VaneHow to Use Your iPhone as a Spirit Level, Accurately
An iPhone makes a good spirit level, once you know its one weakness: a phone can be off by a degree or so out of the box. A ten-second check finds the error, and zeroing removes it.
Updated 22 October 2026
Two ways to hold the phone
- Flat, screen up, for a table, a floor, a washing machine or a caravan: a round bubble shows the tilt in both directions at once.
- On an edge, for a shelf, a picture frame or a door frame: a beam level shows the tilt along that edge. Stand it on its long edge for a long surface.
Every iPhone has a basic level built in: open the Measure app and tap Level. Recent versions of iOS can also show a level line in the Camera app, for straight horizons, under Settings, Camera.
How accurate is it?
The level reads the phone's accelerometer, which senses which way gravity pulls. Studies that compared phones with clinical instruments found them close, and their readings very consistent. But consistent is not the same as accurate: tested on certified angle blocks, one of two iPhones in a study read about 1.1° low on every measurement. That is a fixed offset in that phone, and a fixed offset is easy to find and remove.
The other errors come from the phone's shape, not its sensor:
- The camera bump. Laid flat on its back, the phone rocks on the camera. Measure screen-down, or use a case with a flat back.
- Rounded edges and cases. A thick or uneven case tilts the phone; so do buttons on the edge you stand it on.
- A short base. The phone is about 15 cm long. On a rough or bowed surface, lay a straight piece of wood or a ruler first and put the phone on that.
The two-way check
Carpenters have always tested a spirit level this way, and it works for a phone:
- Put the phone on a surface and read the angle, say +0.8°.
- Turn it end for end, 180° on the same spot, and read again, say −0.4°.
- The surface's real tilt is half the difference of the two readings: (0.8 − (−0.4)) ÷ 2 = 0.6°. The phone's own error is half their sum: (0.8 + (−0.4)) ÷ 2 = 0.2°.
If both readings are the same size with opposite signs, the phone is true. If not, either correct by the error you found, or zero the phone on a surface you know is level and measure from there.
Degrees, percent and mm per metre
Builders, plumbers and road signs describe slope in different units. They convert like this:
| Degrees | Percent | mm per metre | Inches per foot |
|---|---|---|---|
| 0.1° | 0.17% | 1.7 mm | 0.02 in |
| 0.5° | 0.87% | 8.7 mm | 0.10 in |
| 1° | 1.75% | 17.5 mm | 0.21 in |
| 2° | 3.49% | 34.9 mm | 0.42 in |
| 5° | 8.75% | 87.5 mm | 1.05 in |
A useful sense of scale: on a shelf one metre long, 1° puts one end 17.5 mm higher than the other, which anyone can see. 0.1° is under 2 mm, which almost nobody can.
When to fetch a real level
A phone is fine for pictures, shelves, furniture, appliances and getting a caravan or camper level enough to sleep in. For work that has to be right to a fraction of a millimetre per metre, such as setting out a long drain fall, laying a long worktop or hanging kitchen units in a run, use a builder's level or a laser: a good one is calibrated, has a long straight base, and does not depend on the shape of a phone.
Sources
- Validation of smartphone inclinometer tools for measuring rib hump in scoliosis patients: an iPhone 4 and an iPhone 5 on certified angle gauge blocks from −30° to 30°; one read 1.1° low on average (0.7 to 1.6), consistently.
- Lau et al. (2020), smartphone inclinometer vs goniometer for shoulder internal rotation, Journal of Physical Fitness, Medicine & Treatment in Sports.
- Conversions: percent grade = tan(angle) × 100; mm per metre = percent × 10.
Common questions
Does the iPhone have a built-in level?
Yes. Open the Measure app and tap Level. Lay the phone flat for a bubble, or stand it on an edge to level a shelf or frame. It shows the angle in degrees and turns green at level.
How accurate is an iPhone level?
Usually within a degree or two, and much better after you check it. Studies found phones close to clinical instruments, but on certified angle blocks one iPhone read about 1.1 degrees low every time. Measure the same spot both ways round, or zero the phone on a surface you trust, and that fixed error goes away.
What is 1 degree in percent or mm per metre?
1 degree is a slope of about 1.75%, or 17.5 mm over a metre (about 0.2 inch per foot). 1% is 0.57 degrees, or 10 mm per metre.