how much simple syrup in an old fashioned

An Old Fashioned cocktail typically uses about 1/8 oz (around 2.5 to 3.75 ml) of simple syrup or roughly a barspoon, especially for a 2 oz spirit base. Some recipes suggest around 0.5 oz of simple syrup for the drink. The key is to add just enough syrup to sweeten the drink slightly without overpowering the whiskey's flavor.