Solid height is the wire diameter times the total coil count. Once the coils touch, the spring is a solid bar — the load path goes rigid with no warning, and the stress check that said it was fine no longer applies.
The stress calculation answers "will this wire survive the load", but geometry answers a different question that usually bites first: whether there is room left to deflect. Free length minus solid height is all the travel there is, and any overtravel, tolerance stack or thermal growth eats into it. Going solid is not a gentle limit — the component behind it suddenly sees whatever force is available.
Springs designed to bottom out on purpose exist — many are used as travel limiters. And a spring that reaches solid gently under a controlled load is fine; the danger is the impact case, where a rigid path passes the shock straight through.
Estimate with the rule, then check it against the calculator that models it properly.
Open Spring Rate Calculator →Solid height is the wire diameter times the total coil count. Once the coils touch, the spring is a solid bar — the load path goes rigid with no warning, and the stress check that said it was fine no longer applies. The stress calculation answers "will this wire survive the load", but geometry answers a different question that usually bites first: whether there is room left to deflect.