One 'generous quart' bag fills at least 1 six-inch pot or 2 to 4 four-inch pots.
One 2-gallon bag fills at least 1 ten-inch pot or 6 six-inch pots.
Because the DrySoil bags vary, I can't be more exact - plan that each bag contains at least one gallon of soil and will fill approximately 3 six-inch pots. You may have some left over, depending on the soil type.
For custom measurements, here's the formula:
Pop in your pot measurements here to get the volume of soil you'll need, then divide by 231 (cubic inches in a gallon). This is how many gallons you'll need :)