Open the app on your phone to use the level. This page needs a phone’s motion sensor.
How the Bubble Level works
This tool turns your phone into a level and inclinometer. Lay it flat on a surface to read the tilt of that surface (plus side to side and top to bottom), or stand it on its edge or hold its side against a post to check level and plumb like a torpedo level. It shows the angle in degrees, the slope as a percentage and the rise per foot in inches.
The phone's motion sensor measures which way gravity pulls, and the angle comes from that. Slope % = tan(angle) × 100, and rise per foot = tan(angle) × 12". So 1° is a 1.75% slope, and 45° is a 100% slope, 12" of rise per foot.
Example: a drain line that falls 1/4" per foot. 0.25 ÷ 12 = 0.0208, so the slope is 2.08% and the angle is arctan(0.0208) = 1.19°. Standing on its edge along the top of the pipe, the phone should read about 1.2°, 2.1% and 1/4" per foot.
Calibrate before work that matters: lay the phone on a surface you know is level and tap Calibrate (set zero). It only accepts a zero within 5° of level, so a stray tap can't zero a tilted phone. Check it with the flip test: read, turn the phone end for end on the same spot and read again; the two readings should match. A case with a camera bump rocks, so set the phone on a straight board. The reading turns green within 0.2° of level, and Hold freezes it so you can read it after you pull the phone away. A phone is a quick check, not a substitute for a good level on finish work.
Frequently asked questions
How accurate is a phone level?
Once calibrated, a phone's motion sensor is good enough for most layout and slope checks, but many phones read a little off out of the box, and a case or camera bump adds error. Calibrate on a known level surface and confirm with the flip test. For finish work such as cabinets and trim, use a quality level.
How do I calibrate my phone level?
Lay the phone on a surface you know is level (checked with a good level) and tap Calibrate (set zero); flat and on-edge modes each keep their own zero. To check it, turn the phone end for end on the same spot: the reading should not change. Reset calibration clears the zero.
What is 1/4 inch per foot in degrees?
1/4" per foot is arctan(0.25 ÷ 12) = 1.19°, a 2.08% slope. It is the usual minimum fall for horizontal drain pipe 2 1/2" and smaller (IRC Table P3005.3); 3" to 6" pipe may run at 1/8" per foot (0.60°, 1.04%). Check your local plumbing code, which may differ.
How do you convert degrees to percent slope?
Percent slope = tan(angle) × 100. 1° is 1.75%, 5° is 8.75%, 10° is 17.63% and 45° is 100%. Degrees and percent are not the same scale: a 10% slope is only about 5.7°.
How do I check plumb with my phone?
Switch to On edge and hold the phone's long side flat against the post or stud. The reading is the angle away from plumb; at 0° (green) it is plumb. Check two neighboring faces, because a post can be plumb one way and leaning the other.