PHYSICALLYBASED

A database of physically based values for CG artists

Shading

sRGB Gamma to Linear

sRGB Gamma

Linear

Formulas:
sRGB = if (Linear < 0.0031308) Linear * 12.92 else (1.055 * pow(Linear, 0.41666) - 0.055)

Linear = if (sRGB < 0.04045) sRGB * 0.0773993808 else (pow(sRGB * 0.9478672986 + 0.0521327014, 2.4))

Sources: [0][1][2]

Shading

Specular to IOR

Specular

Index of Refraction

Formulas:
Specular = pow((IOR - 1) / (IOR + 1), 2) / 0.08

IOR = 1 / (2 / (sqrt(0.08 * Specular) + 1) - 1)

Sources: [0][1]

These reference values are based on the most accurate data available at the time, and are continually updated whenever new measurements are available. Depending on the material and the property you're after, there is more or less variability to take into account. Use these values as a starting point, or as validation for your existing materials, and then tweak further using your artistic freedom.

Data from https://api.physicallybased.info/