There are formulas for calculating spring rate.

For example:

https://suspensionsecrets.co.uk/how-to-calculate-coil-or-leaf-spring-rates/
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Or from another website.

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What you won't know exactly is the material used. So you would likely have to use a ballpark guess for a typical spring steel, like the 78.5 GPa used in the example. Paint thickness has a small impact on the result too.

From my understanding, heavier vehicles like a land cruiser can use spring rates around 130,000 N/m (or 13.25 kg/mm).

Another way, if it is a single rate spring, is to measure the original height, then add weights, measure the compressed height.

The added weights mass (kg) x 9.813 (gravity acceleration m/s^2) = Force (N).

Divide the 'Force' value by the distance the spring compressed in metres. Gives you the spring rate in N/m.

From the above spring rate though, you can see you might need to add around 130 kg just to get 10 mm of compression for a high rate spring.