Spring Constant Calculator
Find a spring’s stiffness (spring constant k) from the force applied and how far it stretches, plus the stored elastic energy.
How it works
k = F / x · elastic energy = ½·k·x²
Hooke’s law says the force a spring exerts is proportional to how far it is stretched or compressed, and the spring constant k is that proportionality — the stiffness. A large k means a stiff spring that needs a lot of force per metre of stretch. The work you do stretching it is stored as elastic potential energy, ½kx², ready to be released.
Worked example
A spring that stretches 0.1 m under a 20 N force has k = 20 ÷ 0.1 = 200 N/m, and stores ½ × 200 × 0.1² = 1 J of elastic energy at that extension.
Sources: Hooke’s law — Wikipedia
Frequently asked questions
What are the units of the spring constant?
Newtons per metre (N/m) — the force needed for each metre of stretch. A car suspension spring might be tens of thousands of N/m, while a soft toy spring is only a few.
Does k change if I stretch the spring further?
Within the elastic limit, no — k is a fixed property of the spring, which is why force stays proportional to extension. Stretch it too far and it deforms permanently, and Hooke’s law no longer applies.
Does compression use the same formula?
Yes. Enter the compression distance in place of the extension; an ideal spring has the same constant whether pushed or pulled.
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