How-To: Calculating Spring Rates
 

If you ever need to calculate spring rates...

The spring rate equation is k=(G*d^4)/(8*n*D^3)
where k is the spring rate, G is the modulus of rigidity, d is the wire diameter, n is the number of active coils, and D is the average diameter (inside diameter plus wire diameter).

Given that G is about 11.4 million psi for steel alloys, that reduces the equation to: k=(1425000*d^4)/(n*D^3),

If that's still too much for you this is about a simple as I can make the equation:
spring rate = (1425000*d*d*d*d)/(n*D*D*D)

OR

Here's a handy online calculator:
http://www.engineersedge.com/spring_comp_calc_k.htm

Hint: As mentioned above, if the springs are pretty much any steel alloy you can use 11,400,000 for G "modulus of rigidity". All other variables can be easily measured at home from your springs.

Or use the fatcat spreadsheet ;)

Oscar,

The above formula has nothing to do with the FatCat calculator. It calculates the spring rate of a given unknown spring from physical measurements.

Since d is to the fourth power, better bring a good micrometer.

Or a ruler and a fat friend.

I know. I thought it had those formulas too. All you had to do was enter the length and wire thickness etc. of the unknown spring and it would do the calculations for you.

or a micrometer and a skinny friend.