Div with Margin-Left and Width:100% Overflowing on the Right Side

A div is a block element and by default 100% wide. You should just have to set the textarea width to 100%.

If some other portion of your layout is influencing the div width you can set width:auto and the div (which is a block element) will fill the space

<div style="width:auto">
    <div style="margin-left:45px;width:auto">
        <asp:TextBox ID="txtTitle" runat="server" Width="100%"></asp:TextBox><br />
    </div>
</div>

If that's still not working we may need to see more of your layout HTML/CSS

<div style="width:100%;">
    <div style="margin-left:45px;">
       <asp:TextBox ID="txtTitle" runat="server" Width="100%"></asp:TextBox><br />
    </div>
</div>

Add some css either in the head or in a external document. asp:TextBox are rendered as input :

input {
     width:100%;
}

Your html should look like :

Note this will affect all your textbox : if you don't want this, give the containing div a class and specify the css.

.divClass input {
     width:100%;
}

I realise this is an old post but this might benefit somebody who, like me, has come to this page from a google search and is at their wits end.

None of the other answers given here worked for me and I had already given up hope, but today I was searching for a solution to another similar problem with divs, which I found answered multiple times on SO. The accepted answer worked for my div, and I had the sudden notion to try it for my previous textbox issue - and it worked! The solution:

add box-sizing: border-box to the style of the textbox.

To add this to all multi-line textboxes using CSS, add the following to your style sheet:

textarea
{
  box-sizing: border-box;
}

Thanks to thirtydot for the solution at

width: 100%-padding?

and

Content of div is longer then div itself when width is set to 100%?