Compression Spring Rate and Stress Calculator
This calculator returns spring rate, load, Wahl-corrected shear stress and solid-height safety factor for a round-wire helical compression spring. Enter the wire and coil geometry, pick the material modulus and see in seconds whether the spring survives being closed solid.
Your numbers
Round wire. Convert from mm by dividing by 25.4.
Outside diameter minus one wire diameter.
Coils that deflect. Total coils minus the dead coils at the ends.
About 2 for closed and ground ends, about 1 or less for open ends.
Typical values; confirm with your wire certificate or supplier data.
Use the minimum tensile for your wire diameter from the material spec table.
About 45% to 50% for static, no set removal; up to 65% to 70% if the spring is preset (scragged).
Your results
Round-wire helical compression spring, static loading, room temperature. Fatigue life, buckling stability, surge and set removal are not evaluated. Allowables are rules of thumb; confirm against your spring maker and the governing spec.
Load and shear stress vs deflection
Sweeps Working deflection from 0.05 to 1.6, holding your other inputs fixed.
Want spring checks run from your ERP drawings?
An optional walkthrough of a read-only assistant that pulls wire, coil and tolerance data from your item master for engineering and quoting checks.
Optional. The tool and every export work without this. No spam.
How to use it
Start with the drawing dimensions: wire diameter, mean diameter (outside diameter minus one wire diameter), active coils and free length. Active coils are total coils minus the dead end coils, so a closed and ground spring with 10 total coils usually has 8 active. Pick the modulus that matches the wire, then enter the tensile strength from the wire table for that diameter. The calculator returns rate, load at the working deflection and the stress at both the working point and solid. Use the chart to see how load and stress climb with deflection and where the working point sits against your allowable.
- Use mean diameter, not outside diameter, in every formula.
- Check solid height against the space in the assembly, not just the working length.
- A safety factor under 1.0 at solid means the spring sets if anyone closes it fully.
The formulas and where they come from
Rate follows k = G d^4 / (8 D^3 Na), the standard helical spring equation found in Shigley's Mechanical Engineering Design and the Spring Manufacturers Institute handbook. Shear stress is tau = Kw x 8 F D / (pi d^3) with the Wahl factor Kw = (4C - 1) / (4C - 4) + 0.615 / C, which corrects for direct shear and wire curvature. Solid height is total coils times wire diameter. Units are inches, pounds-force and psi. Allowable stress is taken as a percentage of minimum tensile strength; typical practice is 45% to 50% for a static spring that has not been preset.
Worked example
With the defaults (0.080 in music wire, 0.720 in mean diameter, 8 active coils, 2.0 in free length) the index is 9.0, the rate is about 19.7 lbf per inch, and 0.8 in of deflection gives roughly 15.8 lbf and about 66,000 psi of shear stress. Solid height is 0.80 in, so closing the spring solid means 1.2 in of deflection, about 23.7 lbf and 98,600 psi, which is a safety factor near 1.3 against 45% of a 290 ksi tensile. Increase free length to 3.0 in and the safety factor at solid drops below 1.0, which is the classic reason springs take a set in the field.
Common mistakes
Most spring complaints trace back to a few repeat errors that this tool makes visible before drawings are released.
- Counting every coil as active; end coils that touch do not deflect.
- Ignoring the tolerance on wire diameter: stress scales with d cubed and rate with d to the fourth.
- Designing the working point near solid height with no travel allowance.
- Using this static check for a cycling spring, which needs a fatigue check with a Goodman or Zimmerli approach.
Frequently Asked Questions
What is a good spring index for a compression spring?
Most makers prefer a spring index between 6 and 12. Below 4 the wire is hard to coil and residual stress is high. Above 12 the spring tangles easily and is hard to hold to tolerance. This calculator flags the index so you can adjust wire or mean diameter early.
What is the Wahl factor and why is it needed?
The Wahl factor corrects the simple torsion formula for two effects: direct shear through the wire and the higher stress on the inside of the coil because of wire curvature. It matters most at low spring index, where the correction can be 20% or more.
Why does safety factor at solid matter if the spring never goes solid?
Springs get overtravelled during assembly, shipping and maintenance. If the stress at solid is below the allowable, a spring that is accidentally closed returns to its free length. If not, it takes a permanent set and loses load, which shows up as a field failure.
Can I use this for conical, rectangular wire or fatigue duty?
No. This is a static check for constant-diameter round wire. Conical, variable pitch and rectangular wire springs need different formulas, and cyclic duty needs a fatigue analysis against the wire's endurance data and surface condition such as shot peening.
If spring quotes and drawing checks live in spreadsheets, we can connect your ERP item and drawing data to a private, read-only assistant that runs these checks when an RFQ arrives.
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