You can measure roof pitch with a smartphone app (free), a basic digital angle finder ($25), or a laser distance meter with built-in inclinometer ($100-300). Which tool is right depends on the accuracy you need, the access you have to the roof itself, and how often you will repeat the measurement.
Spoiler: the cheap tools are good enough for almost everything residential. The expensive tools are worth their cost only in specific professional contexts.
Phone apps: free, accurate enough, terrible workflow
Modern smartphones include a 3-axis accelerometer accurate to about 0.5° in the orientation that matters for roof pitch (lying flat against a rafter or roof surface). Built-in level apps and free third-party apps like RoofPitch, Pitch Gauge, and Measure all read the angle directly. The hardware is good enough for residential pitch measurement.
The workflow is the problem. Phones are slippery, screens are hard to read in bright sun, and the apps that exist are mostly bloated with ads or weirdly-designed UI. You also do not want to lay your phone face-down on a rough roof surface, which limits the practical placements.
Use a phone app for occasional measurement, when you are inside an attic measuring a rafter, or as a sanity check on another tool. Do not use it as your primary measurement tool if you are doing this work weekly.
A $25 digital angle finder is the right answer for most people
A basic digital angle finder with a magnetic base is the right tool for most residential pitch measurement. Wixey, AccuMaster, and Bosch all make capable models in the $20-50 range. Accuracy is typically ±0.1° in good models, which is more than adequate.
The magnetic base is the key feature. You can stick it to the bottom edge of a steel-frame square set against the roof, to a rafter, or to a metal-roof panel directly. Hands-free operation, easy-to-read display, and durable enough for daily use.
For roof pitch specifically: place a level or a long straight edge against the roof surface (or under a rafter), set the angle finder on the level, and read the angle directly. Convert to rise/12 with the calculator on this site if your work requires it.
Laser distance meters with inclinometer: worth it for pros
Premium laser distance meters from Leica, Bosch, and Stanley include built-in inclinometers and can compute pitch directly from a single laser shot to a roof surface. The Leica DISTO D2 and S910 are standout examples in the $300-800 range.
For a professional roofer measuring 50+ roofs a year, the workflow advantage is substantial. One-handed operation, instant readout, automatic conversion between rise/12, percent, and degrees, and the ability to measure across distances where you cannot place a tool against the surface.
For a homeowner measuring one roof, this is overkill. The $25 angle finder produces equivalent accuracy with a few extra steps.
My recommendation
For one-time homeowner measurement: a phone app works in a pinch. A $25 digital angle finder is worth the small investment if you are doing any DIY framing or roofing work.
For a contractor doing this weekly: a $25-50 magnetic-base digital angle finder is the right primary tool. Add a phone app as a backup. A $300 laser distance meter is worth the upgrade only if your work involves measuring across distances where you cannot easily access the roof surface.
No tool is a substitute for the underlying math. Use the calculator on this site to convert between formats, validate your measurements with a quick rise/run sanity check, and document everything for the file.