Almost every sour, Old Fashioned and iced coffee on a menu leans on the same quiet ingredient: sugar syrup. Bartenders make it two ways. Simple syrup is one part sugar to one part water. Rich syrup is two parts sugar to one part water. Recipes just say "syrup" and leave you to guess.
The difference matters more than it looks. Rich syrup is not "twice as sweet". It is thicker, it adds less water to the glass, and it keeps better. Once you know the numbers you can swap one for the other without wrecking the balance.
The best way to make syrup is by weight, on a kitchen scale: 200 g of white sugar and 200 g of water for a 1:1, or 200 g of sugar and 100 g of water for a 2:1. Measuring by cup gives a weaker syrup, because a cup of sugar weighs less than a cup of water. Difford's Guide puts a volume-measured 1:1 at about 48 °Brix and a volume-measured 2:1 at about 65 °Brix; by weight they come out at 50 and 66.7.
°Brix is just the percentage of sugar by weight in the liquid. A 50 °Brix syrup is half sugar. That is the number that sets how sweet a spoonful tastes.
Rich syrup is about 1.35 to 1.45 times sweeter than simple syrup, not twice as sweet.
Why not double? Sugar is dense, so each millilitre of rich syrup is heavier, but it is also a bigger share of sugar and a smaller share of water, and what you taste is sugar per millilitre. Difford's Guide measures the gap at about 1.35 times for volume-measured syrups; the calculator below works by weight and lands a little higher. Either way, it is well short of two.
Simple syrup is the everyday choice. You can make it with no heat at all: shake sugar and room-temperature water in a clean jar until it clears. It pours easily, mixes into cold drinks and iced coffee at once, and is forgiving to measure. It does add more water: Difford's Guide estimates about 5 to 8 per cent more dilution than rich syrup in a drink.
Rich syrup suits spirit-forward drinks, where you want sweetness without softening the drink. It is the natural fit for an Old Fashioned, where a small spoonful does the job. It needs gentle warming to dissolve all that sugar, and it is thicker, so let it cool before you measure. You do not need to boil it; heat just to the point the grains disappear.
For a sour, 1:1 is easier to balance, because the volumes are bigger and a few millilitres either way matters less. For an Old Fashioned, 2:1 gives you better control. For hot tea or coffee, either works, as long as you remember rich syrup goes further.
Use plain white sugar for a clean, neutral syrup. Raw or demerara sugar gives a caramel note that suits rum and whisky drinks. Use a scale, stir until no grains remain, and pour into a clean bottle. Keep it in the fridge and use it within a few weeks; rich syrup tends to last longer because there is less free water for anything to grow in. If it turns cloudy, smells off or grows anything on top, throw it away and start again.
Sugar is for balance, not for volume. Most drinks need a teaspoon or two of syrup, not a splash. Sweet drinks are easy to over-pour, so measure it like any other ingredient.