A device attached to the surface of a piece of equipment to measure the amount of strain acting on the surface. A strain gage is a piece of extremely fine wire bonded (glued) to the surface, with the length of the wire parallel to direction of the strain to be measured. When the surface is stressed, it is also strained (deformed), and it either stretches or compresses. Since the strain gage is bonded to the surface, it also stretches or compresses.

The resistance of the strain gage is measured with a precision bridge-type instrument, and the resistance change caused by the wire stretching or compressing is measured. Strain gages are calibrated so the change in resistance indicates the amount of strain acting on the surface.