What next?
Strain pairs with stress to give stiffness.
How to use this calculator
- Enter the change in length (ΔL) — how much it stretched.
- Enter the original length (L₀) before loading.
- Read the strain, a dimensionless ratio.
What your result means
Strain is the fractional change in length — how much a material deforms relative to its original size. It has no units because it is a length divided by a length; engineers often quote it as a percentage or in microstrain.
Why this one is different
Strain is a bare ratio, so what look like three measurements are one quantity at three scales: multiplied by a hundred, and again by ten thousand. Beyond roughly one per cent, most metals have left the elastic region where this number means anything, and that limit is flagged next to the figure rather than buried in the small print.
A tiny number that tells you a lot
Steel typically yields at a strain of about 0.002 — just 0.2%. Strain values look small because they compare deformation to the whole length of a part, which is exactly why microstrain (millionths) is such a handy unit in the lab.
How it works
Strain equals the change in length divided by the original length. Both lengths use the same units, so they cancel and leave a pure ratio. Multiply by 100 for percent, or by one million for microstrain.
Formula
Worked example
A 2,000 mm bar that stretches by 0.6 mm under load:
ε = 0.6 ÷ 2,000
ε = 0.0003
= 300 microstrain = 0.03%
Strain is dimensionless — a length divided by a length — which is why it is usually quoted in microstrain (×10⁻⁶) or as a percentage.
Engineering strain and true strain
The strain calculated here is engineering strain: the change in length divided by the original length. True strain divides by the instantaneous length instead, integrating over the deformation, and the two are almost identical while strains are small. They diverge once deformation becomes large — at 20% engineering strain, true strain is about 18%; at 100%, engineering strain is 1.0 while true strain is 0.69. For elastic work in metals and concrete, where strains are fractions of a percent, engineering strain is the correct and conventional choice. For metal forming or polymer testing, it is not.
Lateral strain and Poisson’s ratio
Stretching a bar makes it thinner as well as longer, and the ratio between that lateral contraction and the axial extension is Poisson’s ratio — around 0.3 for most metals, 0.2 for concrete, and approaching 0.5 for rubber, which is nearly incompressible. It matters whenever a member is restrained in more than one direction, because preventing the lateral movement generates stress that a one-dimensional calculation never sees. Strain gauges are also affected: a gauge mounted transversely reads the lateral strain, not the axial one.
Frequently asked questions
Why does strain have no units?＋
It is one length divided by another, so the units cancel. That makes strain a dimensionless ratio, valid whether you measure in millimetres, metres or inches.
What is microstrain?＋
Microstrain is strain multiplied by one million. A strain of 0.000001 equals one microstrain — a convenient scale for the very small deformations measured by strain gauges.
Is strain the same as the amount something stretched?＋
No — strain is that stretch divided by the original length, which is what makes it comparable between parts of different sizes. A 0.6 mm extension is serious in a 20 mm specimen and trivial in a 20 m cable, and only the strain figure tells you which is which.
How is strain actually measured?＋
Usually with a bonded strain gauge, a foil grid whose resistance changes very slightly as it deforms. The change is tiny, which is why gauges are read through a Wheatstone bridge.
What strain do real materials tolerate?＋
Structural steel yields at roughly 0.2 percent strain, or 2,000 microstrain, while rubber can stretch several hundred percent. The elastic limit, not the breaking point, is what design works to.
What is Poisson's ratio?＋
The ratio of transverse contraction to longitudinal extension. Most metals sit around 0.3, meaning a bar stretched lengthwise gets measurably thinner as it does so.
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Assumptions & limitations
Engineering formulas are exact; the situations they model are not. Read your result with these limits in mind:
- This is engineering strain, measured against the original length. True strain uses the instantaneous length and differs noticeably at large deformations.
- Assumes uniform deformation along the whole gauge length.
- Valid in the elastic region; beyond yield, deformation localises and the assumption of uniformity fails.
- Both lengths must be in the same unit — the result is a ratio, so the unit itself cancels out.