How do I manually calculate the number of BTUs to correctly size a furnace for my home? I'm sure I need to know the type and size of window (whether single or double pane), insulation R-value, etc. to determine this number.

I know computer programs are available (such as the one on this site), but I'm a homeowner and only plan to do this once for this house so I don't see the point in dealing with another program. I know if it can be written into the code of a computer program one can surely figure it out using one's own head.

I'm asking this because I've had many, many individuals give me bids on a new furnace. I've been told I need a furnace from 50,000 BTUs to 80,000 BTUs. I know undersizing and oversizing a furnace tend to degrade them faster. I live in a 1,000 sq ft ranch style house in a rural area outside of Chicago, IL. I know I have minimal insulation in the attic - maybe 2 to 3 inches. The house has a basement as well as 2 doors to the outside and 8 windows with a large picture window (facing south) with 2 adjacent windows on the sides.

Would someone please help! I really don't want to buy more or less than I need.

Thank you!