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We know, that one inch contains 25.4 mm. So, to convert mm to inch, we should made division of value and 25.4. To convert inch to mm, we should made multiplication of value and 25.4.
For example:

// MmToInch
procedure TForm1.Button1Click(Sender: TObject);
begin
  Label1.Caption:=FloatToStr(StrToFloat(Edit1.Text)/25.4);
end;

// InchToMm
procedure TForm1.Button2Click(Sender: TObject);
begin
  Label2.Caption:=FloatToStr(StrToFloat(Edit1.Text)*25.4);
end;

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