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Spring Index

Definition of Spring Index: Spring index is the correlation between the mean diameter of a spring and the wire diameter of a spring.  This proportion will determine the strength of the spring, the stress induced on the spring, and the manufacturability of the spring. On this page you will discover how to calculate your index mathematically using the spring index formula, how to use the index in spring design, how to measure a spring in order to determine index, and the most desirable range for a spring index to be in.   

Formula For Calculating Spring Index:

Index = Mean Diameter (D) / Wire Diameter (d)

Mean Diameter (D) = Outer Diameter (OD) - Wire Diameter (d)


Outer Diameter: 1.000 inches

Wire Diameter: 0.080 inches

Free Length: 2.500 inches

Total Coils: 10

Ends: Closed and Square

Measuring Your Spring

In order to properly calculate the spring index formula you will need to know how to measure a spring.  Please see the images below to see how to properly measure a compression, extension, and torsion spring.  For a more detailed explanation on how to accurately measure your spring please visit our spring measurement page.

      how to measure a compression spring                       how to measure an extension spring

how to measure a torsional spring


Using Spring Creator Calculator to Determine Index:

To see how decreasing or increasing your index will effect the strength of your spring please use our Online Spring Calculators.  Simply enter the dimensions you have measured using the diagrams above and hit calculate.  The spring index will be in the "physical dimensions" section.  Remember, to change your index you will need to manipulate the outer diameter of your spring and/or the wire diameter of your spring. 


Spring Creator spring index calculator

Manufacturability of Your Spring Based on Spring Index:

The spring index on your spring is one of the first things a spring engineer will evaluate when deciding if your spring is manufacturable and what the cost will be to manufacture your spring.  If you stick to the numbers below you will have lower production costs and more availability of spring makers to manufacture your spring.

Spring Index: 0 - 3.9 : Acxess Spring Cannot Manufacture
Spring Index = 4 - 5 : Your Spring Falls in the Difficult Category, higher cost
Spring Index = 6-12 : Best Manufacturing Range Lower Cost
Spring Index = 13 - 15: Your Spring is Just Above Good But Not Quite in The Difficult Range
Spring Index = 15 - 25: Spring Falls in the Difficult Category, higher cost
Spring Index = Above 25 : Very expensive and difficult to manufacture, tolerances are larger

torsion spring index examples; spring index less than 4 is hard to manufacture

How Does My Index Effect The Strength Of My Spring:

The index of your spring is in direct correlation with how strong your spring will be.  If you need to INCREASE the strength of your spring, you will need to reduce the index.  There are two ways to reduce the index.  The first way is to INCREASE the diameter of the wire.  The second way is to DECREASE the outer diameter of the spring.  If you need to DECREASE the strength of your spring, you will need to enlarge the index.  There are two ways to enlarge the index.  The first way is to DECREASE the diameter of the wire.  The second way is to INCREASE the outer diameter of the spring.  Please see the chart below for further information.  

explanation of how a smaller index gives you a stronger spring and a larger index gives you a weaker spring

Acxess Spring, 2225 E. Cooley Dr. Colton, CA 92324

Ph: (951)276-2777 Fx: (951)240-3334 Email: sales@acxesspring.com