How this instrument works
Spring rate, often written k, is the slope of a spring's force-versus-deflection line: how many newtons or pounds-force it pushes back for every unit it is compressed or stretched. Hooke's law states it plainly, F = k·x — double the squeeze, double the push, provided the spring stays in its linear range. A soft coilover front spring might sit near 1 N/mm; a stiff rear spring on a track car can run past 10 N/mm. The number alone says nothing about wire diameter or coil count — it is purely how stiff the finished part feels.
The three units on this page measure that identical physical slope, just scaled by different length and force conventions. A pound-force is fixed by international agreement at 4.4482216152605 newtons — a pound-mass times standard gravity, 9.80665 m/s² — and an inch is fixed at exactly 0.0254 metres. Divide the first by the second and a rate quoted per inch converts to newtons per metre at a factor of 175.126835, or per millimetre at 0.175126835 — the constant this instrument divides by to move from N/mm to lbf/in. Going from N/mm to N/m needs no such derivation; a millimetre is simply a thousandth of a metre, so the figure is multiplied by 1000.
A converted number still describes only the spring, at the point on its travel where it was measured. Two limits are worth knowing. First, this is a linear-rate conversion: a progressive spring's rate climbs as it compresses, so a single N/mm or lbf/in figure holds near the deflection it was tested at, not across the whole stroke. Second, spring rate is not wheel rate — once a spring sits behind a lever, a bell crank, or an angled shock, the rate felt at the wheel equals the spring rate times the installation ratio squared, and no unit conversion changes that geometry.
- Enter your spring's stiffness into Spring rate, N/mm — the single field this instrument reads from.
- Spring rate, lbf/in updates automatically, giving the figure a US or SAE-spec catalog would print for the same part.
- Spring rate, N/m updates too — the plain SI value, useful for feeding into a natural-frequency or vibration formula.
- Already have a rate in lbf/in or N/m? Multiply lbf/in by 0.175126835, or divide N/m by 1000, to get N/mm first.
Worked example — a 1 N/mm coilover spring in three units
Take a coilover spring rated at 1 N/mm — a soft front spring on a street-driven track car, the kind of figure printed on the spring's ID sticker. Enter 1 into Spring rate, N/mm: Spring rate, lbf/in reads 5.71014716277, the exact figure a US-spec parts catalog would print for the identical part, and Spring rate, N/m reads 1000.0, its plain SI equivalent — no rounding, since a millimetre-to-metre step is exact.
Scale the same spring rate up and the three units scale together. A 5 N/mm spring — five times stiffer — converts to 28.5507358138 lbf/in and 5000 N/m; a 10 N/mm spring, near the firm end of a performance suspension setup, converts to 57.1014716277 lbf/in and 10000 N/m. Every output here is a fixed linear multiplier of the N/mm input, so doubling the rate simply doubles every converted figure.
Questions
Why not just use 175 as the lbf/in-to-N/m conversion factor?
Because 175 is only accurate to three significant figures, and spring-rate work often chains several conversions — rate, load, deflection — where that rounding shows up as a real discrepancy. The exact factor, 175.126835 N/m per lbf/in, comes from two internationally fixed definitions, the pound-force (4.4482216152605 N) and the inch (0.0254 m), divided against each other, not from a measured or approximate constant.
Is spring rate the same thing as wheel rate?
No. Spring rate is the stiffness of the spring alone, force per unit of its own compression. Wheel rate is what the wheel feels once the spring sits behind suspension geometry — a bell crank, rocker, or angled shock — and equals spring rate multiplied by the installation ratio squared. Converting N/mm to lbf/in here changes only the unit, never that geometric relationship.
Does the converted rate apply across the spring's whole travel?
Only for a linear spring, where the rate is constant by design. A progressive or variable-pitch spring's rate increases as it compresses, so a single N/mm or lbf/in figure describes just the point, or narrow range, where it was measured — usually stated on a spec sheet as an average or initial rate, not a constant for the full stroke.
Where does 0.175126835 actually come from?
From dividing the exact newton value of one pound-force, 4.4482216152605 N — itself one pound-mass times standard gravity, 9.80665 m/s² — by one inch, exactly 0.0254 m. That division gives 175.126835 N per metre for every lbf/in, or 0.175126835 N per millimetre, the constant this instrument divides by to convert N/mm into lbf/in.
Why is the N/mm-to-N/m conversion an exact factor of 1000?
Because a millimetre is defined as exactly one-thousandth of a metre — no measured constant is involved, unlike the pound-force conversion. A spring rate of 1 N/mm always equals precisely 1000 N/m with no rounding at any stage, which is why that output carries none of the leftover precision the lbf/in figure shows.