FedEx Dimensional Weight Calculator: Shipping Cost Estimation, Rate Comparison, Invoice Accuracy & Packaging Optimization A FedEx dimensional weight calculator applies FedEx’s specific divisor formula to a package’s length, width, and height, converting size into a billable weight figure that determines actual shipping charges. Since FedEx bills based on whichever is greater — dimensional or actual weight — this tool is essential for accurate cost planning. Anyone shipping packages in the USA can use the DimMath calculator for free — including individual online sellers, small business owners, e-commerce store operators, freight coordinators, warehouse and fulfillment teams, and logistics managers at larger companies.