One teaspoon is equivalent to 4.92892 milliliters which is very close to 5 milliliter. So the steps in looking for the milligrams contained, first divide the 325 milligrams to the 1 teaspoon then multiplying it to 1 teaspoon divided by 5 milliliters then multiplying that to the 0.4 milliliter of potassium iodide.
Milligrams contained = (325 mg / 1 tsp) x (1 tsp / 5 mL) x 0.4 mL = 26 milligrams.