How to Measure a Spring You Want to Replace

A spring breaks or wears out in a machine, there is no drawing, and the part number from the original order is long gone. For maintenance teams and machine builders this is one of the most common situations there is. The good news: with a caliper, a steel rule and a few minutes of care, you can describe almost any compression, extension or torsion spring well enough to find a matching standard spring. This guide shows what to measure, in which order, and how to check your numbers before you order.

Before you start: tools and ground rules

You do not need a laboratory. A practical set of tools is:

• Digital caliper for outer diameter, free length and wire diameter.
• Micrometer (optional) for a more exact wire diameter, especially on thin wire.
• Steel rule and a flat surface for long springs.
• Protractor for the leg angle of torsion springs.
• Marker pen to mark the start point when you count coils.

Three ground rules make the result far more reliable:

1. Measure the spring unloaded, never while it is installed or squeezed by the caliper jaws.
2. Take each measurement at least three times at different points. Use the average, or the largest value for outer diameter.
3. If you can, measure a spring that has not been in service, for example the twin on the other side of the machine. A spring that has worked for years may have lost part of its free length.

Step 1: Identify the spring type

The type decides which dimensions matter. A compression spring has open space between the coils and is pushed together in use. An extension spring has coils that touch each other when unloaded and hooks or loops at the ends, and it is pulled apart. A torsion spring has legs sticking out from the coil body and works by rotation around its axis. Die springs, disc springs, wave springs and gas springs are identified by other data and are not covered in detail here.

Step 2: Measure a compression spring

Wire diameter (d)

Measure the wire on a straight part of a coil, away from the ground ends. Turn the spring and take several readings. If the spring is painted or coated, the coating adds to the reading, so measure where it is worn or subtract its thickness. Be precise here: wire diameter has the biggest effect on the spring rate, and a 5% error in wire diameter changes the calculated rate by more than 20%.

Outer diameter (De)

Measure across the coils at several points along the length and note the largest value. Outer diameter is what must fit inside a bore. Inner diameter (Di) is the outer diameter minus two wire diameters and matters if the spring sits over a rod or pin. The mean diameter used in calculations is D = De minus d.

Free length (L0)

Stand the spring on a flat surface and measure the overall length with no load. For long springs, use a steel rule against a square. Also note the installed length in the machine and the length at the end of the working stroke. These two numbers define the working range the replacement must handle.

Number of coils and end type

Mark the tip of the wire at one end, then count each full turn along one side of the spring until you reach the other tip. Include half and quarter turns. This gives the total number of coils (nt). Then look at the ends:

• Closed and ground: the end coil touches the next coil and is ground flat, so the spring stands upright on its own.
• Closed, not ground: the end coil is closed but not flat.
• Open: the pitch continues all the way to the tip.

For closed ends, the number of active coils is normally the total minus two (n = nt minus 2), because the closed end coils do not deflect. Active coils are what you need for the rate calculation.

Winding direction and material

Hold the spring upright and look at the coils facing you. If they rise to the right, like a normal screw thread, the spring is right hand wound. Most compression springs are right hand, and for a single spring the direction rarely matters. It does matter when two springs are nested inside each other or when the spring is screwed onto a threaded part.

For material, a magnet gives a first hint. Carbon spring steel is strongly magnetic. Stainless spring wire such as 1.4310 is usually only weakly magnetic, but cold drawing makes it partly magnetic, so a magnet test alone does not prove the grade. Look at the environment too: a spring in a wet or food related area was most likely stainless.

Step 3: Measure an extension spring

Wire diameter and outer diameter are measured the same way as for a compression spring. The differences are in the length and the ends.

• Free length (L0): measured on the unloaded spring from the inside of one loop to the inside of the other. This is the convention used in EN 13906-2 and in most catalogues, so always state it this way.
• Body length (LK): the length of the coiled part only. With touching coils it is roughly the number of body coils plus one, multiplied by the wire diameter, which is a useful check on your coil count.
• Loop type and position: note whether the ends are full loops, hooks or plain ends, whether the loop sits in the centre or at the side, and the angle between the two loops (in line or turned 90 degrees).
• Loop opening: measure the gap and the inner diameter of the loop so the replacement fits the pin or bolt in your machine.

Extension springs also have initial tension, a built in force that keeps the coils closed. You can estimate it by hanging the spring and adding known weights until a thin gap just appears between the coils. The estimate is rough, but it tells you whether the replacement should open easily or stay tight. The extension spring calculator lets you check force, initial tension and extension for a given size.

