This article describes the use of a spreadsheet to reinforce basic calculus that is expected of all university-level chemistry majors. The example provided shows a calculation using Excel to estimate, using a Riemann summation, the radiant exitance of a hot object using Planck's Law of Blackbody Radiation. The approach reinforces the elementary calculus topic of integrating an area under a curve in an applied chemistry setting and, additionally, requires students to understand how units propagate through integrals in order to calculate correct numerical values. This manual calculation in Excel is pedagogically more satisfying than using black-box type software to calculate areas under curves, and is widely available to almost all students through the use of Excel (or similar spreadsheets software packages).