Hi Ann. Cook's Illustrated did a thing on kosher salt and table salt.
The two main brands of kosher salt measure differently. Both of them measure differently than table salt. Here's the scoop:
Kosher salt is a coarse salt that gets its name from its use in the preparation of meat according to Jewish dietary guidelines. The coarse texture is useful in koshering, which is intended to draw blood out of the meat. With their large surface area, crystals of kosher salt do this more effectively than the small, compact crystals of table salt. Kosher salt is often perceived as being saltier than table salt because the large, flaky crystals dissolve more quickly on the tongue. But the truth is that salt is salt; 1 pound of kosher salt contains just as much sodium chloride as 1 pound of table salt. In fact, because kosher salt weighs less than table salt by volume (that is, when measuring with a spoon), more kosher salt must be used to get the same salty effect.
The two most popular brands of kosher salt are made differently and therefore measure differently: If substituting Morton's Kosher Salt for table salt in specific recipes, increase the measurement by 50 percent (for example, from 1 teaspoon to 1 1/2 teaspoons); if using Diamond Crystal Kosher Salt, increase by 100 percent (from 1 teaspoon to 2 teaspoons).