These calculators are a great way to experiment with antenna designs before you go out and start building. You can try different configurations and get an idea on what the compromises are. Of course, real life is always more complicated, so, for example, a short vertical antenna may have a slightly different capacitance to that predicted by the calculator because of the enviroment it is in. You will not know the losses in advance, but you could try a few values to get a feeling of the effect. The small loop calculator does not calculate the inductance of the loop - there are many formulas, for single turn or multiple turns, and they all may have some error. You can also get an idea of the voltage and current developed across the components - an important parameter when selecting components and insulators.
For all calculators, the user variables are above the "Calculate" button and the computed variables are below.