The measurement tool turns any note into something you can measure. Set a scale once, and every line or box you draw shows its real length or area.
Availability
Create measurements: iPad only
Requires Notability Cloud
How it works
The Ruler 
has three modes:
Straight edge: The same Ruler you already know, for drawing straight lines.
Calculate length: Draws a line that shows its real-world length.
Calculate area: Draws a box that shows its real-world area.
Before you measure, you calibrate the scale by telling the note how long one thing is. Each note has one scale, and each measurement can show its own unit.
Calibrate the scale and start measuring
Open the note you want to measure.
In the toolbox, tap the Ruler
.Tap Calculate length or Calculate area.
If the note doesn't have a scale yet, you'll calibrate one first. Draw a line over something you already know the length of, like a wall on a floor plan.
Enter the real length and choose a unit to calibrate the scale. This sets the scale for the whole note.
Draw a line or drag out a box. The measurement appears automatically.
Supported units: millimeters, centimeters, meters, kilometers, inches, feet, yards, and miles.
To change the Ruler's default units between imperial or metric, go to Library > Settings > Note Editor > Handwriting & Drawing > Ruler Units.
Edit a measurement
Tap a measurement to select it and open the side panel. This switches you to the measurement tool automatically.
Drag a handle to resize a line or box. The value updates as you go, and boxes snap into place.
Change the unit in the side panel. The value converts automatically.
Tap Copy in the side panel to copy the value.
Move or delete a measurement like any other item on your note.
Change the scale
Select any measurement to open the side panel.
Tap Reset scale.
Draw a new reference line, then enter its real length and unit.
Every measurement on the note updates to the new scale.
Hide or show measurements
Tap the eye icon in the toolbox to hide or show all measurements on the note. On shared notes, this hides measurements for everyone. Nothing is deleted, and you can show them again anytime.
Troubleshooting
I don't see Calculate length or Calculate area.
Measurements can only be created on iPad. Make sure you're on the latest version of Notability and using Notability Cloud.
A measurement looks too big or too small.
Your scale may be incorrect. Check that your reference line matches a real, known dimension, then reset the scale and draw a new one.