Here are a few links....

This is from the Poison Spyder Customs site:

For figuring spring rate with two coil springs- (dual rate setup)
(spring rate 1 x spring rate 2) / (spring rate 1 + spring rate 2)

Example for calculating the overall spring rate in a dual rate (two coil) setup:

Spring 1 = 250 lb coil
Spring 2= 300 lb coil

rate= (spring rate 1 x spring rate 2) / (spring rate 1 + spring rate 2)
rate= (250 x 300) / (250 + 300)
rate= 75,000 / 550
rate= 136.36 lbs

Your actual spring rate with a 250 lb coil and 300 lb coil is approximately 136 lbs; remember that the force on the coil is divided between the coils which is why the spring rate is a lower number than the actual individual coil rate.

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FOA's calculator-----> Clicky
This one is kinda fun to play with.
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Rancho's resource-----> CLICKY
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