Spirit level online

Inside a phone, an accelerometer feels which way gravity pulls and a gyroscope tracks how fast the phone turns. The phone combines the two into one orientation sensor reading: how far it is tipped forwards or back and from side to side. The browser passes those angles to the page, which works out where straight up lies and draws the bubble where a real air bubble would float.

The W3C DeviceOrientation Event Specification defines the three angles a browser reports. Two of them, the tip front to back and the tip side to side, are all a level needs. The third, a turn about the vertical, makes no difference to which way is up, so the level leaves it out.

Browsers send a new angle only when it changes, so the display moves on the screen's own clock and eases towards the latest value. The Steadiness setting decides how quickly. Quick follows at once but trembles with the hand, Steady smooths out shaking but lags behind a fast change, and Normal sits between the two.

The Flat, Edge and Angle views each expect the phone in one position, and each reads a different line. A fourth, Convert, needs no sensor at all.

Flat is the round vial of a surface level, often called a bullseye. The phone lies on its back and the bubble floats towards the higher side. Rings mark 0.5°, 2° and 5°, and the rim stands for 10°. In between, the bubble moves by the square root of the tilt, so a small tilt still shows clearly.

Under the bubble the page gives the tilt of the surface, which is its steepest slope in whatever direction that runs. It also splits the slope into left to right and bottom to top, each with the side that is higher. That shows which table leg or washing machine foot is the low one.

Edge works like the tube level of a builder's level. The phone stands on any edge, in portrait or landscape and upside down too, and a vial is drawn along the edge it stands on. Where the screen does not rotate, the drawing turns with the phone. The reading is that edge's slope, with the end that is higher.

The same reading checks plumb. A side edge held flat against a door frame or a post keeps the bottom edge at right angles to it. A frame leaning out of true then shows as a slope of the same size. It is the check a plumb line makes, read as an angle instead of a gap. A second line gives how far the screen itself leans from upright, for a wall with the back of the phone pressed on it. With no lean at all it reads upright (plumb).

Angle is an inclinometer, the tool some shops sell as an angle finder. It gives the angle of the phone's long side from level, up to 90 degrees, and says whether its top end is higher or lower. The figure is the same with the phone flat on a ramp or standing on an edge along a rafter or a stair. Lying flat, it also shows the steepest slope beneath it.

In Flat and Edge, a reading that shows 0.0° turns green. Beep when level adds a short tone, and a short buzz on phones whose browser offers vibration; Safari on an iPhone does not vibrate. The length of a shelf is a job for the online ruler, which draws real centimetres once it is set against a bank card or a coin.

Tap Start the level, lay the phone on the surface or stand it on an edge, and read the figure once it settles. Nothing is read from the sensor before that tap, and on Apple's phones and tablets the browser asks a question first.

1. Choose Flat, Edge or Angle and tap Start the level.
2. On an iPhone or iPad, Safari asks for access to motion and orientation, and that question can only come from the tap. Chrome on Android starts reading straight away.
3. Lay the phone face up for the bubble, on an edge for a shelf or a frame, or with its long side along a slope for an angle.
4. Let the figure settle, then tap Hold to freeze the tilt so the phone can be picked up and read. Release lets it move again.

Zero here makes the current surface the reference. Every later reading in that position then shows the difference from it, for example to set a second shelf to match the first. It lasts until Clear zero is tapped or the page is closed.

Full screen covers the page with the level alone, and Close full screen or the Escape key ends it. While the level runs, the page asks the browser to keep the screen awake where it can, so the display does not dim halfway through a job.

If no reading arrives within 1.5 seconds, the page says the device or browser gives no orientation reading and offers the converter. Most laptops and desktop computers have no such sensor. The page address keeps the view, the unit, the steadiness, the chosen rule and the converter's values, so a link opens the same setup on another phone. A reading never goes into the link or leaves the browser.

A 1 in 12 slope is 4.76°, a gradient of 8.33% or 83 mm/m. A ratio of 1 in n, written 1:12 on drawings, means one unit of rise for every n units measured horizontally. At 1 in 12, a rise of 0.5 m needs 6 m of run.

Smaller numbers are steeper: 1 in 5 is 11.31°, the 20% of a steep hill sign, and 1 in 1 is 45.0°.

A percent slope is the rise per 100 of run, so 100% is 45.0°, not a vertical wall. Degrees measure the angle itself and grow more slowly than the percentage: 10.0° is 17.63%, and 1.0° is 1.75%. Millimetres per metre give the same gradient per 1,000 of run. They are a handy way to state a fall, the drop a pipe or a patio needs for water to run off.

The Convert view turns any one of these into the others on any device. It takes an angle, a percentage, millimetres per metre, a ratio, or a rise and a run in the same unit. It opens on 1:12, which gives 4.76°. The large figure is the angle, or the percentage when degrees are typed, and a table lists every UK limit below in all four forms. The worked cases run those limits through it, together with the Highway Code's own example.

A phone level is accurate enough for everyday jobs, not for setting out building work. Browsers report the orientation angles to 0.1°, the precision the W3C specification limits them to. Chromium has rounded them that way since 3 March 2020, in a change made to protect privacy. One step of 0.1° is 1.75 mm/m, so the reading moves in jumps of that size.

