Manual J is a product of the Air Conditioning Contractors of America for determining the heating/cooling load to size equipment for a home. They have a spreadsheet available on their website that requires filling in all the product info for the doors and windows in the house. It seems a bit overwhelming to me. I also just found a free online Manual J calculator which is pretty flexible and easy to use. http://www.loadcalc.net/
There are several online references to Wrightsoft Manual J software which is a pro tool that costs about $500. I looked for something else that would be free or low cost and came up with HVAC-Calc Residential 4.0. This program costs about $400 but there is a temporary license for $49 if you are figuring just one home, which is what I purchased. There is a great do it yourself flavor to the website, including the information about how to use this PC only software on my Mac. I installed Crossover and bought it immediately for $60. It has worked to run other PC programs as well as HVAC-Calc. I printed all the HVAC-Calc reports to PDF after I entered the information about our house into the database. I found that now that the paid period is over, I can still go in and print the reports again. I have 22 reports both combined and room by room info on the BTU requirements for those rooms. These numbers are critical in a correctly sized and designed system.
Each Manual J is based on the Design Temperatures of the location, both summer and winter. For Denver those are 91 degrees and 1 degree. In some calculations, zero is used for the Denver winter temp. but this program uses 1. Other variables include the number of people typically in the house–2 for us, the square footage of each room, ceiling heights, and the type and size of windows and doors.