Every modern phone has an accelerometer — the same sensor that flips your screen between portrait and landscape. A bubble level app reads that sensor to work out which way is "down," and shows how far your phone is tilted from flat or plumb.
How accurate is it?
Once calibrated, a phone's accelerometer is typically accurate to within about 0.3–0.5 degrees. That's close to what a basic physical spirit level offers, and good enough for hanging a shelf, checking a picture frame, or levelling furniture.
For comparison:
- A cheap bubble spirit level: accurate to roughly 0.5–1 degree.
- A calibrated phone accelerometer: roughly 0.3–0.5 degrees.
- A laser level: typically under 0.1 degree — the choice for structural or professional work.
Why calibration matters
No two phones sit perfectly flat inside their case. The sensor itself might be mounted a fraction of a degree off, or your specific unit might have accumulated a small offset over time. That tiny error carries into every reading unless you correct for it.
Calibrating is simple: place your phone on a surface you're treating as your reference (a surface you know is flat, or simply the exact spot you want "level" to mean), then zero the reading there. Every reading after that is measured relative to that exact point, not an assumed factory-perfect zero.
When to use a physical level instead
A phone bubble level is great for quick, everyday checks — but skip it for anything load-bearing or safety-critical, like structural beams, plumbing slopes, or professional installations. For those, a proper spirit level or laser level, ideally checked against a second reading with the level flipped 180°, is still the standard.
Try it
The free Bubble Level tool has three modes — Surface for flat objects, Vertical for walls and frames, and Angle for reading the exact slope of a ramp or board — all running on your phone's sensors, with one-tap calibration.