How to auto fit/scale DBGrid's (or other similar) columns widths according to its contents?

What you have to do is to use the grid's canvas to measure the contents of each column and set the column's width accordingly. You can either iterate through the dataset or use the OnColumnDraw-Event to adjust the width on the fly.

Here's a sample (I had to use an offset of 5 pixels)

procedure TForm7.DBGridDrawColumnCell(Sender: TObject; const Rect: TRect;
  DataCol: Integer; Column: TColumn; State: TGridDrawState);
Var
  w : Integer;

begin
  w := 5+DBGrid.Canvas.TextExtent(Column.Field.DisplayText).cx;
  if w>column.Width then Column.Width := w;
end;

procedure TForm7.FormActivate(Sender: TObject);
Var
  i : Integer;

begin
  // Initialize width
  for I := 0 to DBGrid.Columns.Count - 1 do
    DBGrid.Columns[i].Width := 5 + DBGrid.Canvas.TextWidth(DBGrid.Columns[i].title.caption)
end;

EDITED:

My first code was about fit the columns inside the grid with, with this new code, AutoSizeColumns reads the records to calc the width of each column until MaxRows or Dataset.Eof:

class function TDBGridHelper.AutoSizeColumns(DBGrid: TDBGrid; const MaxRows: Integer = 25): Integer;

var
  DataSet: TDataSet;
  Bookmark: TBookmark;
  Count, I: Integer;
  ColumnsWidth: array of Integer;
begin
  SetLength(ColumnsWidth, DBGrid.Columns.Count);
  for I := 0 to DBGrid.Columns.Count - 1 do
    if DBGrid.Columns[I].Visible then
      ColumnsWidth[I] := DBGrid.Canvas.TextWidth(DBGrid.Columns[I].Title.Caption + '   ')
    else
      ColumnsWidth[I] := 0;
  if DBGrid.DataSource <> nil then
    DataSet := DBGrid.DataSource.DataSet
  else
    DataSet := nil;
  if (DataSet <> nil) and DataSet.Active then
  begin
    Bookmark := DataSet.GetBookmark;
    DataSet.DisableControls;
    try
      Count := 0;
      DataSet.First;
      while not DataSet.Eof and (Count < MaxRows) do
      begin
        for I := 0 to DBGrid.Columns.Count - 1 do
          if DBGrid.Columns[I].Visible then
            ColumnsWidth[I] := Max(ColumnsWidth[I], DBGrid.Canvas.TextWidth(
              DBGrid.Columns[I].Field.Text));
        Inc(Count);
        DataSet.Next;
      end;
    finally
      DataSet.GotoBookmark(Bookmark);
      DataSet.FreeBookmark(Bookmark);
      DataSet.EnableControls;
    end;
  end;
  Count := 0;
  for I := 0 to DBGrid.Columns.Count - 1 do
    if DBGrid.Columns[I].Visible then
    begin
      DBGrid.Columns[I].Width := ColumnsWidth[I];
      Inc(Count, ColumnsWidth[I]);
    end;
  Result := Count - DBGrid.ClientWidth;
end;

I call it in the DataSet.AfterOpen event:

TGridHelper.AutoSizeColumns(MyDBGrid);

Why to use so complicated code? :D Just use this. Its also have 5 px offset.

procedure TForm1.FormActivate(Sender: TObject);

var

  i:integer;

begin

  for i :=0 to DbGrid1.Columns.Count - 1 do
   DbGrid1.Columns[i].width :=5+dbgrid1.Canvas.TextWidth(DbGrid1.Columns[i].Title.Caption);

end;