open a diagram and place the vertex and rays on its corner. Turn a ray with the left/right arrow keys and compare the smaller angle with its reflex; together they make a full turn of 360 degrees. Snap can hold rays to 1, 5 or 15 degree steps. This is a measuring manipulative, not a scored quiz or game.

Can I use a camera image with the online protractor?

Take a photo with your device, save it, and choose Open a photo or drop the file here. The tool measures the saved image; it does not stream your camera or correct perspective. An oblique photograph may change the visible angle.

Online Protractor

Open a photo or use the blank grid, then place a vertex and ray handles A and B along the angle you want to measure. Read the angle, its reflex and degrees/minutes/seconds, with a note that a photo reading is in the image plane. Copy the figures or save an annotated PNG.

60.0°

Angle

60.0°

reflex 300.0° · vertex 480, 280 · image plane

Angle: 60.0°
an acute angle

Reflex: 300.0°

Vertex: 480, 280
canvas pixels

Snap

Open a photo

Measuring on the blank grid. Open a photo, or drop one on the stage, to measure an angle in a picture.

Press or tap anywhere on the stage to bring the nearest of the three points to it, then drag. With a point focused, the arrow keys move it: left and right turn a ray by a degree, up and down make it longer or shorter, and shift with an arrow takes a bigger step. Snap holds both rays to whole multiples of 1, 5 or 15 degrees. Nothing you open here leaves your browser.

Two raysfrom the vertex you placed
60.0°, an acute angle
Reflexthe other way round the vertex
300.0°
Degrees, minutes, secondssnap off, the rays sit where you put them
60° 00′ 00″

Accuracy. Stable floating-point angle calculation on the two rays you place, reported to a tenth of a degree (0.1°). Placement, lens distortion and perspective limit the reading: it measures the image plane, not necessarily the angle of the real object. The angle is the one between the two rays as they lie in the picture, so a flat drawing or screenshot, or a photo taken square to a flat surface, reduces perspective distortion. Two things are worth watching. A ray only a few pixels long carries the error of a single pixel into the reading, so drag both handles well out from the vertex. And a wide lens close to the subject bends straight lines near the edges of the frame, which moves the angle you measure there.

Common questions

How do I measure an angle on a photo?: Press Open a photo, or drop the file anywhere on the dark stage. Three points appear: the vertex, and the two ray handles marked A and B. Press or tap anywhere on the stage and the nearest of the three jumps to that spot, which beats dragging across a large picture, and from there you can drag it to fine tune. Put the vertex on the corner you are measuring, lay handle A along one edge and handle B along the other, and the angle, its reflex and the vertex position update as you move.
How accurate is a protractor on a screen?: The tool uses a stable floating-point calculation and reports the angle to a tenth of a degree (0.1°). It takes the arctangent of the cross product against the dot product of the two rays, which avoids the loss of precision the cosine formula can have near zero and 180 degrees. That reporting interval does not promise the same accuracy for your placement or the object in a photo. A ray only a few pixels long turns a one pixel slip into a degree or more, so drag both handles well out from the vertex. Zooming in does not change the angle, because the angle between two rays does not depend on their scale, but it does help you place a handle on the intended pixel.
Why does the angle not match the real corner in my photo?: Because a photograph is a flat projection of a solid scene. If the camera was not square to the surface, that surface is foreshortened, and foreshortening changes angles: a corner that is a true right angle can measure 53 degrees in a photo taken from the side. This tool measures the angle as it appears in the image plane and never claims otherwise. For the reading to be the angle of the real object, the surface needs to be flat and the camera roughly square to it, which is the ordinary case for a drawing, a screenshot, a plan or a photo taken straight on.
What is the reflex angle it shows?: The angle measured the other way round the vertex. The two always add up to 360 degrees, so an opening of 62.4 degrees has a reflex of 297.6 degrees. It sits beside the main reading, and the Show and Hide buttons in the Reflex cell control it. That choice is remembered in this browser for next time.
Can I measure an angle without a photo?: Yes. The tool opens on a blank grid with a 60 degree angle already set, and everything works the same way. It is the quick way to check an angle you have on paper against the screen, or to see what a given number of degrees actually looks like: set snap to 15 degrees and swing a ray around the dial. Pressing Blank grid after opening a photo goes back to it.
Does it work with a keyboard, or on a phone?: Both. Tab to any of the three points and the arrow keys take over: left and right turn that ray one degree around the vertex, shift with an arrow turns it ten, and up and down make the ray longer or shorter. On the vertex the arrows move it a pixel at a time, ten with shift. On a narrow screen each handle is a 44 pixel target, and a tap brings the nearest of the three points to your finger.
Does my picture get uploaded anywhere?: No. The file is decoded by your own browser from a local reference and drawn on the page. There is no server in this tool, no account, and nothing about the picture, the file name or the angle goes to analytics. It is also why no size limit is imposed: the only ceiling is the memory your device can give the tab, and a very large file gets a note saying so rather than a refusal. The one thing kept is your snap and reflex choice, in this browser's storage, and the Start over button above the tool clears it.
Can I save or copy the measurement?: Two ways. Copy puts a single line on the clipboard: the angle, the reflex, the same angle in degrees, minutes and seconds, the vertex position and the note that it was measured in the image plane. Save PNG writes the picture back out at its own resolution with both rays, the arc and the figure drawn over it, named after the picture and the angle, for example beach-photo-62.4-degrees.png. On the blank grid the PNG is the grid with the angle on it.
What does the snap setting do?: It holds each ray to a whole multiple of 1, 5 or 15 degrees, which makes the angle between them a multiple of that step as well. A pair of rays sitting near 58 degrees reads exactly 60.0 with a 15 degree snap, and the handles move to show where that is. Snap is off by default, so the rays stay exactly where you put them, and the Snap cell shows which step is on.
How can I practise using this interactive protractor?: Choose Blank grid to explore an angle without a photo, or open a diagram and place the vertex and rays on its corner. Turn a ray with the left/right arrow keys and compare the smaller angle with its reflex; together they make a full turn of 360 degrees. Snap can hold rays to 1, 5 or 15 degree steps. This is a measuring manipulative, not a scored quiz or game.
Can I use a camera image with the online protractor?: Take a photo with your device, save it, and choose Open a photo or drop the file here. The tool measures the saved image; it does not stream your camera or correct perspective. An oblique photograph may change the visible angle.

Stable floating-point angle calculation on the two rays you place, reported to a tenth of a degree (0.1°). Placement, lens distortion and perspective limit the reading: it measures the image plane, not necessarily the angle of the real object.