Discrepancies between estimated and actual thermal solution performance may negatively bias early system design decisions and ultimately result in costly redesigns. In particular, an overreliance on manufacturer-provided performance data and insufficiently accurate prediction methods are two potential causes of large discrepancies. To demonstrate deviations between predicted and end-thermal-performance, three methods of predicting thermal performance were investigated. The reference design was a forced convection air-cooled computer chassis, consisting of four printed circuit boards (PCBs), each populated with four processors that utilize heat sinks. The three predictive methods used were rudimentary analytical calculations, spreadsheet based modeling, and computational fluid dynamics (CFD) simulation. The results from these methods were compared to measured results taken from the physical hardware test bed. A thermal resistance network model was used to describe the problem, and the comparison between the measured and calculated results for each element of the model was reported as a percent error. Note that in cases in which physical measurements could not be obtained, the comparison was made between the CFD results and the other calculations. All predictive methods using manufacturer-supplied thermal interface material (TIM) data indicated adequate thermal margin. When corrected TIM values were used, both the hand calculation method and the spreadsheet modeling method indicated adequate thermal margin; however, the CFD analysis predicted significant thermal issues, which were observed in the thermal test bed measured results.