This bit of code here

if( calcStack.size() > 2 )                      
{
    cout << "Error: too many operands" << endl;
    return 1;
}

Needs to move to main and slightly transform

else if(isOperator(input))
{
    performOperation(input, calcStack);
}
else if(input == "=")
{
    if (calcStack.size() != 1)
    {
        cout << "Error: too many operands" << endl;
        return 1;
    }
    else
    {
        cout << "Result: " << calcStack.top();
        // now decide whether or not you are preserving the result for
        // the next computation
        calcStack.pop(); // Assuming not keeping result
    }
}

And that means you’ll need to rethink this loop while(input != "=")

You’re really close.

Two suggestions:

You can optimize the isOperator function.

bool isOperator(const string& input)
{
    static const string operators ="-+*/";
    if (input.length() == 1) // right size to be an operator.
    {
        return operators.find_first_of(input[0]) != string::npos;
        // look in the operator string for the first (and only) character in input
    }
    return false;
}

And since you know there is only one character in the operator, you can use something more elegant than if/else if:

switch (input[0])
{
    case '-':
        result = firstOperand - secondOperand;
        break;
    case '+':
        result = firstOperand + secondOperand;
        break;
    case '*':
        result = firstOperand * secondOperand;
        break;
    case '/':
        if (secondOperand == 0)
        { // moved this test to here because it's the only place it matters.
            cout << "Error: Division by 0.\n";
            return -1;
        }
        result = firstOperand / secondOperand;
        break;
}

addendum to deal with comment

By this point your code should have changed quite a bit and you might want to start a new question with your current code. If not, this is what I’m talking about in the comments.

else if(isOperator(input))
{
    if (performOperation(input, calcStack) != 0)
    {
        // empty out the stack and delete all pending user input.
        calcStack.clear(); 
        cin.ignore(numeric_limits<streamsize>::max(), '\n'); 
        cout << "Bad input. Try again." << endl;
        break; // exit input loop.
     }
}
else if(input == "=")
{
...