Knowing how to measure an extension spring correctly saves you from the most expensive mistake in spring sourcing: a quote built on the wrong numbers. Extension springs look simple, but they hide more variables than compression springs do. The hooks, the initial tension, and the way the coils are wound all affect how the spring performs in your assembly.

This guide walks engineers and purchasing agents through every measurement you need, in the right order, with the right tools. It also covers how to calculate spring rate and initial tension from two load readings, which mistakes derail RFQs, and which specifications drive your cost. If you are working with compression springs instead, start with our companion guide, How to Measure a Compression Spring.

Why Extension Springs Are Harder to Measure Than Compression Springs

A compression spring has a solid height that acts as a natural stop. An extension spring does not. It can be pulled past its design limit and permanently stretched, so you need to know exactly how far it is allowed to travel.

In addition, most extension springs are wound with initial tension. This is a preload built into the coils during coiling that holds them tightly together. As a result, the spring needs a certain amount of force before the coils even begin to separate. You cannot see initial tension with calipers, and you cannot find it by measuring free length alone.

Finally, the hooks are where extension springs usually fail. Hook style, hook length, and hook orientation all need to be captured. Miss any one of them, and the replacement spring may not install, or it may break early in service.

Tools You Need to Measure an Extension Spring

You do not need a metrology lab, but you do need more than a tape measure. Gather the following before you start:

• Digital calipers with 0.001 inch resolution for outside diameter, free length, and hook dimensions.
• A micrometer for wire diameter, since calipers are not precise enough on small wire.
• A spring tester or calibrated force gauge with a length scale for load readings.
• Pin gauges or drill blanks for checking the inside opening of the hooks.
• A protractor or optical comparator if hook orientation matters in your assembly.

If you only have calipers, you can still measure the extension spring’s geometry. However, you will not be able to verify rate or initial tension, and those are the values that decide whether the spring does its job.

How to Measure an Extension Spring in 7 Steps

Work through these steps in order. Record every value on a single sheet, and measure at least two or three samples if you have them. Spring dimensions vary slightly from part to part, so one reading can mislead you.

Step 1: Measure the Wire Diameter

Wire diameter is the first dimension to measure on an extension spring. Place the micrometer across a single coil in the body of the spring. Avoid the hooks, because the bending process can slightly flatten or distort the wire there. Take readings on two or three different coils and average them.

Next, compare your reading to standard wire sizes. Most springs are wound from standard gauges such as 0.041, 0.047, 0.054, or 0.062 inch. If your reading falls between two standard sizes, the spring may be plated or coated. A zinc or powder coat adds thickness, so note the finish on your record.

Step 2: Measure the Outside Diameter

Measure across the body of the extension spring with calipers. Close the jaws gently, since squeezing the coils will give you a false low reading. Take readings near both ends and in the middle, then record the largest value as the outside diameter.

To find the mean coil diameter, subtract one wire diameter from the outside diameter. For example, a spring with a 0.625 inch OD and 0.062 inch wire has a mean diameter of 0.563 inch. You will need the mean diameter for rate calculations, and it also tells you the spring index, which affects manufacturability.

Step 3: Measure the Free Length Inside the Hooks

This is the step most people get wrong. For an extension spring, free length is measured from the inside of one hook to the inside of the other hook, with the spring at rest. It is not the overall length from the outside of one hook to the outside of the other.

The inside-hook measurement matters because that is where the spring actually bears on the pins, posts, or eyelets in your assembly. Manufacturers and catalogs specify extension springs by length inside hooks for this reason. If you send an overall length instead, your spring will come back too short by two wire diameters.

Step 4: Measure the Body Length and Count the Coils

The body is the coiled section between the hooks. Measure its length with calipers, then count the full turns of wire in the body. Do not count the hooks as coils.

On a close-wound spring, you can check your count with a simple formula. The body length equals the wire diameter multiplied by the number of body coils plus one. So, a spring with 0.062 inch wire and 20 body coils should have a body length of about 1.302 inches. If your count and your measurement disagree, recount before you move on.

The number of body coils is important because it drives the spring rate. Fewer coils mean a stiffer spring, and more coils mean a softer one.

Step 5: Identify the Hook Type and Orientation

Extension springs come with many end configurations. The most common include:

• Machine hook, a full loop bent up over the center of the spring. It is the most economical because it is formed right on the coiling machine.
• Crossover center hook, where the wire crosses to the center before forming the loop. It centers the load well and is common in precision applications.
• Side hook, a loop formed over the side of the spring rather than the center.
• Extended hook, a loop set away from the body on a straight length of wire to reach a distant mounting point.
• Special ends, such as V-hooks, rectangular hooks, threaded inserts, or swivel ends.

After you identify the style, measure the hook opening, which is the gap between the end of the wire and the spring body. Then note the relative position of the two hooks. Hooks can be in line with each other, or turned 90, 180, or 270 degrees apart. If orientation matters for installation, record it. If it does not, say so on the drawing, because a hook position requirement adds cost.

Step 6: Measure Loads and Calculate Spring Rate and Initial Tension

This is the step that separates a usable spec from a guess. Mount the spring in a spring tester or on a force gauge. Pull it to a first test length and record the load. Then pull it to a second, longer test length and record that load too.

Both test lengths should fall within the spring’s working range, and they should be well separated. Ideally, use the lengths the spring actually sees in your assembly, such as the installed length and the fully extended length.

With two load readings, you can calculate both key values. Spring rate equals the difference in load divided by the difference in length. Initial tension equals the first load minus the rate multiplied by the first deflection, where deflection is the test length minus the free length.

