Пример расчета величины в отдельном потоке

unit uPiThread;

interface

uses
  Classes;

type
  TPiThread = class(TThread)
  private
    { Private declarations }
  protected
    procedure Execute; override;
  end;

var
  GlobalPi: Extended;
  GlobalCounter: Int64;

implementation

uses uMain;

{ Important: Methods and properties of objects in VCL or CLX can only be used
  in a method called using Synchronize, for example,

      Synchronize(UpdateCaption);

  and UpdateCaption could look like,

    procedure TPiThread.UpdateCaption;
    begin
      Form1.Caption := 'Updated in a thread';
    end; }

{ TPiThread }

const
 //better to choose odd value, to avoid monotonic increase/decrease ef-fect
  UpdatePeriod = 999999;

procedure TPiThread.Execute;
var sign: Integer;
  PiValue, PrevValue: Extended;
  i: Int64;
begin
 { Place thread code here }
  PiValue := 4;
  sign := -1;
  i := 0;
  repeat
    Inc(i);
    PrevValue := PiValue;
    PiValue := PiValue + sign * 4 / (2 * i + 1);
    sign := -sign;
    if i mod UpdatePeriod = 0 then
    begin
      GlobalPi := PiValue;
      GlobalCounter := i;
      Synchronize(fmMain.UpdatePi);
    end;
  until Terminated or
    (Abs(PiValue - PrevValue) < 1E-19);
end;

end.

unit uMain;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls,
  uPiThread;

type
  TfmMain = class(TForm)
    cbCalculate: TCheckBox;
    Label1: TLabel;
    Label2: TLabel;
    laBuiltIn: TLabel;
    laValue: TLabel;
    laIterNum: TLabel;
    procedure FormCreate(Sender: TObject);
    procedure cbCalculateClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    PiThread: TPiThread;
    procedure UpdatePi;
  end;

var
  fmMain: TfmMain;

implementation

{$R *.dfm}

procedure TfmMain.UpdatePi;
begin
  if IsIconic(Application.Handle) then Exit;
  LaValue.Caption := FloatToStrF(GlobalPi, ffFixed, 18, 18);
  laIterNum.Caption := IntToStr(GlobalCounter) + ' iterations';
end;

procedure TfmMain.FormCreate(Sender: TObject);
begin
  laBuiltIn.Caption := FloatToStrF(Pi, ffFixed, 18, 18);
end;

procedure TfmMain.cbCalculateClick(Sender: TObject);
begin
  if cbCalculate.Checked then
  begin
    PiThread := TPiThread.Create(True);
    PiThread.FreeOnTerminate := True;
    PiThread.Priority := tpLower;
    PiThread.Resume;
  end
  else
  begin
    if Assigned(PiThread) then PiThread.Terminate;
  end;

end;

end.