See: https://www.calculator.net/btu-calculator.html

I believe it is the best BTU calculator available for general BTU numbers,
HOWEVER a quick call to me can be better because I can usually tell you
the best size for YOUR application; taking into consideration square footage,
your climate and how well your home is insulated plus what species of wood
you are burning PLUS (probably the most important) how often YOU want
to load your outside furnace or boiler.

Go to the second box or form labeled General Purpose AC or Heating BTU Calculator
and filling out the house width and length and ceiling height but paying close attention to
the Desired Temperature Increase section.

This is how warm you want your home to be, relative to the coldest expected nighttime low temperature, in Jan or Feb.

Hit the Calculate button, get your number and then call me at 828-683-0025 until 9PM ET, six days a week!