A spring constant is a physical attribute of a spring. Each spring has its own spring constant. The spring constant describes the relationship between the force applied to the spring and the extension of the spring from its equilibrium state. This relationship is described by Hooke's Law, F = -kx, where F represents the force on the springs, x represents the extension of the spring from its equilibrium length and k represents the spring constant.
Attach a rod to a pair of laboratory retort stands using clamps so that the rod is held horizontally between the two laboratory retort stands. Suspend the spring you wish to calculate the spring constant of from this fixed horizontal rod.
Measure the length of your spring while it is suspended. Measure it from the top end of the spring to the bottom. Make a note of the value of the length of the spring.
Attach to the opposite end of the spring your weight of known mass. Wait until the spring stops oscillating and is stationary.
Measure the length of your spring with the known mass attached to it. As before measure from the top end of the spring to the bottom. Make a note of the measured length of the spring.
Subtract the initial length of the spring from the eventual length of the spring and make a note of this value. This value is the extension of the spring resulting from the applied force.
Multiply the mass of your known weight by the gravitational strength of the Earth. If your known weight is in kilograms, this value will be 9.8 meters per second squared. If your known weight is in pounds, this value will be 32.2 feet per second squared. This gives you the force acting on the spring.
Divide the force acting on the spring by the extension of the spring. This value is the spring constant.
Ensure you use consistent units, so if you are using pounds for your known mass, use feet for your measuring of the spring extension, if you are using kilograms for your known mass, use meters for your measuring of the spring extension.