Ask three party guides how much ice you need and you get three answers, from about half a kilo per guest to two kilos or more. They are not wrong, they are answering different questions. Ice does two separate jobs at a party: it goes into glasses, and it fills the tub that keeps the bottles and cans cold. Count each job on its own and the number stops being a guess.
Set your guest list, the length of the party and how much of the drinking is over ice. The planner splits the result into the ice that goes into glasses, the ice in the tub and an allowance for melt, then turns it into bags. Drinks here means every drink: water, soft drinks, alcohol-free options and alcohol alike.
A planning estimate, not a measurement. Round up, and keep one spare bag in the freezer.
Three rules do almost all the work, and you can run them on the back of an envelope.
Glasses. A glass of ice holds somewhere between about 85 g (three cubes) and 200 g (a tall glass packed to the top). The planner starts at 150 g. Multiply by the number of drinks that go over ice. Drinks per guest depends on the party: one an hour is a sensible planning figure, and it covers water and soft drinks too. Planning for enough does not mean encouraging people to drink more; if anything, the more cold alcohol-free drinks on offer, the easier it is for guests to pace themselves.
The tub. A common cooler rule is to pack twice as much ice as there are drinks, measured by space: drinks fill a quarter of the tub, ice fills half. Loose cubes weigh roughly 0.5 to 0.7 kg per litre of space, so the planner uses 0.6 kg for every litre of ice. That gives about 1.2 kg of ice for every litre of drinks you want to keep cold.
Heat. In a hot room or outdoors in the sun, planners commonly double the base figure. The three buttons are a blunt allowance for that, not a measurement of your weather.
Most of the ice in a tub is not chilling the drinks. It is keeping them chilled.
Melting ice soaks up a large amount of heat: about 334 joules for every gram, the same energy that would warm a gram of water by roughly 80 °C. Water warms by 1 °C for every 4.18 joules per gram. So cooling a litre of drinks by 14 °C (from a warm room at 20 °C to a serving temperature of 6 °C) needs only about 175 g of ice to melt. Eight litres needs 1.4 kg. That is the physics floor, and it is small.
The reason a real tub needs several times that is everything the floor leaves out. Glass and aluminium warm up too, the air around the tub keeps pushing heat in for hours, and every time someone digs out a can, they lift the ice and let warm air in. A tub that starts with only the bare minimum of ice turns into a tub of cool water long before the party ends.
The planner assumes the ice goes into the glass and stays there. If your party is mostly shaken cocktails, remember that the ice used to shake or stir is thrown away, and it can easily match what goes into the glass. Raise the ice-per-drink slider for a cocktail-heavy evening. If most guests drink wine or beer from the tub, set the iced share low and the tub size high. If it is a long hot afternoon, move the heat button up one step and plan to refill the tub rather than buy it all at once.
Finally, ice is food. Use ice from a clean bag or from trays you trust, keep scoops for ice only, and never use the scoop to cool a bottle and then put it back in the drinks.