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Extension Spring Design Calculator (Tension Spring Calculator)

Use this free extension spring calculator to size ordinary tension springs and calculate spring diameter, spring rate, number of coils, preload range, spring index, free length and body length.

This calculator is intended for ordinary extension springs with twisted ends where the hook mean diameter equals the spring mean diameter.

It is useful for engineers, designers, students and buyers who need a fast starting point for extension spring design.

What this extension spring calculator does

This tool helps you calculate the main dimensions and stiffness of an extension spring from different known parameter sets. You can work from spring rate and free length, spring diameter and free length, number of coils and free length, preload and spring rate, or spring diameter and spring rate.

The calculator reports mean diameter, outer diameter, inner diameter, active coils, body coils, body length, preload range, spring index and material modulus values.

How to use it

  1. Select the calculation mode that matches your known inputs.
  2. Enter only the active input fields.
  3. Choose input and result units.
  4. Select a standard material or enter your own values.
  5. Click Calculate to view the results below.

Extension spring design basics

An extension spring, also called a tension spring, stores energy when it is stretched. Important design parameters include wire diameter, mean coil diameter, spring rate, number of active coils and spring index.

k = (d4 × G) / (8 × D3 × Na)

Where k is spring rate, d is wire diameter, D is mean diameter, Na is active coil count and G is shear modulus.

Extension spring terminology

This figure shows the main terms used in the calculator so it is easier to match each input to the spring drawing. Use it to check wire diameter, free length, outside diameter, inside diameter, mean diameter and body length before you calculate.

Tip: choose the same diameter type here and in your drawing. If your drawing gives outside diameter, select Outer diameter, OD. If it gives inside diameter, select Inner diameter, ID.

Extension spring design terminology showing wire diameter, free length, body length, outside diameter, inside diameter and mean diameter
Ordinary extension spring with twisted ends and the main design dimensions used on this page.

1. Calculation mode

Known parameter set

2. Spring inputs

Only the needed inputs are active for the selected mode.

Always required.
Active only in diameter modes.
Active in free-length modes.
Used in modes 1, 4 and 5.
Used only when body coils are known.
Active only in preload mode.

3. Units and output format

Input units are separated from result units so the screen is easier to read.

4. Material data

Choose a standard spring material or enter your own modulus values.

Editable only for user-defined material.
Editable only for user-defined material.

How to use this calculator

Pick a calculation mode, enter the active input fields, then click Calculate. The inputs are grouped into spring inputs, units and material data to make entry easier.

Results

Mean diameter, D mm—
Spring index, C—
Spring rate, k N/mm—
Body coils, Nb—

Geometry

Number of active coils, Na—
Number of body coils, Nb—
Spring index, C—
Spring rate, k N/mm—
Wire diameter, d mm—
Spring outer diameter, OD mm—
Spring mean diameter, D mm—
Spring inner diameter, ID mm—
Free length inside hooks, Lo mm—
Spring body length, Lb mm—

Material and preload

Preload range, minimum N—
Preload range, maximum N—
Shear modulus, G GPa—
Elastic modulus, E GPa—
Material ASTM No.—