How Much Milk a Baby Needs by Weight

For formula there is a published sum, and it is the American Academy of Pediatrics': on average about 2½ ounces — roughly 75 millilitres — of formula a day for every pound of body weight, and usually no more than about 32 ounces, or 960 millilitres, in 24 hours. That gives you a figure for a day rather than for a bottle, and the step from one to the other is where most of the error lives. There is no matching sum for a breastfed baby. That is an answer rather than a gap, and the second half of this page is about why.

Both routes through the arithmetic are worth having in front of you before anything else. Weight in pounds, multiplied by about 2½, gives ounces a day. Weight in kilograms, multiplied by about 165, gives millilitres a day — the same rule of thumb converted, because the AAP prints a pound as 453 grams. Then the ceiling comes down on top of whatever either sum produced. The milk intake calculator runs both and draws the ceiling as a line you can see.

What the number is for

It is a check on a week, not an instruction for tonight. A baby who takes noticeably less than the sum on Tuesday and noticeably more on Wednesday has done nothing unusual — the AAP's own wording is that babies regulate their intake from day to day to meet their own needs, and a baby regulates her own intake from day to day whether or not there is a figure written on the fridge.

Where the number earns its place is at the extremes. If a week's average is a long way under what the sum suggests, or a long way over it, that is worth a sentence to your pediatrician or health visitor at the next weigh-in. That is the whole job. It cannot tell you what any single bottle should hold.

Turning a day into a feed, which is the shaky step

The obvious move is to divide: a day's total, shared out over the feeds you expect. Do it once so you have a rough size of bottle to make up, and then hold it loosely, because the division assumes three things that are usually false.

It assumes you know how many feeds there will be. You do not, and the AAP's published answer to how often is to feed whenever she seems hungry rather than to a clock. It assumes every feed is the same size. They are not — a baby who feeds hard at seven and dozes through the nine o'clock one has still had her day. And it assumes the day is the unit that matters to her, when the unit that matters is the week.

So a divided figure is a bottle size to start from. The honest version of the instruction is: make up roughly that much, offer it, and let her stop where she stops. The NHS is blunt about the other direction — do not force a baby to take more milk than she wants, because that can lead to overfeeding — which is the subject of the paced bottle feeding page.

The ceiling, and why it is printed at all

No more than about 32 ounces in 24 hours, on average. It is there because the weight sum runs away with itself: a bigger baby multiplied by 2½ eventually asks for a volume that no publisher supports, and the ceiling is the point at which the arithmetic stops being useful. If your sum lands above it, the answer is not to pour the extra — it is to mention the appetite to whoever weighs her.

The unit traps, which are the ones that actually hurt

Every mistake worth worrying about on this page is a unit mistake, and each one is silent.

Kilograms read as pounds is the big one. A baby of 7 kg and a baby of 7 lb are not near each other — one is a newborn and one is around six months old — and running the pounds sum on a kilogram figure produces a fraction of the right answer, while running it the other way produces roughly double. Weighing scales at a clinic usually give kilograms in the United Kingdom and pounds and ounces in the United States, and the number written on your phone rarely says which.

Millilitres read as ounces is the same error in the bottle. An ounce is about 30 millilitres, so a 4 on a bottle marked in ounces and a 4 on a bottle marked in millilitres differ by a factor of seven. Bottles sold in the same shop carry different scales on the two sides.

The habit that fixes both: write the unit down every time, and say the answer out loud in both — "about 24 ounces, so about 700 millilitres" — because a figure that survives being converted is usually a figure you have read correctly.

What changes when solids start

Less than the internet suggests. Solid foods start to be added at around six months, and milk stays the main event for the rest of the first year while food is practice. The AAP's own by-age figures do not fall away at six months: it still publishes 6 to 8 ounces at each of four or five feeds at that age, which is roughly where the weight sum has been sitting anyway.

What does change is that some days will look different from others. A big lunch and a smaller evening bottle is information, not a problem to correct. The formula amounts by age page has the published stages laid out in order.

Why there is no version of this sum for breastfeeding

Because the calculation does not exist, and every site printing one has taken formula arithmetic and quietly relabelled it. You cannot see what went in, supply moves with demand, and the same organisation that publishes the weight rule for formula answers the breastfeeding question completely differently: feed whenever she seems hungry. Newborns nursing every two hours or so, ten to twelve sessions in 24 hours, is the norm the AAP prints — a frequency, not a volume. The how often to breastfeed page works through it.

This is not squeamishness about the subject. It is that the checkable signal for a breastfed baby is on the other end. The NHS's own list of what tells you enough is going in is weight gain and nappies — around six wet nappies (diapers) a day from a few days after the birth, and at least one dirty one a day — and the wet nappy counts page has the thresholds and the point at which they say to ring somebody.

The one figure the AAP does publish for breastfed babies is worth knowing precisely because of how wide it is: most take about an ounce at a feed in the first few weeks, rising to roughly four and a half ounces at a year. That is a range across a whole year of a baby's life, offered as context. It is not a target for this evening, and nobody should be measuring against it.

What to do with the number you now have

Write it on the feeding chart as a starting point for a week and then stop looking at it for a week. Watch the three things that are actually measurable instead: what she takes across several days rather than one, what comes out, and what the scales say at the routine weigh-in.

A baby who wants less at one feed and more at the next is doing exactly what the AAP says babies do. A baby whose weight gain has stalled, whose nappies have dropped off, who is hard to rouse for feeds, or who never seems satisfied however much is offered, is a phone call to your pediatrician, GP or health visitor — today rather than next week. Nothing on this page is medical advice and none of it can see your baby.

One thing that does not change with any of this: however the milk goes in, she goes down on her back for every sleep, day and night, on a firm flat mattress, in a cot clear of blankets, pillows, bumpers and soft toys, in your room for the first six months at least.