Spring Index Calculator

Calculate spring index C = D/d from wire diameter and mean, outside or inside coil diameter — with OD/ID conversion and Wahl factor. Metric and imperial. Free, no signup.

Inputs

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Known coil diameter

Choose whether the entered coil diameter is mean, outside or inside diameter.

Coil diameter (D)

Mean, outside or inside diameter according to the selector above.

Wire diameter (d)

Diameter of the spring wire.

Results

Default result

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Spring index(C)

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Also computed

Mean coil diameter(D)10mm

Outside diameter(OD)11mm

Inside diameter(ID)9mm

Wahl factor(Kᵥᵥ)1.145

Method notes 3 notes

Spring index is C = D / d, mean coil diameter divided by wire diameter.
Low C values are hard to manufacture and raise stress concentration; high C values can buckle and tangle more easily.
Wahl factor is shown because it is the stress-correction term used by the spring calculators.

Spring index is C = D/d, mean coil diameter divided by wire diameter. It is a quick manufacturability and stress-concentration screen: very low C raises the Wahl factor and is hard to wind, while very high C can be slender and unstable. This calculator also converts mean, outside and inside coil diameters.

How to use this calculator

Choose diameter basis. Select whether your coil diameter is mean, outside or inside diameter.
Enter coil diameter. Enter that diameter in the active unit system.
Enter wire diameter. Enter the spring wire diameter.
Read index and conversion. Read spring index, mean diameter, OD, ID and Wahl factor.

How it works

Spring index is the ratio of mean coil diameter to wire diameter: C = D / d If outside diameter is known, D = OD - d. If inside diameter is known, D = ID + d. The calculator converts those diameters before calculating C.

The Wahl correction factor is also shown: K_w = (4C - 1)/(4C - 4) + 0.615/C Lower spring index increases this stress correction, so the result is flagged when the geometry falls outside a practical range.

Worked example

Verified against the live calculator

A spring with D = 10 mm mean coil diameter and d = 1 mm wire has C = 10 / 1 = 10. The outside diameter is 11 mm, the inside diameter is 9 mm, and the Wahl factor is about 1.14.

Frequently asked questions

What is spring index?

Spring index is C = D/d, the mean coil diameter divided by wire diameter. It describes how tightly the wire is coiled.

What is a good spring index?

Many practical compression springs fall around C = 4 to 12. Below that the spring is hard to wind and stress rises; far above that the spring becomes slender and may buckle or tangle.

Is D outside diameter or mean diameter?

In spring formulas, D is the mean coil diameter. If you know outside diameter, mean diameter is OD − d. If you know inside diameter, mean diameter is ID + d.

Why does this calculator show Wahl factor?

The Wahl factor corrects shear stress for wire curvature. It rises sharply at low spring index, which is why a very tight coil is a stress problem.

Does spring index set spring rate?

Not by itself. Rate is k = G·d⁴/(8·D³·Nₐ). Spring index helps judge manufacturability and stress concentration for a chosen D and d.

Method & assumptions

Round-wire helical spring geometry.
Spring index is a geometry screen, not a complete strength or fatigue check.
Use the compression, extension or torsion spring calculator for load and stress calculations.

Spring Index Calculator

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