Cornstarch or Corn flour (for thickening): 1 tablespoon = 2 tablespoons all purpose flour or. 1 tablespoon potato starch or rice starch or flour 1 tablespoon arrowroot 2 tablespoons quick-cooking (instant) tapioca For other uses such as the main flour it's rather tricky I think it's best to only substitute Corn flour if it's for thickening.