Food Security Improvement Act of 2018 This bill amends the Food and Nutrition Act of 2008 to require Supplemental Nutrition Assistance Program (SNAP, formerly known as the food stamp program) benefits to be calculated using a low-cost food plan. The Department of Agriculture must determine the requirements for the low-cost food plan using the diet described in the Center for Nutrition Policy and Promotion's publication entitled "The Low-Cost, Moderate-Cost, and Liberal Food Plans, 2007" and adjustments specified in this bill. The bill revises the amounts of nutrition assistance block grants for Puerto Rico and American Samoa and requires the amounts to be modified based on adjustments to the low-cost food plan.