In the United States, intercity long-haul trucks idle approximately 1,800 hrs per year primarily for sleeper cab hotel loads, consuming 838 million gallons of diesel fuel . The U.S. Department of Energy's National Renewable Energy Laboratory (NREL) is working on solutions to this challenge through the CoolCab project. The objective of the CoolCab project is to work closely with industry to design efficient thermal management systems for long-haul trucks that keep the cab comfortable with minimized engine idling. Truck engine idling is primarily done to heat or cool the cab/sleeper, keep the fuel warm in cold weather, and keep the engine warm for cold temperature startup. Reducing the thermal load on the cab/sleeper will decrease air conditioning system requirements, improve efficiency, and help reduce fuel use. To help assess and improve idle reduction solutions, the CoolCalc software tool was developed. CoolCalc is an easy-to-use, simplified, physics-based heating, ventilating and air conditioning (HVAC) load estimation tool that requires no meshing, has flexible geometry, excludes unnecessary detail, and is less time-intensive than more detailed computer-aided engineering (CAE) modeling approaches. It is intended for rapid trade-off studies, technology impact estimation, and preliminary HVAC sizing design. It also complements more detailed and expensive CAE tools by exploring and identifying regions of interest in the design space. CoolCalc is built on NREL's OpenStudio platform and is a plug-in extension of Google's SketchUp software. This paper describes the CoolCalc tool, provides outdoor long-haul truck thermal testing results, shows validation using these test results, and discusses future applications of the tool.