Would like to find out the BTU required for my garage. It is 50'x26'x16' tall. Two overhead doors insulated. One is 10'x8' tall and the other is 12'x14' tall. Walls and ceiling are insulated.
You need to figure out how much insulation you have, the space you expect to heat, the outside temp and the inside temp, and then calculate it from there.
Q= btu
U= 1/r. Where r is your insulation value.
A= area of wall or roof
Delta t is the temperature difference outside and inside.
You can figure out the btu loss thru each wall and roof and then you have the total btu loss for your garage. There is a little more to it than this but this is a simple method to get you in the ballpark of the btu you need.
That calculator is a piece of ****! Way overestimates. Better off using the 30 btu/SF rule for rough numbers and then an actual heat load calc to choose equipment.