What tool is use to measure a strap on a backpack?

Answer 1

It depends on what you want to measure: its length. width, thickness, strength, elasticity, resistance to abrasion. Since you have not bothered to provide that crucial bit of information, I cannot provide a more useful answer.

Answer 2

A tape measure is commonly used to measure the strap length on a backpack. Simply extend the tape measure along the length of the strap to determine its measurements.