Elongation Calculator — Tensile Test Ductility
Enter the original and final gauge lengths from a tensile test to get percentage elongation, engineering strain, true (logarithmic) strain and — if you supply cross-section areas — reduction of area.
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mm
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Percentage increase in gauge length after fracture
- 1
Elongation ΔL = Lf − L₀
62.5 mm − 50 mm = 12.5 - 2
Engineering strain ΔL ÷ L₀
12.5 ÷ 50 = 0.25 - 3
Elongation % = strain × 100
0.25 × 100 = 25
How does this calculator work?
Elongation % = (Lf − L₀) / L₀ × 100 where L₀ is the original gauge length and Lf is the gauge length after fracture. Engineering strain = (Lf − L₀) / L₀; true strain = ln(Lf / L₀). Optional: reduction of area = (A₀ − Af) / A₀ × 100. Standard gauge lengths are 50 mm (ASTM) or 5.65√A₀ (ISO); compare only values measured at the same standard.
Formula
How this is calculated
Percentage elongation is the primary ductility metric from a tensile test. A specimen with a defined gauge length L₀ is pulled to fracture; the two halves are fitted back together and the extended gauge length Lf is measured. Elongation % = (Lf − L₀) / L₀ × 100 gives the permanent plastic deformation as a percentage of the original length. Standard gauge lengths are 50 mm (ASTM E8) or proportional (5.65√A₀ under ISO 6892-1).
Engineering strain treats the original length as the fixed reference: ε_eng = ΔL / L₀. True (logarithmic) strain integrates over the continuously changing length: ε_true = ln(Lf / L₀). For small strains the two are nearly equal; they diverge for large plastic strains — true strain is the physically rigorous measure used in metal forming and FEA constitutive models.
Reduction of area (RA%) measures the necking at the fracture point: RA% = (A₀ − Af) / A₀ × 100. Together, elongation and RA% characterise ductility. Note that elongation depends on the gauge-to-diameter ratio, so always compare values at the same standard gauge length. This calculator uses the basic formula without the short-gauge correction factor.
Frequently asked questions
Mild steel (e.g. A36) typically shows 20–23% elongation at a 50 mm gauge. High-strength low-alloy (HSLA) steels range 15–22%. Aluminium 6061-T6 is about 10–17%. Cast iron is brittle at under 2%. Always compare values measured at the same gauge length standard (ASTM vs. ISO).
Most plastic deformation concentrates in the necking region near the fracture. A short gauge length encompasses a larger fraction of that localised deformation relative to its total length, so the calculated percentage is higher than for a long gauge length on the same specimen. Standards use proportional gauge lengths to make values comparable.
At small strains (below ≈ 5%), the two differ by less than 0.1%. At 20% engineering strain, true strain is ≈ 18.2%, a 1.8% gap. At 50% engineering strain, true strain is only ≈ 40.5%. For work-hardening models and forming simulations, always use true (logarithmic) strain.
Also known as
TG we-Calculate Editorial Team. (2026). Elongation Calculator — Tensile Test Ductility [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/elongation-calculator
TG we-Calculate Editorial Team. "Elongation Calculator — Tensile Test Ductility." TG we-Calculate. 2026. https://we-calculate.com/calculator/elongation-calculator.
TG we-Calculate Editorial Team, "Elongation Calculator — Tensile Test Ductility," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/elongation-calculator
@misc{wecalculate_elongation_calculator, title = {Elongation Calculator — Tensile Test Ductility}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/elongation-calculator}}, year = {2026}, note = {TG we-Calculate} }
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