Ask three cafés for a cappuccino and you will get three different drinks. This is not sloppiness. The drink is defined by a measurement that almost nobody measures, and the definition has drifted differently in different countries.
The traditional Italian answer is about one centimetre of foam on a small drink, roughly 150 to 175ml in total. A flat white uses the same espresso with far less foam, around half a centimetre, in a similar or slightly larger cup. Foam depth, not size and not milk quantity, is the variable that separates most milk-and-espresso drinks from each other.
Why depth is the definition
Every drink in this family is the same two ingredients. Espresso and milk. What changes is the ratio and, crucially, what proportion of the milk has been turned into foam.
Foam is not a garnish. It changes the drink physically. It insulates, so the coffee stays hot longer. It slows the drink down, because you sip through it. And it dilutes the perceived strength, because a centimetre of foam is mostly air, so a cappuccino tastes milder than a flat white made with the same shot.
That last point is why people who order a cappuccino and want to taste the coffee are often disappointed, and why the flat white exists at all.
The rough shape of the family
A cappuccino is traditionally a small drink with a distinct foam layer, drunk in the morning and, in Italy, essentially never after lunch. A latte is larger, with more milk and usually a thinner foam layer, so the coffee is more diluted overall. A flat white is closer to a cappuccino in size but with the foam folded thoroughly through the milk rather than sitting on top, which gives a glossier texture and a stronger coffee flavour. A macchiato, in its original form, is an espresso with a spoon of foam, and almost nothing else.
The measurements above are conventions rather than standards. There is no enforcement body, and a great deal of national and café-level variation is entirely legitimate.
Microfoam is the part that matters more
The depth conversation distracts from the texture one. Well-steamed milk is microfoam: bubbles so small the surface looks like wet paint and pours as a single glossy liquid. Badly steamed milk separates into hot milk with a raft of large bubbles on top, and it will not pour, will not hold a pattern, and tastes thin.
The difference is made in the first two seconds of steaming, when air is introduced, and then in the following twenty seconds of spinning the milk to break that air down. That is a skill, not equipment, and it is the single largest gap between a home drink and a café one.
What foam depth will not tell you
It will not tell you whether the espresso underneath is any good. A perfectly proportioned cappuccino built on a sour, under-extracted shot is a sour drink with correct foam. The milk hides a surprising amount, which is exactly why so many cafés with mediocre espresso do a brisk trade in large milky drinks.
It will not tell you the milk is good either. Fresh whole milk foams differently from milk near its date, and plant milks vary enormously between brands, with some formulated specifically to foam and others that will not hold structure at all.
And measuring it is not a useful thing to do at home. Nobody should be putting a ruler in a cup. The number is worth knowing to understand what you are ordering, not to audit it.
The honest recommendation
Order by what you want to taste, not by the name. If you want the coffee to lead, ask for a flat white or a small cappuccino with less foam, and most baristas will simply do it. If you want a longer, milder, more comforting drink, a latte is the honest answer and there is nothing unserious about preferring one.
At home, the equipment that actually changes this is a steam wand with enough power to spin the milk, which is the single most common thing missing from cheaper machines. Several of the machines we sell steam adequately and none of them will make a café flat white in your first week, because the limiting factor is your hands. That takes a few kilos of wasted milk and it is worth knowing before you buy.