Step 4: Measure a torsion spring

For a torsion spring, record wire diameter, outer diameter and the number of body coils as above, then add:

• Inner diameter and shaft diameter. The coil diameter gets smaller under load, so there must be clearance to the shaft or pin.
• Leg lengths, measured from the centre axis of the coil body to the end of each leg, and the leg shape (straight, bent or hooked).
• Free leg angle: the angle between the two legs with no load, measured with a protractor.
• Winding direction: left or right hand. For torsion springs this is critical, because the spring must be loaded in the direction that winds the coils up. A spring of the opposite hand will not work in the same mounting.
• Working angle: how far the legs rotate in the machine between the open and the closed position.

Step 5: Check your numbers with a calculation

Before you search, check that the measurements make sense. For compression and extension springs, the spring rate follows from the dimensions with the standard formula in EN 13906:

R = G × d⁴ / (8 × D³ × n)

R is the rate in N/mm, G the shear modulus (about 81,500 N/mm² for spring steel and about 70,000 N/mm² for stainless 1.4310), d the wire diameter, D the mean diameter and n the number of active coils. The spring calculator does this for you and also helps you find matching sizes. For the theory behind it, read What Is Spring Rate and How Do You Calculate It?

Worked example

A technician measures a worn compression spring from a guard door:

Measurement	Value
Outer diameter De	20.0 mm
Wire diameter d	2.0 mm
Free length L0	50.0 mm
Total coils nt	9.5
Ends	Closed and ground
Material	Strongly magnetic, spring steel
Installed length / end of stroke	40 mm / 30 mm

Mean diameter D = 20.0 minus 2.0 = 18.0 mm. Active coils n = 9.5 minus 2 = 7.5. The rate is R = 81,500 × 2.0⁴ / (8 × 18.0³ × 7.5) = 1,304,000 / 349,920, which is about 3.7 N/mm.

At the installed length the spring is compressed 10 mm and gives about 37 N. At the end of the stroke it is compressed 20 mm and gives about 75 N. The solid length of a closed and ground spring is roughly nt × d = 19 mm, so 11 mm of margin remains at 30 mm. The numbers are consistent.

A quick physical check: press the spring down to 40 mm on a bench scale. It should read close to 3.8 kg (37 N). If the reading is far off, recount the coils or remeasure the wire. With a confirmed rate of about 3.7 N/mm you can now look for a spring with De 20 mm, d 2.0 mm and L0 around 50 mm, and compare the forces at 40 mm and 30 mm.

Step 6: Choose the replacement

A replacement rarely needs to match every number exactly. Work through the requirements in this order:

1. Fit: outer diameter must clear the bore and inner diameter must clear the rod, with room for the diameter change under load.
2. Force: the force at the installed length and at the end of the stroke should be close to the original.
3. Travel: the replacement must not reach solid length (or maximum extension) within the working stroke.
4. Material and ends: match the environment and the seating or attachment.

With these data you can filter the compression springs, extension springs or torsion springs by diameter, wire, length and rate. If the original spring broke early, treat the replacement as a chance to improve: a slightly larger wire, a lower working stress or a corrosion resistant material can give a much longer life.

Quick checklist

Spring type	Measure
Compression	d, De, L0, total coils, end type, installed and working length
Extension	d, De, L0 inside loops, body coils, loop type, position and opening, initial tension estimate
Torsion	d, De, Di, shaft diameter, body coils, leg lengths and shape, free leg angle, winding direction, working angle

FAQ

What if the old spring is broken or has lost length?

Measure wire diameter, outer diameter and coils on the largest intact piece. For free length, measure an unused twin spring if there is one, or work backwards from the installed length and the force the mechanism needs. A spring that has taken a permanent set is shorter than when it was new.

How exact do my measurements need to be?

Wire diameter should be measured to 0.01 mm where possible, because it dominates the rate. Outer diameter and free length to about 0.1 mm are normally enough. Standard springs are made to defined tolerance classes, for compression springs in EN 15800, so small differences between your reading and the catalogue value are expected.

Can I use a spring with slightly different dimensions?

Often yes. If the fit is right and the forces at the installed and working lengths are close to the original, a small difference in free length or coil count is usually acceptable. Check the travel margin to solid length again with the new spring.

Does winding direction matter?

For a single compression or extension spring it rarely does. For torsion springs it always does, and for nested compression springs the inner and outer spring should be wound in opposite directions so they do not tangle.