(updated August 24th, 2026)
Milk. A cappuccino is 25 ml of espresso and 100 ml of steamed milk, and there’s no version of it made with water, because the foam on top is made of milk. Water can’t foam.
That sounds like a technicality. It isn’t: it’s the whole reason the drink exists in the shape it does.
Why can’t you make a cappuccino with water?
Because foam needs something to hold the bubbles up, and water has nothing.
When you steam milk you are forcing air into it. What traps that air is protein: heating unravels the whey proteins so they can wrap around each bubble and hold its shape. Perfect Daily Grind sets out the chemistry, and the short version is that the bubbles survive because the proteins form a skin around them.
Water has no protein, so blow air through it and the bubbles collapse as fast as they appear. You can froth washing-up water into a head of suds because the detergent does the same job the milk protein does. Plain hot water does nothing at all.
There is more on why a cappuccino has foam in the first place if that is the part you are curious about.
What do you get if you use water instead?
An americano, and a perfectly good one.
Espresso plus hot water is a recognised drink with its own name. It is not a failed cappuccino, it is a different order. What it lacks is any of the three things people mean when they say cappuccino: the creaminess, the sweetness milk brings, or the dome of foam.
So if you’re out of milk, you haven’t lost your coffee. You’ve just lost the cappuccino.
Where does water belong in a cappuccino?
In the espresso, where it already is.
This is the bit that confuses the question. A cappuccino is mostly water by weight already, because espresso is water forced through ground coffee, and milk is itself mostly water. Nobody adds water to a cappuccino because it is already the main ingredient of both halves.
There is one real exception, and it is probably why the question gets asked at all. Instant cappuccino sachets are made with water on purpose, because the milk is already in the sachet as powder. You are not replacing the milk, you are rehydrating it. That is a different product built to a different plan, and I’ve been through what’s actually in those sachets separately.
Which milk makes the best cappuccino?
Whole milk, and the reason is worth getting right because it is usually explained backwards.
The foam comes from protein, not from fat. A University of Camerino study of cappuccino milk specifies a minimum of 3.2% protein and 3.5% fat, and the fat is in there for flavour and body rather than for the foam. Perfect Daily Grind puts it more bluntly: fats destabilise milk foam, and you get better foam from skimmed milk.
So whole milk is the right answer, but not because it foams better. It is a trade: you give up a little foam stability and you get a rounder, sweeter drink. Skimmed milk will give you a taller, stiffer head on something thinner underneath, which is why it looks impressive and tastes hollow.
So what should you actually do?
Use whole milk, cold, straight from the fridge, and don’t think about water at all.
The proportions are worth knowing though, because the version most people have in their heads is wrong. A cappuccino isn’t equal thirds of espresso, milk and foam. It’s 25 ml of espresso to 100 ml of milk, which is closer to one part coffee to four parts milk. The foam is not a separate ingredient you add, it is the same milk with air worked into it.
Which loops back to the original question rather neatly. You can’t make it with water because there is no third ingredient to make with water. There is coffee, there is milk, and the foam is just what the milk does when you steam it.
