There are hundreds of different pasta shapes and sometimes, the same pasta goes by different names, as is the case with rotini, which also is known as fusilli. Even though ziti is a tubular pasta and rotini is a spiral, rotini can substitute for ziti with no problems.
What's in a Shape
Although pasta connoisseurs argue that rotini and ziti have shapes too different to be interchangeable, for most uses, you won't be dissatisfied with either pasta. Changing a pasta sauce because you don't have the "correct" pasta shape is unnecessary despite minor differences between rotini and ziti. For baked pasta dishes, pasta salad or pasta with a tomato or cream sauce, go ahead and use the same amount of rotini in place of the ziti called for in your recipe.
Rotini for Chunky Sauces
Tubular pastas such as ziti can gather bits of sauce and other ingredients in the openings of the tubes. With its angled ends, ziti does a passable job of catching sauce. But it also is a small and thin tube, so sauce with chunky ingredients doesn't really travel inside the tube. Spiral shaped pastas such as rotini do a better jot of gathering bits of sauce between the twists of the spirals.
Rotini for Soups
Ziti comes in lengths from 2 to 12 inches, while rotini typically is 1 to 2 inches long. Rotini's shorter length is better for use in soups, where it fits more easily on a spoon. You could use ziti for soup if you had pieces on the short side or didn't mind breaking them to spoon size. But breaking pasta not only is messy, but you won't have similarly sized pieces.
No Change for Cooking
Both ziti and rotini are small pastas and so have similar cooked and uncooked measurements and cooking times. Two ounces of uncooked pasta is the equivalent of 1/2 cup of dry pasta and yields 1 cup of cooked pasta. Consider a single serving of ziti or rotini to be about 1/2 cup of dry pasta. Both pastas cook in about seven to eight minutes and are best when cooked al dente, or slightly firm when you bite into a piece.