How to calculate WPF TextBlock width for its known font size and characters?

Use the FormattedText class.

I made a helper function in my code:

private Size MeasureString(string candidate)
{
    var formattedText = new FormattedText(
        candidate,
        CultureInfo.CurrentCulture,
        FlowDirection.LeftToRight,
        new Typeface(this.textBlock.FontFamily, this.textBlock.FontStyle, this.textBlock.FontWeight, this.textBlock.FontStretch),
        this.textBlock.FontSize,
        Brushes.Black,
        new NumberSubstitution(),
        VisualTreeHelper.GetDpi(this.textBlock).PixelsPerDip);

    return new Size(formattedText.Width, formattedText.Height);
}

It returns device-independent pixels that can be used in WPF layout.