Limitations of a Roof Calculator
Roof calculators are useful tools. But they are not infinitely powerful or remarkably accurate. Most roof calculators are planned to include all major kinds of roofing materials. Price ranges that are provided may have a small margin of error. Generic and specialty roofing will have variables that differ in cost across geographic locales, as well as based upon the kind of roof they will be used on. A degree of disparity between what you compute and what your roofing contractor presents to you is only to be expected.
The type of roofing to be installed will also skew the accuracy of a roof calculator. For instance, with shingle roofing a contractor may be able to use existing chimneys or flashing, while for metal roofing this will need to be removed and replaced. So the estimated costs calculated with a roof calculator will be off, because they cannot automatically factor in these variations.
Tear off costs will also differ by the type of roofing previously installed. Flat roofs cost more to tear down than shingle roofs. The process is more complicated for flat roofing, and disposal of the waste is another expense. So a generic roof calculator will be reasonably good at making roof estimates, but not 100% reliable in all respects.