With regard to the manual the Specular value for the new Principled BSDF node can be computed with following formula:

$\text{specular}=\left(\frac{\text{ior}-1}{\text{ior}+1}\right)^2/0.08$ For example:

water: ior = 1.33, specular = 0.25
glass: ior = 1.5, specular = 0.5
diamond: ior = 2.417, specular = 2.15

But I am getting other numbers. Is the formula wrong? If so, what is the correct one?

  • $\begingroup$ The formula works for me, I get the same results. Are you maybe doing a sqrtf() instead of sqr()? $\endgroup$ Jul 19, 2017 at 13:27
  • $\begingroup$ Yes... I was using square root by mistake. Thanks for pointing that out. It works perfectly. $\endgroup$
    – Tomas
    Jul 19, 2017 at 15:04

1 Answer 1


The formula above is correct. Just don't mistake square with square root as I did before.


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