When you’re writing an RFQ for a replacement compression spring or verifying dimensions on a sample, accurate measurement is critical. Get the dimensions wrong, and you’ll end up with a spring that doesn’t fit, costs more than expected, or fails to perform as required. This guide walks you through the measurement process step-by-step, explains what each dimension means, and shows you how to avoid the most common mistakes.

Whether you’re measuring an existing spring to create a specification or verifying that an incoming sample meets your requirements, you’ll need the same basic measurements. Let’s start with what you need to measure and why it matters.

The Five Critical Measurements for a Compression Spring

Every compression spring can be defined by five core dimensions. If you get these five measurements right, a manufacturer can quote and build an accurate replacement or new spring. These are:

Free length: The overall length of the spring when it is completely unloaded (no weight on it). This is the most fundamental measurement because it determines the spring’s solid height and how much it can compress in your assembly.

Outer diameter: The widest measurement across the outside of the coil. This tells you whether the spring will fit in the space allocated for it.

Wire diameter: The thickness of the wire used to wind the spring. This is one of the most critical dimensions because it directly affects the spring’s stiffness and load capacity.

Number of coils: The total count of complete coil revolutions. Combined with wire diameter and coil diameter, this determines the spring rate.

End condition: How the ends of the spring are finished (open ends, closed ends, closed and ground ends, or closed and squared ends). This affects how the spring sits in your assembly and whether it will rock or stand flat.

If you can provide these five measurements, you’re 80% of the way to getting an accurate quote. But let’s walk through how to measure each one accurately.

Tools You’ll Need

You don’t need expensive precision equipment to get reliable measurements. A basic set of tools will work fine for initial assessment and RFQ purposes.

Digital calipers: A 6-inch digital caliper (about 25 dollars) is your most important tool. It measures outer diameter, wire diameter, and free length quickly and with sufficient accuracy for spring specification. Look for one with both inside and outside jaws, and a depth rod for measuring shaft diameters if you’re also checking bore holes.

Scale or balance: A small digital scale accurate to 0.1 gram helps verify wire diameter indirectly (if you know the material, length, and diameter, you can cross-check the weight). This is optional but useful for catching errors.

Ruler or measuring tape: A regular ruler or steel tape measure works for free length when you don’t have calipers handy. Accuracy within 1/16 inch is usually sufficient for initial quotes.

Flat surface: A granite countertop, machined plate, or even a flat table works fine. You’ll use this to sit the spring on when measuring free length.

Magnifying glass (optional): For very small springs (wire diameter under 0.040 inch), a 10x magnifying glass helps you read wire diameter more accurately on your calipers.

Step-by-Step Measurement Process

Measuring Free Length

Free length is the easiest to measure but often done incorrectly. Place the spring on a perfectly flat surface. Don’t compress it or press it down. If the spring has slightly open ends and rocks a little, place a light weight (like a pencil) across the top to keep it from rolling, but don’t add downward pressure. Now measure from the bottom flat surface of one end coil to the top flat surface of the other end coil.

Common mistake: Measuring the spring while it’s tilted or compressed. Always measure on a flat surface with zero load. Measure vertically, not at an angle.

Measuring Outer Diameter

Outer diameter is the widest point across the outside of the spring coil. Use the outside jaws of your digital calipers. Measure in at least two places—top and middle—because springs can be slightly irregular. Take the larger of the two readings. This is the dimension that matters for fit in your assembly.

Pro tip: If the spring is small enough, slide it onto the calipers so you’re measuring through the center of the coil. This reduces the chance of measuring at an angle.

Measuring Wire Diameter

Wire diameter is critical and often measured incorrectly. Use the inside jaws of your digital calipers to measure the thickness of the wire itself. Take measurements at two or three different points along the spring to catch any variation. All readings should be within 0.001 inch of each other. If they’re not, the spring may have manufacturing defects or may be a different size than you think.

Common mistake: Measuring between coils instead of the wire itself. The gap between coils (the pitch) is not the wire diameter. Make sure you’re measuring the actual wire thickness.

Counting Coils

Count every complete revolution of the wire. Start at the bottom and work your way up. For a spring with 10 coils, you’ll count 10 complete circular paths. End coils (the flat sections at the top and bottom) count as partial coils, but if they’re fully closed and ground, they typically count as 0.5 to 1 coil each depending on the end condition.

This is where confusion happens. Ask yourself: Are the end coils closed (wrapped tightly) or open (loose)? If closed, do they count as full coils or partial? For specification purposes, report the total number of coils as you count them, and separately note whether the ends are open, closed, or closed and ground. The manufacturer will understand.

Identifying the End Condition

Look at both ends of the spring. Are the coils loose (open ends) or tightly wrapped (closed ends)? If the ends are wrapped tightly, is the end surface flat and ground smooth, or is it still showing the wound coil shape? This tells you the end condition.

Open ends: The coils at the end are not closed. The spring may rock slightly when placed on a flat surface.

Closed ends: The end coils are wrapped tightly, but the surface is not ground flat. You’ll see the spiral shape at the end.

Closed and ground ends: The end coils are wrapped and then ground flat on a grinding wheel. The end surface is perfectly flat.

End condition matters for cost and fit. Ground ends are more expensive and add lead time, but they’re necessary if the spring must sit perfectly flat or bear load precisely against another surface.

