Another solution, if you know from the beginning that you want a sum, is use a vector. Prefix your list of numbers with [, finish with ], and sum using calc-reduce with the + operator (v R +):
v R +
Summing the whole stack of calc can be achieved using a keyboard macro.
Define the macro (which does just the + operation): C-x ( + C-x ).
BTW this definition already sums two elements of the stack as a side effect.
Then run the macro until an error occurs with C-0 X.
That's it! The error is expected to occur when one element remains on the stack.