Spring Rate Calculator
This spring rate calculator finds the stiffness of a spring from the force applied and how far it compresses, using the spring rate formula. Enter the force and the displacement and the spring rate calculator returns the rate in newtons per millimeter and newtons per meter. It works for coil, coilover, and suspension springs, so the same spring rate calculator covers a car, a bike, or a machine.
Results
The Spring Rate Calculator is part of our Force & Motion collection. Browse more force & motion to size, convert, and check your work alongside this tool.
What the spring rate calculator does
Spring rate is how much force it takes to compress a spring by a set distance, and this spring rate calculator works it out from a force and the deflection it causes. A stiff spring has a high rate and barely moves under load; a soft one has a low rate and compresses easily. Enter the two values and the spring rate calculator applies the spring rate formula, giving the stiffness in N/mm, the unit a coil spring calculator and a coilover spring rate calculator both use.
The spring rate formula
The spring rate formula is Hooke's law rearranged for stiffness, force divided by displacement:
where F is the force and x is the deflection. A spring rate of 10 N/mm means every 10 newtons compresses it one millimeter. The spring rate calculator reports both N/mm and N/m, and because the relationship is linear for a normal coil spring, the same spring rate formula holds across its working range. A torsion spring rate formula uses torque and angle instead, but the idea is the same.
Spring rate formula example
A spring compresses 25 mm under a 100 N load. The spring rate formula gives:
So every 4 newtons moves it a millimeter. Enter your own force and deflection and the spring rate calculator returns the stiffness, exactly as a coil spring calculator would for the same measurements.
How to use the spring rate calculator
Enter the force applied to the spring and the distance it compresses under that force. The spring rate calculator divides one by the other to give the rate. To find the rate of an unknown spring, hang a known weight and measure the deflection, then enter both. Used as a coil spring rate calculator or a suspension spring rate calculator, the method is identical whatever the spring does.
Reading the spring rate calculator result
A higher N/mm means a stiffer spring. Whether that is right depends on the job: suspension wants a rate matched to the sprung weight for a controlled ride, while a valve spring needs enough rate to close fast without float. Compare the result to the rate you expected, since a spring that reads far off may be fatigued or mislabeled. The spring rate calculator gives a number you can match against a target, which is what a coilover spring rate calculator, a coil over spring calculator, or a motorcycle spring rate calculator is really for.
When to use the spring rate calculator
- Rating an unknown coil with a coil spring rate calculator
- Tuning a car with a coilover spring rate calculator
- Setting up a bike with an mtb spring rate calculator
- Checking an engine with a valve spring rate calculator
- Rating a truck spring with a leaf spring rate calculator
- Converting units with a spring rate conversion calculator
Common spring rate formula mistakes
- Measuring at the extremes. Near coil-bind a spring stiffens, so the spring rate formula reads high; measure in the normal range.
- Ignoring progressive springs. A progressive spring has no single rate, so the spring rate calculator gives the average over your measurement.
- Mismatched units. Keep force in newtons and deflection in millimeters for a clean N/mm result.
Tips for accurate results
- Measure deflection near the real working load for the truest spring rate.
- Use a known weight and a ruler to rate an unknown spring in the calculator.
- Match the rate to the sprung weight when tuning suspension.
What the spring rate formula assumes
The spring rate formula treats a spring as linear, meaning it compresses the same amount for each added newton across its working range, and for a normal coil spring that holds well. This is why you can measure the rate with a single weight and trust it: double the force and you double the deflection. The linearity breaks down at the extremes, when a spring is nearly coil-bound at full compression or when it is a progressive spring wound to stiffen as it compresses. For those, the rate the spring rate calculator gives is the average over the movement you measured, so measure near the load the spring will see for the most useful figure.
Matching the coil spring calculator to the job
The right spring rate is always about the load and the movement you want, not a single best number. In vehicle suspension, the rate is matched to the sprung weight so the ride is controlled without being harsh, which is the whole point of a coilover spring rate calculator when tuning a car or an mtb spring rate calculator for a bike. Rider or driver weight shifts the target, so a heavier setup needs a stiffer spring for the same feel. In an engine, a valve spring rate calculator sets a rate high enough to keep the valve following the cam at speed without float. Different jobs, one formula: measure or specify the rate, then match it to the load.
Spring rate conversion chart
Spring rate in different units for common values, a spring rate conversion chart from the calculator:
| N/mm | N/m | lb/in (approx) |
|---|---|---|
| 1 | 1000 | 5.7 |
| 4 | 4000 | 22.8 |
| 10 | 10000 | 57.1 |
| 25 | 25000 | 142.7 |
| 50 | 50000 | 285.4 |
Spring Rate Calculator FAQ
What is the spring rate formula?
The spring rate formula is k = F / x, force divided by displacement. The spring rate calculator applies it and reports the stiffness in N/mm and N/m.
How do I measure the rate of an unknown spring?
Apply a known weight, measure how far the spring compresses, and enter both. The spring rate calculator divides force by deflection to give the rate a coil spring calculator would.
What does a higher spring rate mean?
A higher N/mm is a stiffer spring that moves less under load. A coilover spring rate calculator is used to match that stiffness to the vehicle weight for the ride you want.
Does it work for suspension springs?
Yes. A suspension spring rate calculator, motorcycle spring rate calculator, or leaf spring rate calculator all use the same force-over-deflection method this tool applies.