What About Spring Rate? Do You Need to Measure It?

Spring rate (how much force is needed to compress the spring by one inch) is a derived specification—it’s calculated from wire diameter, coil diameter, number of active coils, and material. You don’t measure it directly. However, if you have an existing spring that’s performing well and you want a replacement, you can measure spring rate empirically.

To measure spring rate, you need a known weight and a way to measure compression. Place the spring on a scale, add a known weight (like a 5-pound dumbbell) to the top, and measure how much the spring compresses. If it compresses 0.25 inch under 5 pounds, the spring rate is 5 pounds divided by 0.25 inch, or 20 pounds per inch.

This is useful verification if you’re not sure about your other measurements, but most manufacturers will calculate spring rate from the dimensions you provide. If you want to be thorough, measure it. Otherwise, provide the five core measurements and let the engineer calculate it.

Material Identification: How to Know What You’re Looking At

If you’re trying to replace an existing spring, you need to know what material it’s made from. This affects cost, lead time, and performance. There are three common possibilities:

Music wire (high-carbon steel): Shiny, bright metallic silver-gray color. Not magnetic (actually slightly magnetic, but you need a strong magnet to detect it). This is the most common material for industrial springs and typically the most cost-effective.

Stainless steel: Brighter, more reflective than music wire. Often has a slightly yellowish or greenish tint depending on the specific alloy (302, 304, or 316). Definitely not magnetic (or very weakly magnetic for certain grades). Used in applications requiring corrosion resistance.

Alloy steel or tempered steel: Darker gray or sometimes slightly rusty/oxidized. May be magnetic. Less common but used in high-stress applications.

When you provide your measurements to a manufacturer, include your best guess at the material. If you’re not sure, say so—they can usually tell from the coiling characteristics and color. But if the original spring is clearly stainless steel and you need it for a corrosion-resistant application, make that clear.

Common Measurement Mistakes and How to Avoid Them

Measuring free length with the spring compressed: This is the most common error. Place the spring on a flat surface with zero load every time. A light touch is fine, but don’t press down.

Measuring wire diameter at the gap between coils: The gap (pitch) is not the same as wire thickness. Measure the wire itself, not the space around it.

Measuring outer diameter at an angle: Always measure across the widest point, perpendicular to the spring axis. If you measure at an angle, you’ll get a false reading.

Confusing total coils with active coils: Report the total number of coils (including end coils). The manufacturer will calculate active coils from the end condition.

Guessing at end condition instead of looking: Open vs. closed vs. ground ends has major cost and lead-time implications. Look closely at the ends and describe exactly what you see.

When Should You Leave Measurement to the Manufacturer?

For most applications, measuring with basic tools and providing those dimensions in an RFQ is sufficient. But there are cases where you should ask the manufacturer to measure for you:

Very small springs (wire diameter under 0.020 inch): Precision measurement is difficult without optical equipment. Send the spring to the manufacturer for measurement.

Mission-critical applications (aerospace, medical, safety equipment): If a spring failure could cause injury or product failure, have a professional measure and test the spring. This costs more but ensures accuracy.

Springs with complex end conditions or irregularities: If the spring looks unusual or has custom end shapes, send it for professional measurement rather than guessing.

When you need verified test data (fatigue life, deflection accuracy to ±0.001 inch): Precision measurement and testing require lab equipment. Budget for this if it’s critical.

Creating Your Measurement Record

Once you’ve measured the spring, document your findings in a format you can include in an RFQ email. A simple table or list works best:

Free Length: [measurement in inches]

Outer Diameter: [measurement in inches]

Wire Diameter: [measurement in inches]

Total Number of Coils: [count]

End Condition: [open / closed / closed and ground / closed and squared]

Material (if known): [music wire / stainless 302 / stainless 304 / other]

Load at specific compression (if measured): [e.g., 10 lbf at 0.25 inch]

Additional notes: [any irregularities, surface finish, coating, or special requirements]

Include this information in your RFQ, and you’ve provided everything a manufacturer needs to quote an accurate replacement or new spring.

Why Accurate Measurement Matters for Your Budget and Timeline

Getting these measurements right isn’t just about technical accuracy—it directly affects your cost and lead time.

If you underestimate wire diameter, the manufacturer will quote for a softer, less capable spring. You’ll get a quote that’s cheaper than the real spring should be, but when it arrives and doesn’t perform correctly, you’ve wasted time and money.

If you overestimate wire diameter, the manufacturer will quote for a stiffer, more expensive spring than you actually need. You’ll pay for more capability than required.

If you get the end condition wrong (say you specify open ends when you really need closed and ground), the spring will rock in your assembly or won’t distribute load properly. You’ll be back to the drawing board.

Accurate measurement takes 15 minutes. Getting it wrong costs days of delay and potentially hundreds of dollars in rush fees or repricing. Spend the time to measure carefully.

Need Help Verifying Your Measurements?

If you’ve measured a spring and want a second opinion before you RFQ, or if you’re not sure whether your measurements are accurate, our engineering team can review your dimensions and help ensure you’re specifying the right spring. Send us your measurements and a photo of the spring, and we’ll confirm the specs or suggest adjustments. This consultation is free—we just want to make sure you get the right spring on the first try.