How to use your phone as a spirit (bubble) level and calibrate it

Short answer: lay the phone flat on the surface you want to check, or stand it on a long edge to check a wall, and read the bubble. For an accurate result, calibrate once with two readings taken 180 degrees apart. This guide explains how it works and how to get the best reading.

How a phone level works

Your phone has an accelerometer that measures the direction of gravity. When the phone is tilted, gravity pulls along the phone's axes in a different mix, and the app turns that mix into an angle. A level app then draws the angle as a bubble that moves the same way as the bubble in a glass vial: it drifts toward the high side.

An app can show the angle in a few ways. Engineer Tools shows a circular level and a horizontal and a vertical vial together, so one placement of the phone gives you every direction at once.

Place the phone correctly

• Flat on a surface (screen up): reads tilt in every direction, like a circular bullseye level. Use it for floors, tabletops and appliances.
• Standing on a long edge: reads one direction only, like a tube level. Use it for shelves, pictures and door frames.
• Remove the case if it is thick or uneven, and keep the phone's edges clean.
• Watch the camera bump. If the phone rocks on a table, stand it on its edge or put it on a flat board and use the board.

Calibrate with a two-point flip

Every motion sensor has a small offset. A calibration measures it and subtracts it from every reading. The two-point flip needs no perfectly level reference:

1. Put the phone on the surface and let go. The app takes the first reading, which is the true tilt plus the sensor offset.
2. Turn the phone 180 degrees on the same spot and let go. The true tilt now has the opposite sign, but the offset is the same, so the second reading is the offset minus the true tilt.
3. The app adds the two readings and halves the result. The tilt cancels out and what is left is the offset, which it then removes.

In Engineer Tools, open Calibration and choose Precise calibration. A Quick calibration is also available for a surface you already know is perfectly level.

Read the result

• Centre the bubble, or read the number. Many apps show degrees. You can also read percent, millimetres per metre, inches per foot or a 1:n ratio. For conversions, see degrees to percent, mm/m and in/ft.
• Wait for the phone to be still. A steady hand and a surface that is not wobbling give a steadier reading, and an auto-hold feature can freeze the value once the phone stops moving.
• Set a zero point to measure an angle relative to a surface rather than to gravity.

How accurate is it?

After calibration, a phone can usually read tilt to a fraction of a degree. For a shelf or a washing machine, that is comfortably enough. For structural work, a roof frame or anything where safety depends on the reading, confirm the result with a dedicated level or a surveying tool.

Tips for the best result

• Calibrate again if you change phone, case or temperature a lot.
• Take the reading twice, once as it is and once turned 180 degrees. If the two agree, you can trust the number.
• Avoid placing the phone on a surface that vibrates, such as a running washing machine.

Engineer Tools also has an angle finder, a compass, a true-size ruler and an on-screen protractor, and works offline.