Strain is the most basic way engineers and materials scientists describe how much something deforms under load. Because it's a ratio rather than an absolute measurement, strain lets you compare the stretch of a tiny wire and a massive steel beam on the same scale.
Why Strain Is Measured as a Ratio, Not a Distance
A 2 mm stretch means very different things depending on what's stretching. Stretch a 10 mm paperclip wire by 2 mm and it has deformed enormously; stretch a 10-metre steel cable by 2 mm and it has barely moved. Strain solves this by dividing the change in length by the original length, producing a dimensionless number that's comparable across any size of object. Because strain is unitless, it's often reported as a plain decimal (0.002), as a percentage (0.2%), or informally as "mm per mm" — all three describe the same physical stretch.
Engineering Strain vs True Strain
This calculator computes engineering strain — the change in length divided by the original length, which is standard for small deformations in structural and mechanical engineering. There's a related quantity called true strain, which instead divides against the instantaneous length at each point during loading (ε_true = ln(L/L₀)). For small strains (well under 5%), engineering strain and true strain are nearly identical; they diverge meaningfully only for large plastic deformations, such as metal forming or elastomer stretching, where true strain is the more accurate measure.
Compressive Strain and the Sign Convention
Strain isn't only for stretching. If ΔL is negative — the object got shorter under a compressive load — the strain is negative too, representing compression rather than elongation. The same formula applies either way: enter a negative change in length to compute compressive strain. Strain is the starting point for stress-strain analysis: pair it with the applied force per unit area (stress) and you can find a material's stiffness via Young's modulus, or its safety margin before permanent deformation or failure.