Online protractor
Drag the arms to measure an angle on screen, mark one in a photo, or lay your phone on a slope.
Angles on this photo
None yet. Tap the corner first, then one point on each side.
The download is the photo at its own pixel size with the marks drawn in, re-encoded in this browser, so camera details (EXIF: location, date, phone model) are not in it.
- Front to back
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- Side to side
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- Since zero
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Lay the phone flat on the surface, screen up, and read its tilt. Press Set zero on one surface, then lay the phone on another to get the angle between the two.
It is only as exact as your phone's motion sensor, which is not a calibrated level: read it as good to about a degree at best. To check it, read the tilt, turn the phone end for end on the same spot and read again; if the two readings differ, the sensor is off by half the difference. A thick or uneven case adds its own tilt.
How to measure an angle
- On screen: drag one arm along one side of the angle and the other arm along the other side. The angle shows to 0.1°, with its supplement (180° minus it). Switch to 360° for angles over 180°.
- To draw an angle instead, type it in Draw an angle: arm A goes to 0° and arm B to your number. Arrow keys turn a focused arm by 1°, with Shift by 0.1°.
- In a photo: photograph the corner straight on, open the photo, and tap the vertex (the corner) first, then one point on each side. Hold your finger down to see the magnifier and slide onto the exact spot before letting go.
- For a slope or a lean, choose Line vs horizontal and tap two points along the edge: you get its angle from horizontal and from vertical. Mark as many angles as you like, then Download marked photo.
- Tilt: on a phone, tap Start tilt finder (on iPhone, allow motion access), lay the phone on the surface and read the tilt from level.
Why true size, and why it does not change the angle
An angle is the same at any size: a 35° corner is 35° whether the protractor is 3 cm or 30 cm across, so the on-screen reading is right without any calibration. True size matters when you hold a physical thing against the screen and want the protractor's radius in millimetres, for example to draw an arc of 50 mm. A web page cannot know how many pixels make a millimetre on your screen, so this page borrows the calibration that the ruler saves on this device when you match a bank card there. Worked example: at 6.00 px/mm, a 50 mm radius is drawn 300 pixels long; without a calibration the page assumes 96 pixels to the inch, which on a typical 15.6-inch laptop panel is 15 % or more off.
What it cannot do
- A photo angle is true only when the camera faces the surface square on. Seen from a slant, perspective squeezes the far side, and a right angle no longer looks like 90°. In the figure, the same square corner photographed at a slant measures 63.4° at the near corner and 116.6° at the far one. Hold the phone parallel to the surface, over the middle of the angle, and keep the corner near the centre of the photo, where wide-angle lens distortion is smallest.
- It is not for certified or legal measurements: it is not a calibrated instrument. A photo angle is as good as the pixels you tap (on a 100-pixel arm, one pixel off is about 0.6°), and the tilt finder is as good as your phone's sensor.
- True size needs a /ruler/ calibration made on this device, in this browser. Without one the scale is a default and the radius in millimetres is approximate (the angle is not affected). Browser zoom changes the scale, so the page rescales a saved calibration for a changed zoom; on another monitor, calibrate again.
- The tilt finder needs a motion sensor. Most laptops and desktops have none, and the page says so instead of showing a made-up number. It measures the phone's tilt, not a wall's angle around a vertical axis (that would need a compass).
- The camera is not live: you take a still photo and mark it. A printed protractor is only right if it printed at 100 %, so measure its check line.
Privacy
Nothing leaves your device. A photo you open or take is drawn in this page and never uploaded; the download is made in your browser. The page saves nothing of its own: it only reads the ruler calibration under polyatic-ruler that /ruler/ saved, and that page's Clear saved calibration deletes it. It sends an anonymous page-view count and one of two fixed words, protractor-open and angle-photo, never an angle or a photo.
Questions
Can I use my phone camera as a protractor?
Yes, through a photo. Open the In a photo tab, tap Take a photo and photograph the corner straight on, then tap the corner point and one point on each side. The page shows the angle and its supplement. The camera is not live: the angle is read from the still photo, and it is true only if the camera faced the surface square on.
How do I measure an angle in an image?
Open the image in the In a photo tab. Tap the vertex (the corner) first, then a point on each arm; hold your finger down to see a magnifier and slide to the exact pixel before you let go. For a slope, choose Line vs horizontal and tap two points along it. You can mark several angles on one image and download it with the marks drawn in.
Is there an angle finder on my phone?
Most phones have a motion sensor that a web page can read. Open the Tilt tab and tap Start; on iPhone, allow motion access when asked. Lay the phone on the surface and read its tilt from level, or press Set zero on one surface and lay it on another for the angle between them. On a laptop or desktop with no sensor the page says so.
Is there a printable protractor?
Yes. Press Print a protractor: it prints a half-circle protractor with an 80 mm radius and a 100 mm check line, sized in millimetres. Choose Actual size or 100 % in the print dialog, not Fit to page, then measure the check line with a ruler: if it is not 100 mm, the printer scaled the page. A paper print is thin and bends, so for woodwork a plastic or metal protractor is better.
Is this online protractor free?
Yes, with no sign-up, no ads and no limit. Everything runs in your browser: a photo you open is read on your device and never uploaded, and the page stores nothing of its own; it only reads the ruler calibration that /ruler/ saved on this device.