Measure ductility using initial and final gauge length. See strain, true strain, and extension instantly. Download clean tables as CSV or PDF in seconds.
These sample measurements show how percent elongation changes with material behavior and gauge length.
| Material (typical) | L0 | Lf | ΔL | Percent elongation |
|---|---|---|---|---|
| Low-carbon steel (ductile) | 50 mm | 62 mm | 12 mm | 24% |
| Aluminum alloy (moderate) | 50 mm | 58 mm | 8 mm | 16% |
| Cast iron (brittle) | 50 mm | 50.3 mm | 0.3 mm | 0.6% |
| Polymer (highly ductile) | 25 mm | 55 mm | 30 mm | 120% |
Percent elongation measures ductility as the percentage increase in gauge length after a tensile test.
Tip: Percent elongation is unit-independent as long as L0 and Lf use the same unit.
Percent elongation describes how much a specimen length increases before fracture, expressed as a percentage of the original gauge length. In tensile testing, it is a direct indicator of ductility. A higher percent elongation generally means the material can undergo more plastic deformation, which is useful for forming, bending, and absorbing energy under load.
Gauge length is the initial measured length over which deformation is tracked. Common lab values include 25 mm and 50 mm, but standards may define proportional gauge lengths depending on specimen geometry. Because percent elongation is based on L0, results from different gauge lengths are not always directly comparable unless the same test practice is used.
Ductile low-carbon steels often show about 20–40% elongation, while many aluminum alloys fall around 8–25% depending on temper. Brittle materials like gray cast iron can be below 1%. Some polymers and elastomers can exceed 100% elongation, especially under slow strain rates. Use these values as context, not strict limits, because heat treatment, microstructure, and strain rate change the outcome.
Engineering strain uses the original length: ε = ΔL/L0. True strain uses the natural logarithm of the stretch ratio: ln(Lf/L0). True strain is often preferred for large deformations because it accumulates incremental stretching more realistically. This calculator reports both, so you can match your analysis method or reporting requirement.
In many procedures, the two fractured halves are fitted together and the final gauge length is measured along the original gauge marks. This reduces error from gaps and misalignment. If necking is severe, measuring carefully is essential because small length errors can create noticeable differences in percent elongation, especially when L0 is short.
A quick check is ensuring Lf is greater than L0 for a valid tensile extension. If you enter extension directly, confirm the sign convention: extension should be positive for elongation. For repeated tests, record specimen ID, gauge length, and test speed because these factors help explain scatter in reported elongation values.
Designers use elongation to judge formability, crash energy absorption, and resistance to brittle failure. For example, structural components that must deform without sudden fracture often require a minimum elongation in material specifications. Comparing elongation alongside yield strength and ultimate tensile strength provides a balanced view of strength versus ductility.
Clear reports include L0, Lf, ΔL, percent elongation, and the unit used for length. Adding engineering and true strain helps when your audience includes analysts or researchers. Use the CSV export for spreadsheets and the PDF export for documentation packets, audits, and lab record retention.
Not if you use the same unit for L0 and Lf (or ΔL). The ratio ΔL/L0 is dimensionless, so mm, cm, meters, or inches all give the same percentage.
That typically indicates an entry error. In tensile testing, Lf should be greater than L0. Recheck measurements, confirm you fitted fracture faces together, and verify you did not swap fields.
Use “Initial + Final” if you measured Lf after the test. Use “Initial + Extension” if your testing system reports extension directly. Both methods produce the same percent elongation when inputs match.
It depends on material and application. Ductile steels often show 20–40%, many aluminum alloys 8–25%, brittle cast irons can be under 1%, and some polymers exceed 100%.
Elongation includes localized deformation near the neck. Shorter gauge lengths capture more of that local stretch, often yielding higher percent elongation. Comparing results is best when the same L0 and standard are used.
True strain is helpful for large deformations and analysis beyond simple reporting. It uses ln(Lf/L0) and can better represent accumulated stretching. For basic specifications, percent elongation and engineering strain are commonly sufficient.
They include your result table: L0, Lf, ΔL, engineering strain, true strain (if valid), and percent elongation. This format is suitable for lab records, reports, and quick comparisons across tests.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.