Adjust Column width DBGrid

The needed Columnwidth is depended of the settings of the Grids canvas and the mamimum length of the displaytext of each field.

procedure FitGrid(Grid: TDBGrid);
const
  C_Add=3;
var
  ds: TDataSet;
  bm: TBookmark;
  i: Integer;
  w: Integer;
  a: Array of Integer;
begin
  ds := Grid.DataSource.DataSet;
  if Assigned(ds) then
  begin
    ds.DisableControls;
    bm := ds.GetBookmark;
    try
      ds.First;
      SetLength(a, Grid.Columns.Count);
      while not ds.Eof do
      begin
        for I := 0 to Grid.Columns.Count - 1 do
        begin
          if Assigned(Grid.Columns[i].Field) then
          begin
            w :=  Grid.Canvas.TextWidth(ds.FieldByName(Grid.Columns[i].Field.FieldName).DisplayText);
            if a[i] < w  then
               a[i] := w ;
          end;
        end;
        ds.Next;
      end;
      for I := 0 to Grid.Columns.Count - 1 do
        Grid.Columns[i].Width := a[i] + C_Add;
        ds.GotoBookmark(bm);
    finally
      ds.FreeBookmark(bm);
      ds.EnableControls;
    end;
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  FitGrid(DBgrid1)
end;

Minor modification of bummi's answer to insure that Title Row (Row 0) is not truncated, and excess space will be allocated on each column

procedure FitGrid(Grid: TDBGrid);
const
  C_Add = 3;
var
  ds: TDataSet;
  bm: TBookmark;
  i: Integer;
  w: Integer;
  a: array of Integer;
begin
  ds := Grid.DataSource.DataSet;

  if not Assigned(ds) then
    exit;

  if Grid.Columns.Count = 0 then
    exit;

  ds.DisableControls;
  bm := ds.GetBookmark;
  try
    ds.First;
    SetLength(a, Grid.Columns.Count);
    for i := 0 to Grid.Columns.Count - 1 do
      if Assigned(Grid.Columns[i].Field) then
        a[i] := Grid.Canvas.TextWidth(Grid.Columns[i].FieldName);

    while not ds.Eof do
    begin

      for i := 0 to Grid.Columns.Count - 1 do
      begin
        if not Assigned(Grid.Columns[i].Field) then
          continue;

        w := Grid.Canvas.TextWidth(ds.FieldByName(Grid.Columns[i].Field.FieldName).DisplayText);

        if a[i] < w then
          a[i] := w;
      end;
      ds.Next;
    end;

    w := 0;
    for i := 0 to Grid.Columns.Count - 1 do
    begin
      Grid.Columns[i].Width := a[i] + C_Add;
      inc(w, a[i] + C_Add);
    end;

    w := (Grid.ClientWidth - w - 20) div (Grid.Columns.Count);

    if w > 0 then
      for i := 0 to Grid.Columns.Count - 1 do
        Grid.Columns[i].Width := Grid.Columns[i].Width + w;


    ds.GotoBookmark(bm);
  finally
    ds.FreeBookmark(bm);
    ds.EnableControls;
  end;
end;

Minor modification of bummi's answer to insure that Title Row (Row 0) is not truncated

procedure FitGrid(Grid: TDBGrid);
const
  C_Add=3;
var
  ds: TDataSet;
  bm: TBookmark;
  i: Integer;
  w: Integer;
  a: Array of Integer;
begin
  ds := Grid.DataSource.DataSet;
  if Assigned(ds) then
  begin
    ds.DisableControls;
    bm := ds.GetBookmark;
    try
      ds.First;
      SetLength(a, Grid.Columns.Count);
      while not ds.Eof do
      begin
        for I := 0 to Grid.Columns.Count - 1 do
        begin
          if Assigned(Grid.Columns[i].Field) then
          begin
            w :=  Grid.Canvas.TextWidth(ds.FieldByName(Grid.Columns[i].Field.FieldName.).DisplayText);
            if a[i] < w  then
               a[i] := w ;
          end;
        end;
        ds.Next;
      end;
      //if fieldwidth is smaller than Row 0 (field names) fix
      for I := 0 to Grid.Columns.Count - 1 do
      begin
        w :=  Grid.Canvas.TextWidth(Grid.Columns[i].Field.FieldName);
        if a[i] < w  then
           a[i] := w ;
      end;

      for I := 0 to Grid.Columns.Count - 1 do
        Grid.Columns[i].Width := a[i] + C_Add;
        ds.GotoBookmark(bm);
    finally
      ds.FreeBookmark(bm);
      ds.EnableControls;
    end;
  end;
end;