Start With the Cup Half Full of Ice
For a standard 16 oz milk iced coffee, a common recipe starts with the cup about half full of standard ice cubes. It then adds 1/2 cup (4 oz) milk or half-and-half and three espresso shots (roughly 3 oz), with room for syrup if desired. Treat that as a practical starting build, not a universal formula. A half-full cup is a visual fill line---about 8 fl oz of the cup's capacity---not 8 fl oz of solid ice, melted water, or loose cubes measured in a kitchen cup.
The important distinction: ice pieces have gaps between them and displace liquid differently depending on their size and shape. There is no single reliable conversion from "half a cup full of ice" to a fixed number of measuring cups of loose cubes. Use the cup's halfway line as your repeatable reference point.
A 16 oz label usually describes the cup's maximum, brimful capacity---not the amount of beverage you can comfortably serve with ice, a lid, foam, and mix-ins. Cup shape and the way the ice settles matter, too.
Use the ice level to make room for the ingredients you care about most. For a simple milk iced coffee, the half-cup baseline is a good start. For drinks with heavy milk, syrup, or cold foam, you will likely need less ice to avoid overflow. A practical range is about one-third to two-fifths full, but adjust to your cup and ingredients.
If you use cold-brew concentrate
Follow the concentrate's own mixing directions first. One concentrate product, for example, directs a 1:2 ratio of concentrate to water, milk, or oat milk before pouring over ice. Based on that product's stated bottle size and servings, one serving works out to 2 fl oz concentrate plus 4 fl oz water or milk---6 fl oz before ice. That 6 fl oz does not include syrup, foam, extra milk, or the space occupied by ice. Build the diluted coffee first if you want accuracy, then choose an ice level that leaves room for it.
Ice shape and melting
Ice shape changes how much surface area is exposed to the coffee. At equal ice volumes, shapes with more surface area relative to their volume tend to melt faster; shapes with less surface area relative to volume tend to melt more slowly. That principle is useful, but it does not produce a universal melt time for every cube, machine, or cup. Choose ice type based on your drinking pace: larger cubes or rounded shapes can slow melting, while smaller or irregular pieces chill quickly but melt faster.
Coffee ice cubes for flavor
If you dislike a coffee that tastes weaker as it sits, coffee ice cubes are the most direct option. As they melt, they add coffee rather than plain water. You do not need a universal cube count. Replace some or all of the regular ice with coffee cubes, then keep a small amount of regular ice if you want an immediate chill. A coffee-ice-cube method can start with coffee brewed stronger than normal, cooled to room temperature, and frozen for at least 4--6 hours.
Know your brew
It also helps to know what you are starting with. Cold brew is coffee brewed with room-temperature or cooler water, while iced coffee is hot-brewed coffee that is cooled afterward, as explained in this overview of cold brew versus iced coffee. Either can work in a 16 oz drink; the ice decision is still about space, strength, and pace.
For a standard milk iced coffee, use the half-full baseline. For a drink loaded with mix-ins, reduce the ice. For longer sipping, choose larger ice pieces or coffee ice cubes. And when preserving coffee strength matters most, coffee ice cubes are a reliable option.
