From my experience (using observation, not engineering data):
1.) 10mm thick is good for 40mm fans, fair for 50mm fans, bad for 60mm fans.
2.) 15mm thick is good for 50mm fans, fair for 60mm fans, bad for 80mm fans
3.) 20mm thick is good for 60mm fans, fair for 80mm fans, bad for 120mm fans
4.) 25mm is good for 80mm fans, fair for 120mm fans
5.) 38mm is good for 120mm fans.
And some fans are thicker than durability demands, for example I have a 40mm fan that's 25mm thick, it uses that extra thickness to add more curvature to the blade for increased flow in a small space.
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