Stabila gives its aluminium spirit levels an accuracy of 0.5 mm/m in the normal position, about 0.029°, and up to 1 mm/m reversed. A calibrated phone therefore suits a picture, a shelf, a washing machine, a caravan or a quick look at a ramp. It does not replace a builder's level where a few millimetres over a long run matter.

The sensor also has to read true. Bosch Sensortec's BMI270 datasheet gives that accelerometer a typical zero-g offset of 20 mg, about 1.15° or 20 mm/m. That is the raw part's figure before the phone maker calibrates it, not the error of a finished phone.

In a 2021 study in Behavior Research Methods, Kuhlmann, Garaizar and Reips measured 56 phones through the browser. They found mean inaccuracies per device of up to 2.1° for pitch and 6.6° for roll, which is why a calibration matters more than the display's last digit.

A phone lying face up also rests on whatever sticks out. A camera bump lifts one end, and a phone case can level that out or add a tilt of its own. Both become part of the phone's error lying flat, and the calibration below measures them together with the sensor.

The reversal method finds the phone's own error from two readings of one surface, so the surface does not need to be level. Turning the phone round reverses the surface's slope but not the phone's error.

Johnson Level & Tool describes the same test for a bubble vial. The level is reversed end for end on the same spot, and a true vial shows the bubble in the same place. Turning the phone round on the spot is the phone's version of that flip test.

1. With the level running, open Calibrate this phone (reversal method) and lay or stand the phone the way it will measure.
2. Tap First reading and keep still: after a 3 second countdown the page averages the readings over 1.5 seconds.
3. Turn the phone half a turn on the spot, the same side down, and tap Second reading.

The error is then half the sum of the two readings, and the slope of the surface is half their difference. A first reading of 0.9° and a second of -0.3° give an error of 0.3° and a true slope of 0.6°. From then on the page takes that error off every reading in the same position.

A reading is refused if the phone moved by more than 0.5° while it was taken, or if the second was made lying or standing another way. An error above 5° is refused too, since the two readings cannot then come from one surface. Lying flat and each edge are calibrated separately, because a camera bump tilts the phone only when it lies flat.

The result stays in the browser's local storage on that device, under the name spirit-level-calibration, and nowhere else. Clear calibration removes it. Zero here is a different thing: it sets a zero relative to one surface and is forgotten when the page closes.

Approved Document M sets the steepest ramp by its going, the length of one flight measured horizontally. It allows 1:12 for a going of 2 m, 1:15 for 5 m and 1:20 for 10 m, which as angles are 4.76°, 3.81° and 2.86°. The Approved Documents are guidance on meeting the Building Regulations 2010 in England, and the figures here were checked on 21 October 2026.

Each limit also caps the rise of one flight, at 166 mm, 333 mm and 500 mm in turn. No flight is longer than 10 m, and goings in between are interpolated. Landings at the foot and head are at least 1.2 m long and intermediate ones 1.5 m.

For wheelchair users, Approved Document M expects a lift or another way up once a ramp system rises more than 2 m. Steps are provided as well once it rises more than 300 mm. A gradient of 1:20 or steeper counts as ramped.

A route counts as level in England when its gradient along the direction of travel is no more than 1:60. Scotland's Technical Handbook sets 1:50 for a level route instead. Across a path or a landing, the cross-fall, the slope at right angles to the way of travel, is at most 1:40.

Approved Document K caps the pitch of a private stair at 42°, the angle between the horizontal and a line through the nosings. Each rise runs from 150 to 220 mm and each going from 220 to 300 mm, with twice the rise plus the going between 550 and 700 mm. Scotland allows the same pitch for a private stair and 34° for any other.

Approved Document H works the other way round: its gradients for foul and rainwater drains are minimum falls, and a paved area falls at least 1:60. A WC branch pipe falls 18 to 90 mm per metre. Against these limits the check reports a pipe that is flatter than the minimum.

In the Angle view the same list sits under Check against. Laid along a ramp, a stair or a pipe, the phone then says whether the angle is within the limit, too steep, or flatter than a drain's minimum. Northern Ireland has its own building regulations, whose figures are not included here.

A steep hill sign gives the slope as a percentage: 20% means the road climbs or drops 20 m for every 100 m measured horizontally. That is 1 in 5, or 11.31°.

The Highway Code notes that gradients may be shown as a ratio and gives 20% = 1:5 as its example. The Department for Transport's Traffic Signs Manual explains why both forms appear: signs now use the percentage, while older signs giving a ratio may remain.

The manual normally places the downhill warning only where the road falls 10% or more. The uphill one starts at 15%, or lower on a long climb. The figure on the sign is rounded to the nearest whole number.

A steep descent also lengthens the distance a car needs to stop. The stopping distance calculator takes the slope of the road in percent and shows how much further a car travels downhill.

An iPhone has a level of its own in the Measure app. Apple's guide "Use iPhone as a level" describes it: open Measure, tap Level, and the display turns green when the phone is level. Apple adds that the measurements are approximate.

Android phones differ by maker, and a level may or may not come with one. A page in the browser needs no app either way. This one runs in Chrome or Safari, keeps its calibration on the device, and gives percent, millimetres per metre and the UK limits as well as degrees.

The same motion sensors drive the online compass, which adds the phone's magnetometer to find north and keep a bearing.