To measure an extension spring, you need five key dimensions: wire diameter, outside diameter, body length, overall length (including hooks), and the number of coils. Getting these right ensures you can find an exact replacement or order a custom spring that fits and performs correctly. A standard ruler or tape measure works for length, but a caliper gives you the precision you need for wire diameter and outside diameter.
Wire Diameter
Wire diameter is the thickness of the metal wire the spring is made from. It’s the single most important measurement because small differences in wire size dramatically change how stiff the spring is. If you have a caliper, measure the wire at one of the hook ends where a single strand is exposed and easy to access.
If you don’t have a caliper, there’s a reliable workaround: count 10 or 20 coils along the body of the spring and measure that total length to the nearest 1/16 of an inch. Then divide the total by the number of coils you counted. Measuring across 20 coils and dividing by 20 gives you a more accurate result than measuring a single wire, because it averages out any small irregularities. For example, if 20 coils span 1.5 inches, your wire diameter is approximately 0.075 inches.
Outside Diameter
The outside diameter (OD) is measured across the widest point of the coil body, from one outer edge to the other. Use a caliper placed around the middle section of the spring rather than near the ends, where the coils can flare slightly. If you notice slight variation from coil to coil, take a few readings along the central portion of the body and average them.
Precision matters here. Manufacturing tolerances for extension springs are tight. A spring with an OD between 0.120 and 0.240 inches, for instance, is typically held to a tolerance of plus or minus 0.005 inches. Larger springs have slightly more allowance: a spring with a 1-inch OD might vary by plus or minus 0.015 inches. So when you measure, try to get within a few thousandths of an inch if possible. A digital caliper makes this straightforward.
Body Length vs. Overall Length
Extension springs have two different length measurements, and confusing them is a common mistake. Body length is the distance covered by the coiled section only, from where the coils start on one end to where they stop on the other. Overall length includes the hooks or loops on both ends. When ordering a replacement, you typically need both numbers.
Measure overall length from the inside edge of one hook to the inside edge of the opposite hook. This is sometimes called “length inside hooks” and represents the functional length of the spring, the distance between the two attachment points. Then measure just the coiled body separately. The difference between the two tells you how much length the hooks add, which varies depending on hook style.
Hook and Loop Styles
The ends of an extension spring come in several configurations, and identifying yours helps narrow down the right replacement. The most common types are full loops and side loops. A full loop is centered in the middle of the coil, sitting directly in line with the spring’s axis. A side loop is offset to the outer edge of the coil body, which changes how the spring attaches and where it sits relative to its mounting point.
Double loops use two wraps of wire instead of one, which reinforces the connection point and handles higher loads. You’ll also see machine hooks (a half-loop that’s more compact) and extended hooks that reach further from the spring body. When measuring, note the hook style and measure the hook opening, the inside gap where the hook attaches to a pin or bolt, since that determines what hardware it fits onto.
Counting Active Coils
The number of active coils directly affects the spring’s stiffness. Count every full coil in the body section, excluding the hooks or transition coils at each end. The transition coils are the ones that curve away from the body to form the hook. Only count coils that are tightly wound together as part of the main body.
This count is essential if you need to calculate the spring rate, which is the amount of force the spring exerts per inch of stretch. The spring rate formula uses wire diameter, the mean diameter of the coils (outside diameter minus one wire diameter), and the number of active coils. More active coils make the spring easier to stretch; fewer coils make it stiffer.
Measuring Initial Tension
Extension springs are unique because their coils are wound tightly together with built-in tension. This is called initial tension: the force you have to apply before the coils even begin to separate. If you pull on an extension spring and nothing happens for the first few pounds of force, that resistance is the initial tension.
To find it, you need a simple test. Hang the spring vertically and add weight until the coils just barely start to separate. That weight is approximately your initial tension. For a more precise calculation, use this approach: stretch the spring a known distance, measure the force required at that distance, then subtract the spring rate multiplied by the distance traveled. For example, if your spring takes 20 pounds of force at 1.5 inches of stretch, and the spring rate is 10 pounds per inch, the initial tension is 20 minus (10 times 1.5), which equals 5 pounds.
Tools and Tips for Accurate Measurement
A digital caliper is the best tool for wire diameter and outside diameter. They’re inexpensive and read out to thousandths of an inch, which is the level of precision spring suppliers expect. A ruler or tape measure works fine for body length and overall length, but round to the nearest 1/16 inch at minimum.
Always measure the spring in its relaxed, unstretched state. If the spring has been in service for years, it may have taken a slight set, meaning it’s a bit longer than it was originally. Note whether the spring appears stretched or if any coils have separated. A properly functioning extension spring at rest should have all its body coils pressed tightly together with no visible gaps.
When recording your measurements, write down all of them: wire diameter, outside diameter, body length, overall length inside hooks, number of active coils, hook style, and the direction of the wind (clockwise or counterclockwise when viewed from the end). Wind direction matters for some applications, particularly garage doors, where left-wound and right-wound springs are not interchangeable.