House's Square Footage...how can I determine it?

If I multiply the total length of one side of my house with the total length of the front or back, will that give me my square footage?
I'm wanting to buy some cellulose insulation and need to determine the number of bags I'll need. I want enough to cover my A/C ductwork as well.
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It will give you a rough idea.. it will not be dead on however..

Just some advice. If your AC ducts are flex, or the old fiberglass, or ductboard, and they were not installed correctly...meaning, mastic, straps, and no tape, something as simple as what you are suggesting can tear a seam, or crack a joint. From someone that has seen it hundreds of times..reconsider the duct cover...
Also, should there ever come a time where work needs to be done on the ducts, covered ducts are a pain in the ass, and will cost you more to repair.
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Ahh. Thanks for the advice. Right now the covering on the ducts is beggining to come lose and condensation is building up on the outside of the duct material that's becoming exposed. This has already caused a couple of small stains on my ceiling from the sweat dripping.
An HVAC company was here last Thursday and he said that the outside of the duct work that's exposed is getting very wet. He suggested re-wrapping the ducts. Also he found only 2 like that and that I needed to look into re-wrapping them.
My house was built in '76 with a Chrystler inside unit. Later that year an outside Whirlpool compressor was added. All of it was replaced in '93 with an entire new Lennox system. At the time they said that I'd get better efficiency if I also installed new ducts.
Kinda wish I had now.
Thanks again for your help.
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