Q.: I often hear the term "hairline crack." How wide and deep is such a crack?
A.: ACI 116R-90, Cement and Concrete Terminology, defines hairline cracks as cracks in an exposed concrete surface having widths so small as to be barely perceptible. Although this definition doesn't give a numerical value for width (or require the viewer to wear bifocals to see the crack), it probably applies to cracks less than about 0.003 inch (0.08 mm) in width.
Hairline cracks can be very shallow or quite deep, depending on the cause. However, there is no way to determine crack depth based on crack width.