Question: I came across a formula while looking at old message here. Just wondering if anyone has used this way of calculating wood stove needs, or if they have an alternate way of figuring their stove requirements. BTU Needs = 4.5 x cubic feet in house cubic feet = each room's length x width x height Any adjustment factors for old or new houses? Answer: It's a good rule of thumb for most construction in temperate climates. However, much more must be considered in order to properly calculate...such as window area, exterior walls, how open to other rooms, etc. Heat loss can vary a lot, as can climate. Some rooms could need double this figure, while others could need LESS than half. Keep in mind that, despite ratings, most wood stoves put out 15-30K BTU on a average basis AND they are not always able to heat the entire house, but can provide 50-80% of the heat load in many cases.