Fold Protractor

Measure with the hinge (iPhone Duo)

1. Open the Duo past 10°.
2. Set it into a corner, or lay the two halves against two edges.
3. Read the angle. On the open Duo it sits right of the fold.
4. Pick Inside or Outside at the top of the right-edge column.
5. Tap Hold (pause icon) to freeze the reading. Tap again to release.

Calibrate the hinge

1. Unfold the Duo partway or fully, then tap Calibrate (sliders icon).
2. Open the phone flat on a table and tap Set as 180° (Open Flat). Or open it inside a square corner and tap Set as 90° Inside (Square Corner).
3. “Calibrated” then shows under the reading.
4. To undo it, tap Reset Calibration.

Use the level

1. Stand the phone on an edge for rotation, or lay it flat for surface tilt. It switches by itself.
2. Tap Lock (padlock, bottom right) to set the current angle as zero. Tap it again to unlock.
3. Tap Hold (pause icon, bottom left) to freeze the reading.
4. A folded Duo works as a level too. Unfold it partway to measure with the hinge.

Use the rulers

1. The rulers run down both long edges of the level. They start at the top edge of the phone.
2. On an open iPhone Duo, rulers also run along the top and bottom of the inner display, starting at the left edge of the phone. Their reset button appears at the top of the right-hand column.
3. To measure past the end of the phone, turn off Lock Ruler to Edge in Settings. Drag a ruler, keep your finger still, and slide the phone along.
4. Tap the reset button between Target and Settings, or double tap a ruler, to put it back.
5. To match your own ruler or tape, tap Adjust Rulers in Settings and drag a ruler to move the marks, up to 1/4 in either way. Touch and hold for a magnifier. Tap Done to save or Reset to go back to 0.

Set a target

1. Tap Target (crosshair, top left on the level).
2. Drag the dot around the dial, tap a preset, or tap the number in the middle to type an exact value.
3. Tap the checkmark to set it. Clear removes the target.
4. The screen is red until the reading is within tolerance, then green.

Settings

• Tolerance: ±0.5°, ±1° (default) or ±2°. How close counts as on target, level or plumb.
• Hinge Reading: inside or outside (iPhone Duo).
• Lock Ruler to Edge: on by default. Turn it off to slide the rulers.
• Adjust Rulers: line the ruler marks up with a real ruler. Reset Offset sets them back to 0.

On the level, Settings is the top-right button. On the hinge gauge it is the Calibrate button.

Requirements

• iPhone with iOS 26 or later.
• The hinge gauge needs iPhone Duo with iOS 27.1 or later.
• On iPad it runs in an iPhone window, without the rulers.
• Works offline. No account.