Every modern smartphone contains an accelerometer — a tiny sensor that measures gravitational acceleration along multiple axes. This is the same physical principle that makes a traditional spirit level work: gravity always pulls straight down, so measuring the angle between your device and that pull tells you exactly how far off horizontal or vertical you are.
You don't need a separate app. Spirit Level Online runs directly in your phone's browser — just open the page and you have a precision bubble level in your hand.
How the accelerometer works
Your phone's accelerometer reports its orientation as three numbers: the tilt from front to back (pitch), the tilt from side to side (roll), and the force in the up-down direction. When the phone is perfectly flat on a horizontal surface, pitch and roll are both 0°. When the phone is perfectly vertical and level, the relevant axis reads 0°. Spirit Level Online reads these values many times per second and displays them as a moving bubble — just like the liquid-filled vial in a traditional spirit level.
Surface Mode vs Wall Mode
Spirit Level Online has two modes, each designed for a different task:
Surface Mode — Place your phone flat on a horizontal surface (a shelf, a table, a floor). The circular dial shows tilt in all directions simultaneously. Both the horizontal and vertical readings need to reach 0.0° for a surface to be truly level.
Wall Mode — Hold your phone upright against a vertical surface (a wall, a door frame, the edge of a picture frame or TV). It measures the single axis that matters for vertical alignment and shows whether you're tilted left or right of true vertical.
Surface Mode — bubble centred means the surface is level
Wall Mode — bubble centred means the vertical surface or frame is straight
How to calibrate for a camera bump
Most modern phones have a raised camera module on the back. When you lay the phone flat on a surface, the camera bump lifts one corner and introduces a small measurement error. To compensate:
Lay the phone on the surface you want to measure.
Let the readings stabilise for a second or two.
Press the Zero button. This tells the app to treat the current position as the reference point, cancelling out the camera bump offset.
Now move the phone to other positions on the same surface — the readings will be accurate relative to that zeroed reference.
Tip: Re-zero each time you switch surfaces. The Zero calibration applies to the current orientation and surface only. If you move to a different surface or flip the phone, zero it again.
What accuracy can you expect?
Modern smartphone accelerometers are excellent — typically accurate to within 0.5° or better for most phones made in the last five years. This is more than sufficient for DIY tasks like hanging shelves, mounting TVs, checking furniture, or leveling a pool table. For tasks that require sub-0.1° precision (precision engineering, optical equipment, scientific instruments), a dedicated calibrated level is still the right tool.
Note: Accuracy varies by device. Flagship phones tend to have higher-quality accelerometers than budget models. If you need the best possible reading from your specific phone, use the 180° rotation calibration method: place the phone, note the reading, rotate it 180°, and note the new reading. The true level is exactly halfway between the two. This cancels out any systematic offset in the sensor itself.
Common tasks you can do with your phone as a spirit level
Hanging picture frames straight on a wall
Leveling a TV wall mount before drilling
Checking shelves are horizontal before fixing them permanently
Leveling a pool or billiard table
Checking appliances like washing machines and refrigerators are stable
Verifying a door frame or window frame is plumb during installation
Checking furniture legs on uneven floors
Open Spirit Level Online in your browser — it works on any modern smartphone or tablet, requires no download, and is completely free.