Compression Springs Explained — Types, Terms, Formulas & Uses
Quick answer: A compression spring is an open-coiled helical spring that resists a squeezing force and returns to its free length when the load is removed; its stiffness is the spring rate k = G·d⁴ / (8·D³·Na), in N/mm.
By Gurudatta Trading Co., Mumbai · Updated 2026-10-11
What is a compression spring?
A compression spring is an open-coiled helical spring wound from round wire. When a load squeezes it, the spring shortens and pushes back; when the load is removed it returns to its free length. Within its working range the force is proportional to the deflection (Hooke's law): F = k × x, where k is the spring rate in N/mm.
Compression springs are among the most widely used springs — in valves, clutches and brakes, suspensions, press tools and moulds, switches, pens, mattresses and appliances.
Main types of compression spring
| Type | Shape | Typical use |
| Cylindrical (straight) | Constant coil diameter | The most common form; general-purpose, steady rate |
| Conical | Tapered coil diameter; coils can nest inside one another | Short solid height or a rate that stiffens as the coils close |
| Barrel and hourglass | Coil diameter bulges or narrows towards the middle | Lateral stability and different vibration behaviour |
| Variable pitch | Coil spacing changes along the length | Progressive rate as coils close in stages |
| Die spring | Rectangular wire, heavy duty | Press tools, dies and moulds — see the die spring guide |
Key terms
| Term | Meaning |
| Free length (L₀) | Length of the spring with no load |
| Solid length (Ls) | Length when all coils touch; roughly total coils × wire diameter |
| Wire diameter (d) | Diameter of the wire — see the SWG chart |
| Outside / inside diameter | OD = D + d and ID = D − d, where D is the mean diameter |
| Mean diameter (D) | Outside diameter minus wire diameter |
| Total coils (n) and active coils (Na) | All coils versus the coils that actually deflect; with closed ends, Na is about n − 2 |
| Spring index (C) | D ÷ d |
| Pitch | Distance between adjacent coils |
| Spring rate (k) | Force per unit of deflection, in N/mm |
| Set | Permanent loss of free length after the spring has been overstressed |
Spring rate formula with a worked example
k = G · d⁴ / (8 · D³ · Na), where G is the shear modulus of the wire (80,000 N/mm² for the mild steel setting in our calculator).
Example: wire d = 2 mm and OD = 20 mm, so D = 18 mm; 12 total coils with closed ends, so Na = 10. Then k = 80,000 × 2⁴ ÷ (8 × 18³ × 10) ≈ 2.74 N/mm. The spring index is C = 9 and the solid length is about 12 × 2 = 24 mm. Our compression spring calculator gives the same rate (2.743 N/mm) and adds the safe load (63.1 N for mild steel) and maximum deflection (22.99 mm). More on the maths in the spring rate formula guide.
Spring ends
Open ends leave the end coils untouched and are the simplest to make. Closed ends turn the end coil in to touch the next coil, and closed-and-ground ends are then ground flat so the spring sits square on a surface. Closed ends that are not ground add roughly one wire diameter to the solid length. Details are in our spring end types guide.
Materials
| Material | G in our calculator (N/mm²) | Typical use |
| Music wire (ASTM A228) | 81,500 | High-strength, small precision springs in dry conditions |
| Hard drawn wire (ASTM A227) | 79,300 | General-purpose, lower cost |
| Chrome vanadium (ASTM A232) | 80,700 | Higher stress, shock loads and moderate heat |
| Stainless steel (ASTM A313) | 73,000 | Corrosion resistance |
| Phosphor bronze (ASTM B197) | 44,000 | Corrosion resistance and non-magnetic duty |
| Mild steel (IS 4454) | 80,000 | Economical general industrial springs, usually plated or coated |
See the material comparison and the IS 4454 guide for how to choose.
Design rules of thumb
- Spring index between about 4 and 12. A very low index is hard to coil and highly stressed; a very high index tangles easily and is less consistent.
- Watch slenderness. A spring whose free length is more than roughly four times its mean diameter can buckle sideways unless it is guided by a rod or a hole.
- Never run to solid. Leave a margin before the coils touch; working at solid height overstresses the wire.
- Cycling needs lower stress. A spring that flexes millions of times must be stressed far less than one that is compressed once and held.
- Allow for tolerance. Wire and coil diameters vary within tolerance, so a real spring’s rate typically varies by roughly ±10% from the theoretical formula.
Why compression springs fail
| Failure | Typical cause | Prevention |
| Permanent set | Stressed beyond the allowable stress, or compressed to solid | Lower the stress, stop before solid, pre-set the spring |
| Fatigue fracture | Repeated cycling at high stress or from surface flaws | Lower stress, clean wire, surface treatment such as shot peening |
| Buckling | Slender spring with no guide | Guide on a rod or in a bore; reduce free length relative to diameter |
| Corrosion | Moisture or chemicals | Stainless steel or a protective finish — see finishes and coatings |
How to specify a compression spring
- Wire diameter, and outside or inside diameter
- Free length and total number of coils
- End type and direction of wind (left or right hand)
- Material, finish and quantity
- The load required at a given length, if it is critical
Not sure how to measure? Follow our guide on how to measure a spring, or read how to choose a compression spring manufacturer.
Need springs made to your specification?
Gurudatta Trading Co. (Mumbai, since 2000, ISO 9001:2015) manufactures compression, extension and torsion springs to drawing or sample. Use the free Spring Rate Calculator or message us on WhatsApp.
FAQs
What is a compression spring?
A compression spring is an open-coiled helical spring that shortens under a squeezing load and returns to its free length when the load is removed.
How do I calculate the rate of a compression spring?
Spring rate k = G·d⁴ / (8·D³·Na), where G is the shear modulus, d the wire diameter, D the mean coil diameter and Na the number of active coils.
What is the difference between a compression spring and an extension spring?
A compression spring is open-coiled and resists being squeezed; an extension spring is close-wound with hooks at the ends and resists being pulled apart.
What is solid length?
Solid length is the length of the spring when all its coils touch, roughly total coils multiplied by wire diameter for closed and ground ends.
Can you make a compression spring from a sample?
Yes. Send the sample or its measurements and we will quote and manufacture a matching spring.