Torsion springs are the hardest of the three common spring types to spec from a sample. A compression spring gives you a length and a diameter. A torsion spring adds legs, angles, a wind direction, and a load measured in torque instead of pounds. Miss any one of those and the replacement part won’t fit, won’t load correctly, or will fail early.

This guide shows you how to measure a torsion spring step by step, the same way our engineers do it when a customer sends in a sample. You’ll get the seven measurements every manufacturer needs, a worked example that turns those numbers into a spring rate, and an RFQ checklist so your quote comes back right the first time.

New to torsion springs? Our overview of torsion spring functionality and applications covers the basics. This article picks up where that one stops. It also completes our measurement series, alongside the guides on how to measure a compression spring and how to measure an extension spring.

What You Need Before You Start

You don’t need a metrology lab. You do need tools with enough resolution to catch small differences, because torsion spring performance is very sensitive to wire size.

• Digital calipers with 0.001″ (0.02 mm) resolution
• A micrometer for wire diameter, if you have one
• A protractor or digital angle gauge for leg angles
• Pin gauges or a set of rods to check inside diameter
• A torque gauge or spring tester with torsion capability, for load verification
• A paint marker and a flat, clean work surface

One more thing: measure more than one sample if you can. A spring pulled from service may have taken a set, so its free angle and torque will read differently from a new part. Note whether your sample is new or used. That context helps your manufacturer decide which numbers to trust.

How to Measure a Torsion Spring in 7 Steps

Here’s how to measure a torsion spring, in order. Each step builds on the one before it, and the first four feed directly into the rate calculation later in this guide.

Step 1: Measure the Wire Diameter

Measure the wire on a straight section of a leg, not on the coil body. Winding can slightly oval the wire in the coils, which skews the reading. Take two or three readings and average them.

Then compare your number to standard wire sizes. If you read 0.0615″, the wire is almost certainly 0.062″. Standard sizes keep cost and lead time down, so a manufacturer will steer you there anyway.

Why so much care? Torsion spring rate scales with wire diameter to the fourth power. On 0.062″ wire, a 0.002″ error changes the calculated rate by roughly 13%. No other measurement on the spring has that much leverage.

Step 2: Measure the Inside and Outside Diameter

Measure the outside diameter (OD) with calipers at several points along the body. Then calculate the other two diameters:

• Inside diameter (ID) = OD − 2 × wire diameter
• Mean diameter (D) = OD − wire diameter

For torsion springs, the ID usually matters most. Most torsion springs ride on a shaft or arbor, so confirm the ID directly with pin gauges if you can. Slide in the largest pin that fits without forcing the coils open.

Keep this in mind: the ID you measure is the free ID. A torsion spring’s diameter shrinks as it winds up under load. We’ll calculate exactly how much in the worked example, because it’s the single most common cause of torsion spring failures in the field.

Step 3: Count the Body Coils

Count the coils in the body, from the point where one leg leaves the coil to the point where the other leg leaves. Mark the starting point with a paint marker. Count full turns, then estimate the remaining fraction by looking straight down the spring’s axis.

Expect a fractional count, such as 6.5 or 8.25. That fraction isn’t sloppy winding. It sets the angular offset between the two legs, which is how a manufacturer controls the free position angle. If you round 8.25 coils to 8, the legs land a quarter turn away from where you need them.

For engineering calculations, the legs also flex a little under load. Designers account for that with an “equivalent active coil” count that adds a small leg term. You don’t need to calculate it to order a spring, but you’ll see it in the example below.

Step 4: Measure the Body Length

Body length is the length of the coiled section along its axis, outside coil to outside coil. Don’t include the legs.

Most torsion springs are close-wound, meaning the coils touch. For a close-wound spring, body length is about wire diameter × (body coils + 1). Use that as a sanity check. If your measured length is noticeably longer, the spring has pitch between the coils. Call that out on your drawing, since space-wound torsion springs need extra specification.

Also note that body length grows as the spring winds up, because the coil count effectively increases. If the spring sits in a tight slot or between two walls, leave room for that growth.

Step 5: Measure the Legs

Leg measurements cause more quoting confusion than anything else on a torsion spring. The fix is simple: always state where you measured from.

