Bubble level, in your browser.

How to use it

1. Flat surfaces (a shelf, a table, a floor): lay the phone on its back. The big number is the total tilt; the line below splits it into left–right and front–back.
2. Edges and frames (a picture frame, a door head): stand the phone on its long or short edge. The page switches to a level line and says which end is low.
3. Hold reading freezes the number, so you can read it after you take the phone away.
4. % slope shows the rise per 100 units of length, which is how drains, ramps and patios are often specified.

The bubble is “level” inside 0.25°. Many phones have a camera bump: lay the phone on a book or a flat case, or use the edge mode, so the bump does not tilt it.

Check your phone in 20 seconds

Every phone's tilt sensor has a small error from the factory. You can measure it on any surface, flat or not:

1. Lay the phone down and read the angle. Draw round it with a pencil, or press it against a straight edge.
2. Turn the phone half a turn (180°) and put it back in exactly the same place. Read the angle again.
3. The average of the two readings is the true slope of the surface. Half the difference is your phone's own error.

Example: 1.4° and then 0.2° the other way round. The surface slopes 0.6°, and the phone reads 0.8° wrong. The Level & Compass app does this sum for you (we call it flip calibration) and then removes the error from every reading.

What the app does better

This page is a quick level. The Level & Compass app is the careful one:

• Flip calibration removes your phone's own error, so readings go from about ±1° to about ±0.2°.
• Every reading shows how far to trust it, as a dashed halo round the bubble.
• It works offline, with a gentle tap when you reach level, and a compass that warns you about magnets.