Why not just measure initial tension directly? You can estimate it by slowly loading the spring until light first appears between the coils. That method is useful as a quick sanity check. However, it is not precise, so the two-point calculation is the better number to put in your records.

Step 7: Determine the Maximum Extended Length

Finally, measure the longest length the spring reaches in your assembly. Do this in the application, not by stretching a sample to see how far it will go. An extension spring pulled beyond its elastic limit takes a permanent set, and the hooks carry the highest stress in the part.

Your manufacturer uses maximum extended length to check that the wire and hooks stay within safe stress levels. Without it, they cannot confirm the design will survive your duty cycle.

Worked Example: Calculating Spring Rate and Initial Tension

Here is a complete example using realistic numbers. Suppose you measured a music wire extension spring with the following geometry:

• Wire diameter: 0.062 inch
• Outside diameter: 0.625 inch, so the mean diameter is 0.563 inch
• Free length inside hooks: 2.40 inches
• Body coils: 20, with machine hooks in line

Next, you run two load tests. At 2.90 inches, which is 0.50 inch of deflection, the tester reads 5.8 lbf. At 3.65 inches, which is 1.25 inches of deflection, it reads 10.3 lbf.

First, calculate the rate. The load difference is 10.3 minus 5.8, or 4.5 lbf. The length difference is 3.65 minus 2.90, or 0.75 inch. Therefore, the rate is 4.5 divided by 0.75, which equals 6.0 lbf per inch.

Then, calculate initial tension. Multiply the rate by the first deflection: 6.0 times 0.50 equals 3.0 lbf. Subtract that from the first load: 5.8 minus 3.0 equals 2.8 lbf of initial tension.

Finally, check the result against theory. The standard rate formula is G times d to the fourth power, divided by 8 times D cubed times the number of active coils. With a shear modulus of 11.5 million psi for music wire, this spring calculates to roughly 6 lbf per inch. The measured and calculated values agree, so you can trust your readings.

Common Measurement Mistakes That Derail an RFQ

Even experienced engineers make these errors when they measure an extension spring. Each one can lead to a wrong quote, a failed first article, or a spring that breaks in the field.

• Measuring overall length instead of length inside hooks. This is the single most common error, and it shortens the spring by two wire diameters.
• Specifying rate and initial tension instead of loads at lengths. Initial tension is hard to control in production. Two loads at two specific lengths give your manufacturer a clearer, more testable target.
• Measuring wire diameter at the hook. Hook forming can distort the wire, so always measure in the body.
• Relying on a magnet to identify material. Music wire is magnetic, but cold-worked 302 stainless steel can be partly magnetic too. When material matters, ask for certifications or a material test.
• Measuring a worn or overstretched spring. A spring that has taken a set will have gaps between coils and lower initial tension. If the old spring is not closed tightly at rest, its measurements no longer reflect the original design.
• Leaving out the maximum extended length. Without it, your supplier cannot verify that the design is safe for your application.

Which Extension Spring Specifications Drive Your Cost

Accurate measurement is only half the job. The other half is knowing which requirements to lock down and which to leave open. Every tight callout on your drawing adds inspection time, setup, or secondary operations.

Hook style has the biggest impact. Machine hooks form on the coiler in one operation. Crossover, extended, and special hooks often require a second forming step, which adds labor and lead time.

Hook orientation is next. If your assembly does not care how the hooks line up, leave the relative position unspecified. Holding a tight hook angle requires extra adjustment and inspection on every part.

Load tolerances matter as well. Tight tolerances at two different lengths are harder to hold than a tolerance at one working length. Specify tight loads only where the function truly depends on them.

Material is the final big lever. Music wire is the most economical choice for most indoor applications. Stainless steel costs more but is necessary where moisture, washdown, or corrosion is a concern. If you are still deciding whether a catalog part will work at all, our guide to custom vs. stock springs covers that decision in detail.

Extension Spring Measurement Checklist for Your RFQ

After you measure the extension spring, and before you send a request for quote, confirm that your record includes each of the following:

• Wire diameter, measured in the body, plus material and finish
• Outside diameter and any maximum OD limit from your assembly
• Free length inside hooks
• Number of body coils
• Hook type, hook opening, and relative hook position, or a note that position does not matter
• Load at two specific lengths, with tolerances only where needed
• Maximum extended length in the application
• Operating environment, cycle life expectations, and annual quantity

With this information, a spring manufacturer can quote accurately the first time. As a result, you avoid back-and-forth emails, revised quotes, and delayed first articles.

Frequently Asked Questions

How do you measure the length of an extension spring?

Measure from the inside of one hook to the inside of the other hook while the spring is at rest. This is called length inside hooks, and it is the standard free length measurement for extension springs.

What is initial tension in an extension spring?

Initial tension is the preload wound into the coils that holds them tightly together. It is the force required to begin separating the coils. You can calculate it from two load readings at two different lengths.

How do you calculate the rate of an extension spring?

Measure the load at two different extended lengths. Then divide the difference in load by the difference in length. The result is the spring rate, usually expressed in lbf per inch or N per mm.

Do the hooks count as coils?

No. Count only the full turns of wire in the body of the spring. The hooks affect overall length and stress, but they are not counted as body coils.

Get Your Extension Spring Specification Right the First Time

Measuring an extension spring carefully turns a worn-out part or a rough prototype into a spec your supplier can build to. Capture the geometry, test the loads, and decide which tolerances truly matter to your application.

Jackson Spring has manufactured custom extension springs, compression springs, torsion springs, and wire forms in Elk Grove Village, Illinois, for three generations. If you have a sample, a drawing, or just a set of measurements, send it to our engineering team. We will review it for manufacturability, flag anything that adds unnecessary cost, and get you an accurate quote.