Some can just look at a job and tell you the necessary size. This is not to say that some contractors don't meticulously hand calculate loads or punch them into a computer. It's just that a majority still rely on quick, rough estimates. The programs are so easy to use, there's even built-in help.
Many contractors are still in the dark regarding computers.
No matter how easy a program is to use, without some minimal knowledge of computers, a contractor can't even get started. Most will admit that if they could get past the introduction, then yes, a computer program would be the best way to perform load calculations. Fortunately, getting started with computers has become increasingly easy. The cost of hardware has dropped dramatically, and software has become very user-friendly. Although HVAC contractors can opt to learn an HVAC load calculation program on their own, most software suppliers offer relatively inexpensive training, particularly some air conditioning equipment manufacturers that supply software.