How to Measure a Compression Spring

Before measuring: be sure which spring you are holding

If there is a gap between the coils and the spring works by being compressed, it is a compression spring. If the coils are closed and there are hooks at the ends, it is an extension spring. If the diameter changes from one end to the other it is a conical spring, and both ends must be measured.

The distinction matters because each type needs a different set of dimensions. The steps below are for a straight cylindrical compression spring.

1. Wire diameter · d

Measure the thickness of the wire itself with a caliper. This is the dimension that affects the result most: spring rate varies with the fourth power of wire diameter, so a ten per cent error means roughly a forty-six per cent error in stiffness.

Take the measurement from a sound coil in the middle of the spring. The end coils may have been ground, leaving a reduced wire section there.

2. Outside diameter · Dd

The outer measurement of the coil, taken with the outer jaws of the caliper. This is the figure chosen to suit the bore.

The mean diameter used in calculation is derived from it: Dm = Dd − d. There is no need to measure it separately.

3. Free length · L₀

The end-to-end length with no load on the spring. Standing it on a flat surface and measuring with a rule is enough.

Take care: if the spring in your hand has fatigued, its free length has already shortened. A new spring made to that figure copies the fatigued state rather than the original. Measure an unused example if you can; if not, say so when ordering.

4. Active coil count · n

The number of load-carrying coils. On a closed-end spring, subtract two from the total coil count — the end coils carry no load and serve to seat the spring.

To count, turn the spring on its side, start at one end and count full turns. Half turns count too: a value such as 8.5 is normal and is used in the calculation.

5. End form and coil direction

There are three end forms: closed and ground, closed but unground, and open. Stand the spring upright on a flat table — if it stands square and steady, the end is closed and ground.

For coil direction, look at the spring from the side: if the coils rise from left to right it is right hand, otherwise left hand. On a spring working alone this often makes no difference; on nested pairs the two springs are wound in opposite directions and the information is required.

The three commonest mistakes

• Measuring wire diameter at a ground endThe section may be reduced at the end coils; take the measurement from a sound coil in the middle.
• Mistaking inside diameter for outside diameterThe calculation is built on outside diameter; if you give the inside diameter, state that clearly.
• Copying the free length of a fatigued springThe shortened length is not the original; a new spring made to it will give too little force.

Once you have the dimensions

With these five values you can enter them into the calculator on our home page and see the spring rate, solid length and spring index. If the index falls outside the 4–12 band the calculator warns you — a chance to correct the dimensions before production.

Where you cannot take measurements, a photograph of the old spring next to a ruler is enough for most items. You can send it through the quote form.