How do i calculate btu's in a residential home for ...

You take the sqft of each area your going to cool off, like the bedroons livingroom diningroom, bathrooms and kitchen if you have walkin closets take that too. to find the heat load add up all the sqft and multiply by 40 that will give you the inpuit of btu needed to heat the area. to cool the area add all the sqft and divided by 500 sqft, of the total area you added. I usaully add another half a ton to my total to make up for the amount of people that will be there. hope this helps

There are alot of variables such as window type and number, age and type of insulation, as well as your particular climate. A good easy rule of thumb would be to multiply the total square footage by 40. This will give you a ballpark btu amount for heating. Divide it in half for the cooling. If still unsure, contact a contractor for an estimate. Also, sometimes the power companies will do this for you. Hope this helps.

As a "rule of thumb" for sizing residential I ahve used the following:

1) For tight residential construction of the home: (ie. heavy insluation in walls, attic, etc and new windows, doors, etc.) use 600 square foot per ton of air conditioning.

2) For an average home some new windows etc, older batting insulation etc....use 500 square foot per ton of air conditioning.

3) For an older leaky home, original windows, no wall insulation light attic insulation, etc.  ....use 400 Square foot per ton of air conditioning. 

That being said.....if you have a two story home, the best investment you can make with your air condtioning is to assure your attic is well insulated (R-30 range is great) A reputable insulation company can give your good advice and a quote ..it should not be overly costly and will pay back in comfort and utility savings all around.  Also, assure the attic is well ventilated. 

A two-story also has challenges with cooling the second floor just due to physics (hot air rises, cold air drops) you will probably need to set the air duct dampers differently in the summer than the winter....reduce flow on the lower floor and open the damper for upstairs fully open.  Reverse this in the winter setting. Once you find the most comfortable settings...write a "winter" and "summer" mark at the damper adjuster and you have an easy fall and sping routine of resetting your dampers. 

I hope that helps!

By the way, assure your heating/cooling contractor does a load calculation on your home. and have them look at the ductwork layout of your system.  A foreced air system is only as good as the sheet metal ducts supplying air to the different areas of the home.  If the airflow is poor the heating and cooling will be also.  Get good references of other customers and call them to talk.  Also, Angies List is a great way to get reputable companies even though there is a slight cost for the on-line service....you can join ans drop out as you need to. 

Matt

i actually completely agree that angie's list is a great resource. 

http://factoidz.com/angies-list-better-alternative-to-craigslist-for-finding-local-contractors/