The standard method measures leg length from the centerline of the spring body (the axis) to the end of the leg, or to the point where the load is applied. That second distance is the moment arm. Since torque equals force times moment arm, the load point is what determines the force your assembly actually sees.

Next, identify the leg style. Common configurations include:

• Straight tangential legs, which leave the body in line with the coil
• Straight radial legs, which point outward from the axis
• Short hooks or hinge ends that wrap around a pin
• Bent or offset legs with one or more formed angles
• Double torsion springs, with two coil bodies joined by a center loop

For any bend, record each segment length, the bend angle, and the inside bend radius. A dimensioned sketch beats a written description every time. If the leg geometry is complex, send the sample along with the sketch.

Step 6: Measure the Free Position Angle

The free position angle is the angle between the two legs when the spring is unloaded. Set the spring on a flat surface, look straight down the axis, and center your protractor on the axis. Measure from the centerline of one leg to the centerline of the other.

Specify a tolerance with this angle, and expect it to be wider than you might assume. Small variations in each coil add up across the whole body, so free angle tolerance typically widens as the coil count increases. The Spring Manufacturers Institute (SMI) publishes commercial tolerance guidelines that most U.S. manufacturers use as a starting point.

If your assembly is sensitive to angle, control the torque at a working position instead of tightening the free angle. That gives you the function you need without paying for a tolerance the process can’t easily hold.

Step 7: Determine the Wind Direction

Every torsion spring is wound either left-hand or right-hand, and the two are not interchangeable. Stand the spring on end and look at the coils facing you. If they rise up and to the right, like the threads on a standard bolt, the spring is right-hand wound. If they rise up and to the left, it’s left-hand wound.

Wind direction matters because a torsion spring should be loaded in the direction that winds it tighter. Loading it in that direction closes the coils and reduces the diameter. Loading it the other way opens the coils and sharply lowers the stress the spring can safely handle. A mirror-image spring may look identical on the bench and still fail fast in your assembly.

How to Measure Torsion Spring Torque and Rate

Dimensions tell a manufacturer what the spring looks like. Torque tells them what it does. Torsion spring load is a torque, usually stated in inch-pounds (in-lb) or newton-millimeters (N·mm). Spring rate is torque per degree of rotation, or sometimes torque per full turn.

To measure it, support the spring on a mandrel that fits inside the wound-up ID, fix one leg, and rotate the other. Record torque at two known angles. Then calculate:

• Rate = (Torque 2 − Torque 1) ÷ (Angle 2 − Angle 1)

Use two readings rather than one. Friction between the coils and the mandrel, plus a little settling at the start of the stroke, distorts the first reading. A dedicated spring tester with torsion capability gives you repeatable numbers for production and incoming inspection.

On your drawing, specify torque at the working angle with a tolerance, not just the rate. Torque at position is what your assembly feels, and it’s what quality teams can verify.

Worked Example: From Measurements to Spring Rate

This is where knowing how to measure a torsion spring pays off. Say you measured the following on a music wire torsion spring:

• Wire diameter (d): 0.062″
• Outside diameter: 0.625″, so inside diameter is 0.501″ and mean diameter (D) is 0.563″
• Body coils (Nb): 8.25
• Leg lengths: 1.00″ each, measured from the axis
• Wind direction: right-hand
• Free body length: 0.574″, which matches 0.062 × (8.25 + 1), so the spring is close-wound

First, check the spring index. Index = D ÷ d = 0.563 ÷ 0.062 = 9.1. That sits comfortably in the 5 to 12 range that winds easily and holds tolerance well.

Second, find the equivalent active coils. The leg term is (L1 + L2) ÷ (3πD) = 2.00 ÷ (3 × 3.1416 × 0.563) = 0.38. Add that to the body coils and you get 8.63 active coils.

Third, calculate the rate. The standard formula is rate per turn = (d⁴ × E) ÷ (10.8 × D × Na), where E is about 29,000,000 psi for music wire. That gives (0.0000148 × 29,000,000) ÷ (10.8 × 0.563 × 8.63) = 8.17 in-lb per turn. Divide by 360 and the rate is 0.0227 in-lb per degree.

