Cookbook | Recipes | Ingredients | Units of measurement
A pinch is a very informal unit, equal to the amount of powder that can comfortably be held between the thumb and forefinger. It is roughly equal to 1/8 teaspoon.
If this were accurate, which it's not, the metric equivalent would be 0.625 mL. Call it 1/2 mL or 2/3 mL if you like.