Calculus problems come in many shapes and sizes, not all of which can be solved online (or at all). If your problem is purely computational, the Wolfram Alpha website is often helpful in computing things like integrals or derivatives, or solving certain differential equations.
Depending on the level of the calculus problem being posed, online calculators like Wolfram Alpha or the Mathscoop Calculus Calculator Online can be used to perform calculations. For high school or early undergraduate problems, these online calculators work well.
For higher level problems, online calculators have more limited usefulness. Some problems, such as integration, differentiation or solving simple differential equations, are easy to state but tedious to work out by hand. Online calculators can be helpful in dealing with these. Others, such as boundary value problems, can be simplified by using an online calculator to do some computations, but still require somebody to work out which computations are needed and/or useful. Still other problems, such as doing surface integrals, involve parameterizing portions of surfaces based on a diagram or verbal description, which online calculators are unable to do as of 2015.
At higher levels, it also sometimes happens that some computations have no answer in terms of the usual mathematical functions, whether you use an online calculator or any other method.