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 R_G 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.