PRACTICAL GUIDE
How to use the phone bubble level
Updated
This online spirit level checks how far your phone tilts on a flat surface. Give the sensor a stable position before following its adjustment guidance.
Get a reading
- Use a phone with a motion sensor. Open the page in a supported phone browser over HTTPS.
- Choose your position. Flat surface needs a screen-up phone resting on the surface. Edge / wall needs the screen facing you and a straight phone edge against the object. A thick case, side buttons or camera bump can keep the body from making even contact.
- Tap Start level. If your browser asks for motion access, allow it. The tool shows a message if access is denied or the device provides no reading.
- Wait for “Steady.” Moving the phone can disturb an accelerometer reading. Once it settles, use the bubble and the high-side message to adjust your surface.
- Check again. Change one side, let the phone settle, and confirm the result. Hold freezes a steady reading so you can lift the phone to inspect it.
What do the numbers mean?
Left · right and top · bottom are signed tilt angles in degrees. The bubble travels toward the higher side of the phone. The “near level” message uses an on-screen guide of 0.5° in each direction. That guide is a display threshold, not a claim about the accuracy of your phone.
In Edge / wall mode, the single angle is the difference from the nearest horizontal or vertical reference. It is intended for levelling a frame or checking whether a wall is plumb, rather than measuring a full-range slope. Keep the screen facing you and follow the clockwise or counterclockwise guidance.
Changing modes or rotating the screen clears Set zero and Hold because the visible axes have changed. Place the phone again and let it settle. The display asks you to reposition the phone if its placement does not match the selected mode.
Measure a slope or match an angle
Choose Slope / angle and rest a long phone edge along the slope. The angle ranges from −90° to 90°; positive means the phone’s top end is higher. Grade is 100 × tan(angle), so 45° is 100%. Rise:run shows the magnitude; the signed angle and grade show direction. Vertical displays “Vertical” instead of an infinite grade.
Open Target angle to enter a value from −90° to 90°, or choose a preset. Guidance and the bubble compare your reading with that target. Positive and negative targets refer to opposite phone placements. Set zero makes angle and grade relative to your chosen reference. Relative angles can extend to ±180°; grade and rise:run are unavailable outside ±90°. Reset zero before measuring an absolute slope.
Read without bending over the phone
Expand tool removes surrounding page content while you work; use Exit expanded view or Escape to return. Hold freezes a steady reading so you can lift the phone. Copy reading copies the angles, mode and reference as a text snapshot. Sound on gives a short tone when a steady reading enters the near-level guide; it is off by default.
Vibration on adds a short pulse on supported browsers when a steady reading reaches level or your target. It is off by default and some browsers do not support it.
Use it offline
While connected, tap Save offline and wait for “Saved offline.” The tool and guides are saved in this browser. Reopen the same address without internet. Motion permission and a supported sensor are still required. Private browsing, storage restrictions or clearing site data may remove saved files.
How to calibrate with Set zero
Use Set zero when you want to compare one surface with a chosen reference. First place the phone on that reference and wait for a steady reading; then tap Set zero. The tool subtracts that reference from future angles. A zeroed display does not prove the reference is absolutely level. Tap Reset zero to return to the sensor’s original reference.
For example, if the sensor initially shows +2° and you set that position as zero, returning to the same position should show about 0° relative to it. The original surface may still be tilted. If you change modes or screen orientation, set the reference again.
To look for placement or sensor bias, take an unzeroed reading, turn the phone 180° on the same spot, and let it settle again. Compare the magnitude using the same physical direction on the surface; screen-axis signs can change when the phone turns. A mismatch is a reason to check the case, contact and reference with a physical level. This comparison does not automatically calibrate or certify the phone.
Placement matters: a flexible case, camera bump or uneven contact can introduce a bigger error than the number shown on screen suggests.
Why is the bubble level not working?
- Use an HTTPS page on a phone or tablet; most laptops have no usable motion sensor.
- Allow motion access when prompted. If denied, review that site’s permissions in your browser settings and tap Try again.
- If the reading stops, return to the page and try again. Some browsers pause sensor events in background tabs.
- Match the placement to the selected mode: screen-up for Flat surface, upright with the screen facing you for Edge / wall.
- On a computer with no motion sensor, open the phone panel and scan the QR code. Localhost previews cannot be opened by another phone; use a published HTTPS address.
How accurate is a phone bubble level?
This tool has not been calibrated against a certified level across phone models. Sensor bias, browser behavior, motion, the case and the surface all matter. It is suitable for quick, noncritical checks. Use a calibrated physical spirit level or other appropriate instrument for construction, safety and precision work.
The calculation uses the direction of the reported gravity-inclusive acceleration vector while the phone rests. Movement adds acceleration and can change the angle, which is why you must wait for a steady reading. The W3C Device Motion specification describes the browser input; support and delivered precision vary by browser and device.