This is how I refer to single strength simple syrup where we are using equal parts sugar and water by volume. This simple syrup recipe has about 45 calories per fluid ounce. Use that as a guide to see if a store bought bottle is single strength.
Almost every drink recipe will refer to simple syrup generically leaving out whether it is single or double sweet. Wherever possible on this site we will specify which type of simple syrup as it can have a large effect on the palatability of the drink.
Heat water and sugar to boiling. Boil for about 1-2 minutes while stirring. Turn off heat and let cool to warm. Add 151 and bottle.
This makes about 12 oz of syrup and will keep in your refrigerator for months. The rum will help preserve the syrup.
I use some raw sugar to give the syrup some character--but not too much character as it is a simple syrup and is meant primarily for sweetening with other ingredients being used for flavor.