Determine how wide a rendered character is in .NET

It depends on the rendering engine being used. .NET may use GDI or GDI+. Switching can be done by setting the UseCompatibleTextRendering property accordingly or calling the Application.SetCompatibleTextRenderingDefault method.

When using GDI+ you should use MeasureString:

string s = "A sample string";

SizeF size = e.Graphics.MeasureString(s, new Font("Arial", 24));

When using GDI (i.e. the native Win32 rendering) you should use the TextRenderer class:

SizeF size = TextRenderer.MeasureText(s, new Font("Arial", 24));

More details are described in this article:

Text Rendering: Build World-Ready Apps Using Complex Scripts In Windows Forms Controls

Note that the above talks about Windows Forms. In WPF you would be using FormattedText

You don’t say how you “render” it, but if you have a string, you can use MeasureString too.

Here's an MSDN piece about determining font metrics. You can use Graphics.MeasureString to do the measurement.