[T]he Avogadro constant to be 6.02214084 times 10^23, with an uncertainty of only 30 parts per billion . . . The . . . [standard kilogram of used as the definition of the metric system kilogram] in Paris, for all its faults, is still a little more reliable than that. Its mass is uncertain to 20 parts per billion.
From here.
Avogadro's number is the number of atoms in a quantity of an element or molecule with a weight in grams equal to the its atomic number (a periodic table quantity).
For example, a mole of silicon would weigh 28.0855 grams and have an Avogadro's number of silicon atoms in it.
We are close to being able to define the kilogram in the metric system more accurately via Avogardo's number than we can via the cylinder of platinum adopted as a standard in 1889, which in turn is supposed to have the mass of 1000 cubic centimeters of water at room temperature at sea level.
The meter is defined to be very close to, but not quite exactly, 1/300,000,000th of one light-second. The American inch is defined to be exactly 25.4 millimeters.
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