The accurate prediction of internal heat gains fromoccupants, lighting, and plug loads inside a building iscritical for mechanical engineers and energy modelers,with implications related to HVAC sizing, thermalcomfort requirements, and energy efficiency. However,internal gain values are often misjudged, stemming fromindustry standard assumptions. This paper describes anempirical approach to calculate plug load density inoffice spaces, and the subsequent simulation of an activeoffice building incorporating different internal gainsassumptions. The outcome is a 24-hour plug loadschedule in the form of W/ft² that assists in predictingpeak heating and cooling loads. The model underwentsix iterations: five with a variety of plug load densitiesbased on current design handbooks, and the last withimproved values determined by this procedure. Thesimulation results are compared to reveal the impact ofrenewed plug load estimates on HVAC demand andenergy use.