A strain gauge is a device used to measure deformation (strain) of an object. The most common type of strain gauge consists of a flexible backing which supports a metallic foil pattern etched onto the backing. As the object is deformed, the foil pattern is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge circuit, can be used to calculate the exact amount of deformation by means of the quantity known as the gauge factor.
The gauge factor of a strain gauge relates strain to change in electrical resistance. The gauge factor GF is defined by the formula 
where RG is the resistance of the undeformed gauge, ΔR is the change in resistance caused by strain, and ε is strain.
For measurements of small strain, semiconductor strain gauges are often preferred over foil gauges. A semiconductor gauge usually has a larger gauge factor than a foil gauge. Semiconductor gauges tend to be more expensive, more sensitive to temperature changes, and are more fragile than foil gauges.