Gauge factor (GF) or strain factor of a strain gauge is the ratio of relative change in electrical resistance R, to the mechanical strain ε. The gauge factor is defined as:

${\displaystyle GF={\frac {\Delta R/R}{\Delta L/L}}={\frac {\Delta R/R}{\varepsilon }}=1+2\nu +{\frac {\Delta \rho /\rho }{\varepsilon }}}$

where

• ε = strain = ${\displaystyle \Delta L/L_{0}}$
  • ${\displaystyle \Delta L}$ = absolute change in length
  • ${\displaystyle L_{0}}$ = original length
• ν = Poisson's ratio
• ρ = resistivity
• ΔR = change in strain gauge resistance due axial strain and lateral strain
• R = unstrained resistance of strain gauge