Do you know how much money I charge for doing this? There are many factors that are taken into consideration: # of windows and doors, which walls they are on, type of building material, type of insulation, etc. etc. etc.
Then there are the heat loss coefficients for all of the various materials. Then there are table, formulas and charts to convert. You need to locate a good heating contractor to work this out for you or you can convert the square footage into BTU's and hope for the best. Sorry!!!