The formula
ml of each = parts ÷ total parts × spirits in the glass
Where it comes from
A cocktail recipe is written in parts because the proportion is what defines the drink, and it is served in millilitres because a glass has a size. What almost nobody counts is dilution: stirring or shaking with ice adds twenty to thirty per cent of the final volume in water, and that is not a flaw but part of the recipe. A negroni served without that water is badly made, and its real strength is not that of its three spirits.
How to work it out by hand
- Add up all the parts in the recipe
- Take off the water the ice will contribute from the glass volume
- Split what is left in proportion to the parts
- The strength is the pure alcohol over the whole glass volume
What is worth knowing
Dilution depends on technique: stirring in a mixing glass adds around twenty per cent, shaking twenty-five to thirty, and serving over ice keeps diluting while you drink. A standard drink is ten grams of pure alcohol, which is what a small beer or a glass of wine holds; a seventy-millilitre short cocktail at thirty per cent is nearly two, which is why they go to your head so fast. The undiluted strength is the number that usually appears in cocktail books.