What is 'dilution' in cocktails and why does it matter?

Dilution is the water added to a cocktail by ice melt during shaking or stirring. A typical cocktail is 20 to 30 percent water by volume in the final glass; this softens the alcohol and brings the flavors into balance.

Cocktail recipes are designed assuming a specific level of dilution. A Martini at 2 oz gin plus 0.5 oz vermouth (2.5 oz total) ends up at roughly 3.5 oz in the glass after a proper stir — about 1 oz of that volume is ice melt. Dilution is doing important work: it (1) softens the alcohol burn, bringing 40 percent ABV spirits down to something approachable; (2) chills the drink to the right temperature; (3) opens up the aromatics so subtle flavors come through. Under-diluted cocktails taste hot and tight; over-diluted cocktails taste flat and weak. Common dilution mistakes: stirring too briefly (under 20 seconds), shaking with crushed ice instead of cubed (way too much surface area, dilutes catastrophically), or making cocktails ahead of time and serving them later (continuing to dilute on the way to the glass). The fix in all three cases is good cubed ice in roughly 1 to 1.5 inch cubes, proper shaking or stirring duration, and serving immediately.