Bubble Level
Lay your phone on a surface to check it is flat, or hold it against an edge to check it is level. The bubble, angle and slope update live, and you can zero the level on a known flat surface.
How to use the Bubble Level
- Open the page on your phone and press Start level. On iPhone, tap Allow when asked about motion and orientation.
- Choose Surface and lay the phone flat to check a table or shelf, or choose Bar and hold the phone's long edge against the object.
- Calibrate once: put the phone on a surface you know is level and press Calibrate.
- Adjust until it says Level: the bubble centers and the angle shows 0.0°.
- Press Hold to freeze the reading when the screen is hard to see.
Degrees, percent and inches per foot
The slope of a surface is percent = tan(angle) × 100 and rise per foot = tan(angle) × 12 in. Small angles are easy to underestimate: a 1° tilt drops a 4-foot shelf by more than three quarters of an inch.
| Angle | Slope | Rise per foot | Rise per meter | Drop over 4 ft |
|---|---|---|---|---|
| 0.25° | 0.44% | 0.05 in | 4.4 mm | 0.21 in |
| 0.5° | 0.87% | 0.10 in | 8.7 mm | 0.42 in |
| 1° | 1.75% | 0.21 in | 17.5 mm | 0.84 in |
| 1.19° | 2.08% | ¼ in | 20.8 mm | 1.00 in |
| 2° | 3.49% | 0.42 in | 34.9 mm | 1.68 in |
| 4.76° | 8.33% | 1 in | 83.3 mm | 4.00 in |
A slope of ¼ inch per foot (about 2%) is the common minimum for drainage on patios and showers, and 1:12 (8.33%, 4.76°) is the maximum slope for a wheelchair ramp under the ADA standards.
How the level reads your phone's tilt
The browser's DeviceOrientation API reports two tilt angles: beta, the front-to-back tilt, and gamma, the left-to-right tilt. In surface mode the total tilt is
tilt = arccos(cos β × cos γ)
and the bubble moves toward the higher side, just like the air bubble in a real vial. In bar mode the tool works out which way gravity points within the plane of the screen, angle = atan2(sin γ × cos β, sin β), and shows how far that is from the nearest horizontal or vertical, so it works with the phone standing on its long or short edge.
Tips for accurate results
- Calibrate on a known flat surface, ideally with the phone out of a bulky case. Then turn the phone 180° on the same spot: if the reading is still near 0°, the calibration is good.
- Calibrate each mode separately: surface mode and bar mode keep their own zero point.
- Use the flat back in surface mode. A protruding camera bump tilts the phone; calibration corrects for it as long as the phone lies the same way.
- For long shelves, put a straight board or ruler on the shelf and the phone on top, so small bumps don't affect the reading.
- Turn on smoothing for a calm bubble, or turn it off for the fastest response.
Frequently asked questions
How accurate is a phone bubble level?
After calibration most phones are accurate to about 0.1° to 0.5°, which is fine for hanging pictures, shelves and furniture. Professional levels are more precise, so use a real spirit level for construction work.
Why do I need to calibrate?
Phone sensors have a small offset, and cases or camera bumps tilt the phone slightly. Place the phone on a surface you trust and press Calibrate to remember that position as zero. For the best result, calibrate, turn the phone 180° and check the reading is still close to zero.
Why doesn't the level work on my laptop?
Laptops and desktops usually have no accelerometer exposed to the browser, so there is no tilt to show. Open the page on a phone or tablet.
How do I use it on iPhone?
Press Start level and allow access to motion and orientation. If you denied it, close the tab and open the page again. The page must be served over https.
What is the difference between degrees and percent slope?
Degrees measure the angle itself. Percent slope is the rise divided by the run times 100, so 1° is about 1.75% or roughly 3/16 inch per foot.
Is any sensor data uploaded?
No. Orientation readings stay in your browser and are not stored or sent anywhere. Only your calibration offset is saved on this device.