Helical Spring Rate Calculator

How it is calculated

The tool applies the standard formula for a round-wire helical compression spring: k = G × d⁴ ÷ (8 × D³ × n). G is the shear modulus of the wire material in GPa, d is the wire diameter in mm, D is the mean coil diameter in mm, and n is the number of active coils. The engine multiplies G by 1000 to turn GPa into MPa, so the answer comes out in N/mm, shown with two decimals. All four values must be greater than zero or no result is given. The result is the force needed per millimetre of compression. It does not check stress, buckling, solid height or the end coils, which are not active.

Common mistakes

• Using the outside coil diameter. The formula needs the mean diameter, which is the outside diameter minus one wire diameter.
• Counting every coil. Only the active coils count; the end coils that sit on the seat, often one or two, should be excluded.
• Entering the shear modulus in MPa or psi. The field is in GPa, so spring steel at 79,300 MPa is entered as 79.3.
• Measuring in inches while the formula's result is labelled N/mm. All lengths must be in millimetres, otherwise the answer is meaningless.
• Underestimating wire size. The rate depends on d⁴, so a measurement error of 5% in wire diameter changes the rate by more than 20%.