 <p>You mixing things. You either do this:</p> <pre><code>private void sumBtn_Click(object sender, EventArgs e) { int counter; int loopAnswer = 0; int number1 = int.Parse(number1Txtbox.Text); for (counter = 1; counter &lt;= 10; counter++) { loopAnswer += number1; //same as loopAnswer = loopAnswer + number1; } equalsBox.Text = loopAnswer.ToString(); } </code></pre> <p>or this:</p> <pre><code>private void sumBtn_Click(object sender, EventArgs e) { int counter = 1; int loopAnswer = 0; int number1 = int.Parse(number1Txtbox.Text); do { loopAnswer += number1; //same as loopAnswer = loopAnswer + number1; counter++; } while (counter &lt;= 10); equalsBox.Text = loopAnswer.ToString(); } </code></pre> <p>Also, the final answer (<code>equalsBox.Text = loopAnswer.ToString();</code>) should be out of the loop.</p>