The short answer, and why it is unsatisfying

Output varies more between people than almost anything else in feeding, and the same person varies across the day, across the week and across a cold. Anyone who gives you one number is guessing. What the health services do give are ranges, and they are wide enough to be reassuring.

The first days: millilitres, not ounces

The HSE publishes figures for expressing for a newborn that most people find surprisingly low, and they are worth reading before you judge yourself. Across the whole of day one, expected total expressed milk is 0ml to 123ml in 24 hours, with a baby taking anything from a few drops to about 5ml at a feed. On day two the 24-hour range is 44ml to 335ml, with 5–15ml per feed. On day three it is 98ml to 775ml, with 15–30ml per feed. The HSE also notes that supply often appears to dip on day two before increasing.

Those ranges start at zero for a reason. Getting almost nothing in the first days is common and is not evidence that you will not make milk. The HSE's advice is that even if you get little or no milk, continuing to stimulate the breast is what builds the supply. It advises expressing at least ten to twelve times in 24 hours in those first days.

Once supply is established

Pumping alongside breastfeeding. Anything from a few millilitres to 120ml per session is ordinary. If you pump straight after a feed, expect much less — the baby has just taken most of it.
Exclusive pumping. A rough total of 750–900ml over 24 hours covers most babies from around one month old, spread across however many sessions you do.
Across the day. Volumes are typically highest in the early morning and lowest in the late afternoon and evening. This is a normal daily rhythm, not a decline.

A baby feeding well at the breast usually removes more milk than a pump does. That is the single most useful thing to know if you have started pumping and been shocked by the bottle.

Why the bottle is a bad measure of supply

Pumps work by suction alone. A baby works by suction, tongue movement, jaw compression and — most importantly — by triggering an oxytocin release with their smell, warmth and weight against you. Some people simply do not let down easily to a machine. The NHS's framing of whether a baby is getting enough milk deliberately uses nappies, weight gain and feeding behaviour, not pumped volumes, and the Breastfeeding Network says the same: what comes out into a bottle tells you about your response to that pump on that day, not about your capacity.

If someone has told you that a low pumping output proves you have low supply, that is not a conclusion any of the guidance supports.

Things that genuinely change output

Fit. The part that touches you matters more than the motor. Too small and the nipple rubs; too large and areola tissue is dragged in. Both reduce how much milk comes out.
Time since the last removal. Two hours after a feed will always yield less than four.
Whether you use your hands. Massaging during the session and hand expressing afterwards reliably increases the total, and gets more of the fattier milk.
Worn parts. Valves and membranes degrade. A pump that quietly stopped working properly is a common and very fixable cause of a sudden drop.
Being stressed, cold, rushed or watched. The HSE lists relaxation and comfort as real variables. Looking at your baby, or a photo of them, helps let-down.
Illness, dehydration, a period returning, and restarting hormonal contraception. All can cause a temporary dip.

Three sessions is a fairer test than one

Judging output on a single session is the fastest route to feeling like a failure. Compare the same time of day across three or four days before concluding anything.

When a low output is worth investigating

Output on its own is not the concern. Output plus one of these is:

Your baby is not gaining weight as expected, or has dropped across growth chart lines.
Fewer wet nappies than expected, or dark, strong-smelling urine.
Your baby feeds constantly and still seems unsatisfied, or is sleepy and hard to rouse.
Your supply drops sharply and stays down for more than a few days.
Pumping hurts.

Ask your midwife, health visitor, public health nurse, GP or an IBCLC lactation consultant. If it turns out that your supply is genuinely low, there are usually reasons for it, and most of them are not things you did. Combination feeding is a legitimate outcome, and giving some formula alongside your milk does not undo the milk you are giving.

Comparing yourself to other people

This is worth naming, because it does more damage than almost anything else. Photographs of full bottles are self-selecting — people rarely post the session that produced 20ml. Someone with a large milk storage capacity can go six hours and pump 200ml; someone with a smaller capacity may make exactly the same amount in a day across eight sessions of 90ml. Storage capacity is anatomical and has nothing to do with breast size or with how good a job you are doing.

The same applies to comparing yourself with yourself. Output falls when you are unwell, dehydrated, premenstrual, back at work, or simply having a bad week, and it comes back. Judge over a fortnight, not a session.

The one number that does matter

If you are exclusively pumping, the useful figure is the 24-hour total set against what your baby is drinking, not any individual session. If you are pumping alongside feeds, there is no number that means anything at all — your baby is the measuring instrument, and their nappies and weight are the readout.

How Much Milk Should I Get When I Pump?