How accurate is a phone ruler compared to a physical ruler?

Under good lighting and for objects under 15–20 cm, a camera-based ruler can get within 1–2 mm. Accuracy drops for longer objects and if the phone is not held perfectly parallel to the surface. Always cross-check critical measurements with a physical ruler.

Why does the spirit level on my phone seem slightly off?

Factory calibration of accelerometers is close but not perfect. Most spirit-level apps, including Carbide's, let you apply a manual offset: place the phone on a surface you know is level, then tap 'calibrate' to zero the reading.

Can I use my phone compass without an internet connection?

Yes. The compass reads the magnetometer chip directly — it needs no GPS signal and no internet. Carbide works fully offline, so your heading is always available even in a basement or a remote location.

What can throw off a phone compass reading?

Metal surfaces, magnetic phone cases, nearby speakers, and even reinforced concrete walls can deflect the magnetometer. Performing the figure-eight calibration gesture and keeping the phone away from these materials will give you a more reliable heading.

Guide

How to measure things with your phone

Your phone already carries a set of sensors — camera, accelerometer, magnetometer, gyroscope — that can stand in for several physical measuring tools. This guide shows you how to use them as a ruler, spirit level, compass, and protractor, what to watch out for, and how to get the most accurate results possible.

Use your camera as a ruler

The Ruler tool in Carbide overlays a calibrated scale on your camera feed so you can measure small objects — a screw, a piece of trim, a gap — without hunting for a tape measure. Lay the object on a flat, well-lit surface, hold your phone directly above it, and align the on-screen guides. Accuracy is best for objects under 20 cm; for anything longer, a physical tape measure is still more reliable. Carbide runs entirely on your device, so no photo is ever uploaded.

Check a surface is level or plumb

The Spirit Level tool reads your phone's accelerometer in real time and shows a bubble dial that settles at centre when the device — and whatever it rests on — is perfectly level. Place your phone flat on a shelf, a picture frame, or a countertop to check horizontal level, or hold it upright against a wall to check plumb. The sensor is already calibrated at the factory; if the bubble never quite centres on a surface you know is flat, go to Settings and use the calibration offset to zero it out. Carbide is free and works fully offline.

Find your heading with the compass

The Compass tool fuses your phone's magnetometer and accelerometer to show magnetic bearing and the cardinal direction you are facing. It is useful for basic orienteering, positioning a satellite dish, or checking which way a room faces for sunlight. For best accuracy, hold the phone flat and away from metal surfaces, speakers, or magnetic cases — all of which can skew the reading by several degrees. On first use, perform the figure-eight calibration gesture the app prompts you to do; it takes about five seconds and measurably improves accuracy.

Measure angles with the protractor

The Protractor tool turns your camera into an angle-finder. Point it at the corner of a frame, a roof pitch, or a stair riser and read the angle in degrees. It works best on high-contrast edges in good light; shadows or low contrast make edge detection unreliable. For very precise work — cutting trim, for example — cross-check with a physical protractor or digital angle gauge. The protractor, like all Carbide tools, needs no internet connection and collects no data.

Tools in this guide

Ruler Spirit Level Compass Protractor

Frequently asked questions

How accurate is a phone ruler compared to a physical ruler?: Under good lighting and for objects under 15–20 cm, a camera-based ruler can get within 1–2 mm. Accuracy drops for longer objects and if the phone is not held perfectly parallel to the surface. Always cross-check critical measurements with a physical ruler.
Why does the spirit level on my phone seem slightly off?: Factory calibration of accelerometers is close but not perfect. Most spirit-level apps, including Carbide's, let you apply a manual offset: place the phone on a surface you know is level, then tap 'calibrate' to zero the reading.
Can I use my phone compass without an internet connection?: Yes. The compass reads the magnetometer chip directly — it needs no GPS signal and no internet. Carbide works fully offline, so your heading is always available even in a basement or a remote location.
What can throw off a phone compass reading?: Metal surfaces, magnetic phone cases, nearby speakers, and even reinforced concrete walls can deflect the magnetometer. Performing the figure-eight calibration gesture and keeping the phone away from these materials will give you a more reliable heading.