I've never used the calculator so can't comment on that.
There are a load of things that can affect the load your motor will move i.e. drive motors for a robot or a joint in an arm with the same motor will be able to move different loads.
Also if they are to be drive motors: Supply voltage (motors don't have to be run at their rated voltage), gearing on the output, efficiency of the gearing, wheel size, will the robot run on gradients, indoors or out, these and other things will affect the load your motors will cope with.
A suggestion, once you have it worked out is to try and build to about 75% of whatever you calculated, I'll bet it ends up heavier than you think.