For $39, I was able to do what none of the three contractors who came to my house was able to do: accurately size a replacement furnace. Each asked me for the square footage of the house, and told me what size I "needed." The first said 120k BTU, but later said he had deliberately over-sized it. The other two said 100k.
After carefully measuring and inputting data into HVAC-Calc, I found that my total heat loss, with no upgrades, is 62k BTU. If I add R30 ceiling insulation and insulate the ducts to R4, its only 50k. With R6 duct insulation, its 48k BTU. I'm no expert, but I do think I can use a tape measure with fair accuracy.
So I need a furnace half the capacity of what three experienced HVAC contractors recommended by rule of thumb. Not only did HVAC-Calc give me an accurate size for my present situation, it also let me compare the result after upgrading my insulation.
While I did not get quotes on A/C, the program says I need 1.5 tons before upgrades or 1.0 tons after.
Great program, try it! http://www.hvac-calc.com