Spring Swap Force Calculator

Working Principle

A keyboard spring is a helical compression spring, and its stiffness follows the standard spring rate equation k = G·d⁴ / (8·D³·n), where d is wire diameter, D is mean coil diameter, n is the number of active coils and G is the shear modulus of the wire material, about 79.3 GPa for stainless steel music wire. The fourth-power term on wire diameter is why tiny gauge changes matter so much: increasing wire from 0.16 mm to 0.17 mm raises stiffness by roughly 27 percent with everything else unchanged. Force at any depth is then k multiplied by the compression at that depth, where compression equals free length minus installed length. Inside an MX switch the spring is preloaded, meaning it is already compressed when the switch is at rest, and that preload is what gives a switch its initial resistance. The calculator asks for free length and installed length so it can compute preload, then adds the actuation depth and total travel to get compression at each point. Two practical consequences fall out of the maths. First, a longer free length at the same rate raises the whole force curve without changing its slope, which is exactly what a long spring does to a switch's feel. Second, progressive and slow springs deviate from this linear model because their coil pitch varies along the length, so the tool reports linear results and notes when a progressive spring will read higher near bottom-out than predicted.

How to Use

1. 01Measure or look up wire diameter in millimetres — vendors list it as 0.15-0.20 mm for typical keyboard springs.
2. 02Enter mean coil diameter (outer diameter minus one wire diameter) and the number of active coils.
3. 03Enter free length and the installed length inside the switch, usually around 11.5 mm for MX.
4. 04Read spring rate, preload force, actuation force and bottom-out force, then compare against your current switch.
5. 05Note: progressive and slow springs are non-linear; treat the bottom-out figure as a lower bound.

FAQ

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