Spring Rate Calculator

A compression spring's rate is how hard it pushes back for each bit you squeeze it, and it comes from four things: the wire, the coil diameter, how many coils flex, and the metal. This works out the rate from those, and the force at any deflection you set, so you can pick or wind a spring that gives the push you need.

Four things set the rate

Spring rate is force per unit of travel, and it rises with the shear modulus of the metal and the fourth power of the wire diameter, and falls with the cube of the coil diameter and the number of active coils. The headline is that fourth power on the wire: nudge the wire a few thousandths thicker and the spring gets dramatically stiffer. Thicker wire and a smaller coil make a stiff spring; thinner wire, a bigger coil and more turns make a soft one.

Measure the mean coil diameter

The diameter in the formula is the mean, taken to the center of the wire, not the outside. So measure the outside diameter and subtract one wire thickness, or the inside and add one. Getting this wrong throws the rate off a lot, because it is cubed. The same goes for counting active coils: the squared-and-ground ends at each tip do not flex, so the active count is usually about two less than the total coils you can see.

Rate, force and solid height

Once you have the rate, the force is simple: rate times how far you compress it. A spring at 30 pounds per inch pushes back 15 pounds at half an inch of squeeze. That holds until the coils touch and the spring goes solid, where it stops being a spring and becomes a stack of metal. Keep your working range well short of solid height, both so the force stays linear and so you do not set the spring by overstressing the wire.

A design value, not a guarantee

The formula is the standard one and it gets you close, but a real spring varies with the exact wire, how it was wound and heat-treated, and the end conditions. Wire grade and modulus differ a little between batches too. So use this to choose a wire size and coil count, then measure the rate of the real spring on a scale if it matters, and adjust the active coils or wire to land it. For most jobs the calculated rate is plenty close.

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