FedEx Dimensional Weight Calculator: Shipping Cost Estimation, Rate Comparison, Invoice Accuracy & Packaging Optimization. Invoice Accuracy: Pre-calculating dimensional weight gives businesses a benchmark to check against FedEx invoices, quickly flagging overcharges or dimension mismatches that would otherwise go unnoticed. Packaging Optimization: Understanding how box size directly affects billable weight encourages tighter packaging choices, reducing wasted cubic space and unnecessary freight costs on bulky, lightweight shipments. 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.