Estimate how much ice you need for a party.
Ice serves two separate jobs and most people budget for only one. Ice in glasses runs at roughly 450 grams per guest per hour, since each refill takes fresh ice and the previous lot has melted. Chilling bottles and cans in tubs is a different and larger demand: you need a substantial initial charge to bring warm drinks down, then continuous top-ups as it melts. Ambient temperature scales both — a 30 degree afternoon melts ice dramatically faster than an 18 degree evening, and the calculator applies about 3.5 percent more ice per degree above 20.
Ice for glasses
kg = guests x 0.45 x hours x heat factor
Ice for tubs
kg = guests x 0.6 x heat factor + guests x 0.15 x hours
Cooling a warm bottle from room temperature takes far more ice than keeping an already-cold drink cold, because the ice has to absorb all that stored heat by melting. A tub of warm cans can consume its own weight in ice before the drinks are properly chilled.
Chill drinks in a fridge beforehand so the tubs only maintain temperature, keep tubs in shade, use larger blocks alongside cubes since they melt more slowly, and add a little water and salt to the tub — a slush transfers heat faster than dry ice cubes, so drinks chill sooner with less total ice.