How is elongation calculated for a tested metal sample?

In the context of materials testing, elongation is expressed as strain, which is defined as the change in length per unit length of a material sample under tension.1

To calculate this value, you measure the initial length of the test section (the gauge length) and the final length after the material has been stretched under a specific load. The formula for strain is:

$$\text{Strain}=\frac{\text{Final Length}-\text{Initial Length}}{\text{Initial Length}}$$

For example, if a 100-millimetre steel bar elongates by 1 millimetre under a given load, the calculation is $(101-100)/100=1/100$, resulting in a strain of 1 percent. During testing, an extensometer is typically used to measure these changes in the gauge length at various loads to determine the material's ductility.12