iOS iPhone Education
A ruler, or rule, is an instrument used in geometry, technical drawing and engineering/building to measure distances and/or to rule straight lines. Strictly speaking, the ruler is essentially a straightedge used to rule lines and the calibrated instrument used for determining measurement is called a "measure". However, common usage is that a ruler is a calibrated straightedge that can be used for making measurements.
You can use this app to measure in cms or inches.