Fourth, find torque at the working angle. If the application winds the spring 90 degrees, torque is 0.0227 × 90 = 2.04 in-lb. With 1.00″ legs, that’s about 2 lb of force at the leg tip. Bending stress at that torque works out to roughly 95,000 psi, well below the allowable for 0.062″ music wire.

Fifth, check the ID under load. Wound diameter = D × Nb ÷ (Nb + turns of deflection). Ninety degrees is 0.25 turns, so the mean diameter becomes 0.563 × 8.25 ÷ 8.50 = 0.546″. The loaded ID is 0.546 − 0.062 = 0.484″.

This is the step that saves designs. The free ID of 0.501″ makes a 1/2″ shaft look like the obvious choice. But at 90 degrees the spring clamps down to 0.484″ and binds on that shaft. A good rule of thumb is to size the arbor about 10% under the loaded ID, which points to a 7/16″ (0.4375″) shaft here.

Finally, check body length at full deflection: 0.062 × (8.25 + 1 + 0.25) = 0.589″. Leave clearance for it.

Keep in mind these are textbook calculations. Coil friction, leg bending, and normal material variation mean real-world rates often differ by a few percent. Verify the final number on a tester before you release a drawing.

Torsion Spring Measurement Mistakes That Drive Up Cost

Knowing how to measure a torsion spring is half the job. The other half is avoiding the handful of errors behind most bad quotes:

• Sizing the arbor to the free ID instead of the loaded ID
• Giving leg lengths without saying where they were measured from
• Skipping wind direction, or loading the spring in the unwinding direction
• Measuring a fatigued sample and treating its numbers as the design intent
• Specifying rate only, with no torque at a defined working angle
• Holding free angle and leg length to tighter tolerances than the function requires

That last one quietly adds cost and lead time. Our guide to spring design for manufacturability explains how tolerance and spec choices show up in your price. If a spring is already failing in service, our article on common spring failures and how to prevent them is a good next read.

Torsion Spring RFQ Checklist

Include these items and you’ll get an accurate quote without a round of follow-up questions:

• Material and finish (see our guide to spring material selection for options)
• Wire diameter
• Inside diameter and the arbor or shaft size it rides on
• Outside diameter, if space is limited
• Number of body coils
• Maximum body length
• Leg configuration, with lengths, bends, and the reference point for each
• Free position angle, with tolerance
• Wind direction
• Torque at one or two working angles, with tolerance
• Maximum deflection angle
• Required cycle life
• Order quantity and estimated annual usage
• Operating environment, such as temperature, moisture, or chemicals

For a deeper walkthrough of what makes a quote request work, read how to write a spring RFQ that actually gets you an accurate quote. Material questions? Start with our overview of spring material selection.

Frequently Asked Questions

How do you measure the leg length of a torsion spring?

Measure from the centerline of the coil body to the end of the leg, or to the point where the load is applied. State which reference you used on the drawing. The distance from the axis to the load point is the moment arm, and it controls the force your assembly sees.

How can I tell if a torsion spring is left-hand or right-hand wound?

Stand the spring on end and look at the coils facing you. Coils that rise up to the right, like a standard bolt thread, are right-hand wound. Coils that rise up to the left are left-hand wound.

What units are used for torsion spring rate?

In the U.S., rate is usually given in inch-pounds per degree or inch-pounds per turn. Metric specs use newton-millimeters per degree. Always confirm which one a datasheet uses, since per-turn values are 360 times larger than per-degree values.

Can I measure a used torsion spring to order a replacement?

Yes, with care. Dimensions like wire size, ID, coil count, legs, and wind direction stay reliable. Free angle and torque may have drifted if the spring took a set. Tell your manufacturer the sample is used so they can design to the original intent.

Send Us the Sample and We’ll Handle the Rest

Now you know how to measure a torsion spring and what a manufacturer needs to quote it. Still, some samples are worn, oddly formed, or missing half the callouts. That’s fine.

Send the part or the drawing to our engineering team in Elk Grove Village, where we build custom torsion springs for OEMs across the Midwest and beyond. We’ll measure it, fill the gaps, and come back with a quote, often with a design-for-manufacturability suggestion or two that lowers your cost. Have a general question first? Our custom spring manufacturing FAQ covers lead times, pricing, and tolerances. Ready to start? Request a quote today.