Food quantities get careful attention on most quotes. Beverages, especially for a hosted or open bar, often get a rough estimate tacked on at the end - which is exactly backwards, since running out of ice or liquor mid-event is one of the most visible failures a guest will notice.
Plan for roughly 1-1.5 pounds of ice per guest for a standard bar service, covering both drink ice and the ice needed to keep beverages cold in tubs or coolers. On a hot outdoor event, push toward the higher end - ice for cooling disappears faster than ice going into glasses.
For a 100-guest open bar running 3-4 hours, a reasonable starting split is roughly 60% of drinkers choosing beer or wine over spirits. Plan for about half a bottle of wine per wine-drinking guest over the course of the event, and a case (24 bottles/cans) of beer per roughly 15-18 beer-drinking guests.
A 750ml bottle of a given spirit pours roughly 16-17 standard 1.5oz drinks. For a full bar serving a range of cocktails, plan for roughly one bottle of your core spirits (vodka, whiskey, gin, tequila, rum) per 20-25 guests who are likely to order cocktails over the course of the event, weighted toward whichever spirits your specific crowd favors.
Mixers run out faster than the liquor they're paired with, especially soda and tonic. Plan mixer quantities against pour count, not bottle count of liquor - a single bottle of vodka supports far more mixed drinks than a bottle of a sipping whiskey does.
These ratios all move together once you know guest count and bar style, which is exactly what Catering Calc Pro's beverage and ice calculator handles - enter the guest count and bar type once, get the full shopping list, and save it as part of the event template for the next job with a similar bar setup.
Plug in your guest count and get exact numbers in seconds.