Stupid problem

Look at the documentation of the Convert.ToDecimal Method (String). See what it says about if you prefer to not handle an exception if the conversion fails? Also see the example that shows use of try/catch. Either one of those two solutions would solve your problem. Just read the documentation.

Try putting the following in a console program:

Console.WriteLine(Convert.ToDecimal(""));

You can create a console program that you can use to test code; I do.

Yeah.. I think I know what the problem is... how can I code it to only take numbers in the textbox.
I know I could use a masked textbox, but I want to know how to do it through code..

Sam Hobbs,
      I dont use commas. So thats not the problem... But thanks for the reply... Im still lost...
I know with decimal we are supposed to use the M, but Im not sure how to implement the M in my code... Any sugestions?

Did you "use the debugger to see specifically what the data is"?

Try this. Don't use a comma; use "100000" instead of "100,000". If that works and you must use a comma in those values in the form, then you can add code that allows that.

Suthish Nair,
    I replaced my code with yours and it apears to work, and then when I delete whats in the downPaymentTextBox I get the same error.
Input string was not in a correct format.
and in the troubleshooting tips I get: Make sure your method arguments are in the right format 
Im really confused because I dont understand what the problem is.. this should be simple data type conversion from decimal to string... and it works then when I clear the textbox i get this error... If anybody can take a look and explain the problem I would greatly appreciate it.
* Also Im using big numbers in the text boxes.. I dont know if it matters. I put in 100,000 for the sales price and 20000 for the down payment. Once I delete the 20000, i get the error..

change your code to..

    protected void downPaymentTextBox_TextChanged(object sender, EventArgs e)
    {
        downPaymentTextBox.Text = downPaymentTextBox.Text == "" ? "0" : downPaymentTextBox.Text;
        decimal Price = Convert.ToDecimal(salesPriceTextBox.Text);
        decimal DownPay = Convert.ToDecimal(downPaymentTextBox.Text);
        decimal Percent = (DownPay / Price) * 100;
        downPaymentPercentTextBox.Text = Convert.ToString(Percent);
    }

Hi friend,

Your program has executed when am using like this

            string a = "12",b="13";
            decimal Price = Convert.ToDecimal(a);
            decimal DownPay = Convert.ToDecimal(b);
            decimal Percent = (DownPay / Price) * 100;
            Console.WriteLine(Percent.ToString());
            Console.ReadLine();

I don't know what the data is and I don't know which line of code in your code is getting the error. You can use the debugger to see specifically what the data is. You can also look at the documentation of the relevant method (Convert.ToDecimal?) to determine what it does when it gets the data that you are encounring. At this moment, the MSDN site is having problems so I can't look at the documentation but it's likely it will provide you the best help.