The Ruler is a very simple tool.
“⇄” button switches units between inches and centimeters.
“⇅” button switches the 0 position of the ruler.
“C” button resets the ruler position and the red line position.
“>>” forward button forwards the ruler.
1. Something is shorter than the ruler.
Place it on your iPhone or iPad.
Drag the red line bar. Align the red line to match the end of the thing.
Read the number.
2. Something is longer than the ruler.
Prepare a small piece of paper like in the picture.
Align the left end of both the ruler and the object.
Align the right end of the paper on the red line.
Keep hold the paper and move the ruler(iPhone) to match the left end of the ruler and the right end of the paper.
Tap Forward button >>.
Keep hold the ruler and move the paper to match the right end of the paper and the red line.
Move the ruler to match the left end of the ruler and the right end of the paper.
Tap forward button (>>).
Finally drag the red line bar and read the number.