How do I convert roof pitch to degrees?

Pitch expressed as rise-over-run converts with the arctangent: a 6:12 pitch is atan(6/12) = 26.57°. This tool does it the other way round — it measures the angle and shows the equivalent pitch in twelfths, rounded to the nearest half, which is how rafters are actually cut.

What is the difference between percent grade and degrees?

Percent grade is rise divided by run, times 100 — so a 45° slope is 100% grade, not 50%. Percent grade is what drainage, paving and accessibility standards use; degrees is what structural drawings use. They are the same angle described differently, and confusing the two is a common and expensive mistake.

Can I measure a roof pitch from inside the loft?

Yes, and it is usually safer and more accurate than working on the roof surface. Lay the phone along the underside of a rafter, tap Lock, and read the value once you are back down. The rafter line is the pitch a roofer needs.

Why does the reading differ from my rafter square?

Almost always because the phone is sitting on the covering rather than the structure. Shingles, tiles and battens all sit proud, and an uneven course tips the phone. Bridge across a few battens with a straightedge and read against that.

What is a minimum drainage slope?

It depends on the pipe and the code in force — the common plumbing rule of thumb for small-diameter waste is a quarter-inch per foot, roughly 2%. Check the drain-pipe slope calculator for the diameter you are running, and confirm against your jurisdiction's plumbing code rather than the rule of thumb.

Does the lock button keep the value if I close the page?

No — the lock holds the reading while the page is open so you can climb down. Write it down or feed it straight into one of the linked rafter calculators.

Can I measure a driveway or a site's fall with this?

Over a short run, yes: lay the phone on a straightedge along the line of fall. Over a long run, a phone measures the local tilt where it sits, and ground is rarely a single plane — take several readings and use the worst case.

Why is the reading always positive?

The tool shows the magnitude of the slope, because up-slope and down-slope are the same pitch from opposite ends. Point the top of the phone up-slope and the reading is the pitch of the surface.

Field tool

Roof Pitch & Slope Clinometer

Lay the long edge of your phone flat against a rafter, drainage pipe, or driveway, with the top of the phone pointing up-slope. Reads the surface's angle relative to level.

Motion sensorsWhat it works from
Phone or tabletAccelerometer · HTTPS

Timber-framed house under construction with ladders, sawhorses and lumber

Checking this device for a motion sensor…

Rise-over-run is rounded to the nearest half-inch, matching how roof pitches are conventionally quoted (e.g. “5:12”). This measures the angle of whatever flat surface the phone rests against — it doesn't verify the surface itself is a true, unwarped plane.

How to use the Roof Pitch & Slope Clinometer

Open the tool on a phone
Motion sensors are required, so use a phone or tablet over HTTPS.
Place the long edge on the slope
Lay the long edge of the phone flat on the rafter, pipe or paving, with the top of the phone pointing up-slope.
Lock the reading
Tap Lock to freeze the value before you move. This is what makes the tool usable overhead or at arm's length in a loft.
Read the three forms
Degrees for drawings, percent grade for drainage, and rise-over-run in twelfths for roofing.

Where this tool stops

It measures the surface the phone is touching. A tile or shingle sits proud of the rafter line, so read against a rafter, a batten or a straightedge for a structural pitch.