Spirit Level (Bubble Level)

About the Spirit Level (Bubble Level)

Turn a phone or tablet into a spirit level. Lay it on a shelf, a table or a floor and the round bull’s-eye bubble shows which way the surface slopes; stand it on its long or short edge against a picture frame, a door or a worktop and the tube bubble levels that edge. The page reads the angle in degrees to the tenth, the slope as a percentage, the rise in millimetres per metre (inches per foot in the United States) and as a ratio such as 1 in 50.

Phones are not built as levels: every sensor has a small offset of its own, and a case or a camera bump tilts the phone itself. So the page has a proper two-step calibration, the way builders check a level: read, turn the device half a turn on the same spot, read again. It cancels the sensor’s offset and tells you how much the surface itself slopes. There is also Set zero here, Hold to freeze a reading you cannot see, and an optional tone when level.

It uses the motion sensors your browser lets a page read, after asking where the browser asks; nothing is sent anywhere.

Lying flat or on an edge

• Lying flat (the bull’s-eye): the device lies on the surface. The bubble shows the direction and amount of the tilt in both directions at once; the readings give the slope of the top edge and of the right side, and the angle in the steepest direction.
• On an edge (the tube): the device stands upright on one of its edges, the screen facing you. The reading is the slope of the edge it stands on: positive when the right end is higher. It works on any of the four edges, and with the screen rotated or not.
• Auto chooses between them from how the device is held: lying flat below 45° from level, on an edge above (with a little room either way so it does not flicker).

Calibration: the reversal method

A level that reads 0.4° on a level shelf is no use, and a phone can do that because of a sensor offset, a case, or a camera bump. The cure is the check builders use on any level: read it, turn it end for end on the same spot, read it again. Turning flips the surface’s slope as the device sees it but not the device’s own error, so the error is the average of the two readings and the surface’s real slope is half their difference.

Calibrate in the case you will use, on a firm surface close to level: once lying flat, and once on an edge for each pair of edges you measure with (the top and bottom edges are one pair, the two sides the other). Each calibration is used only in its own position, because a case or a camera bump tilts a phone lying flat but not one standing on an edge. On an edge the second reading stands the device on its opposite edge, so use edges that are straight and free of buttons. The page waits a moment after each tap so that the tap does not count, checks that the device was still and was really turned, and stores the calibration in this browser only. Set zero here is the quick alternative: it treats the current surface as level, which is right only when you know it is, and it works within 5° of level.

Slope, percent and ratios

The same slope is written several ways, and the page shows them all: degrees, a percentage (100 × tan of the angle), a rise per run (millimetres per metre, inches per foot) and a ratio (1 in N, the run for one unit of rise). Some values to recognise:

• 1 in 100 = 1% = 10 mm per metre = 0.57°
• 1 in 50 = 2% = 20 mm per metre = 1.15°
• 1 in 12 = 8.3% = about 83 mm per metre = 4.76°
• 1 in 1 = 100% = 45°

For roofs and ramps there are dedicated calculators: Slope Calculator and Roof Pitch Calculator.

How accurate is a phone level?

Browsers report the device’s tilt in steps no finer than a tenth of a degree, as the W3C DeviceOrientation specification asks them to, so 0.1° is the finest step the page can show. In practice the limits are the phone’s own sensor and how it sits: calibrate it, keep it still, and make sure it rests on its back or edge rather than on a camera bump or a button. For work where tenths of a degree matter, check the result with a good spirit level.

Sources

• W3C, DeviceOrientation Event Specification: the angles browsers report, their precision and the permission prompt.
• The slope conversions are plain trigonometry: percent = 100 × tan θ, rise per metre = 1000 × tan θ mm, 1 in N with N = 1 ÷ tan θ.

Limitations

• The page can only read the sensors the browser exposes: a device without motion sensors, such as a desktop computer, cannot be used as a level.
• Readings come in steps of 0.1°, and each phone’s sensor has its own small error: calibrate before you rely on the tenths.
• A curved back, a camera bump or a case changes how the phone sits. Calibrate with the device exactly as you will use it.
• Browsers stop sending sensor readings while the page is in the background; the level resumes when you come back.
• Phones cannot measure a level line across a room; for that you need a laser level or a water level.

Privacy

On this device: nothing is sent

Sensor readings stay in this browser: they are drawn on the page and nothing is uploaded. Your calibration and settings are saved only in this browser. Motion access is asked for by the browser, not by us.