Breast Milk to Formula Conversion – 8 Influencing Factors

For parents who have chosen to breastfeed their infants, there may come a point when formula is needed either to supplement a low supply or just substitute a feeding in a pinch when the mother or pumped milk is not available.

This can cause some worry or questions for parents but does not need to be a complicated process. For starters, what is the conversion rate from one to the other?

4 oz. breast milk equals how much formula? As both breast milk and formula (once ready to use) are liquids, 4 oz. breast milk is equivalent in terms of volume to 4 oz. formula. This does not make them equal, however. Breast milk is richer in nutrients and easier to digest, leading babies to feed more often than on harder-to-digest formula.

Most formula instructions state that 2 scoops of powder mixed with 4 ounces of water will result in the equivalent of the 4 fluid ounces of breast milk your baby is used to.

However, there may be some differences in your baby’s response to formula, how they digest it, and how their stool looks after consuming formula.

Unfortunately, the conversion isn’t as simple as this due to the differences between breast milk and formula and how your baby processes them.

We’ll look at 8 reasons why this conversion is so puzzling so you’ll have a better understanding of the differences between breast milk and formula.

Breast Milk to Formula Conversion – Why the Answer Isn’t Easy

An ounce of breast milk is the same fluid volume as an ounce of formula. However, a number of factors will influence how much formula your baby needs and the frequency of feeds.

As a starting point, the correct formula amount will depend on your baby’s age and weight, but the following points also need to be taken into consideration…

1. Breast Milk Is More Easily & Quickly Digested

Breast milk contains the enzymes amylase and lipase that help aid digestion. Human milk also contains more whey than formula, which babies can digest much more easily. Better digestion tends to mean less gas and spit-up too.

2. Formula Is Digested Less Efficiently

Formula takes longer to digest due to higher levels of casein, making it sit in baby’s tummy a little longer than breast milk.

This may cause a baby to go longer in between feedings than they normally would with breast milk as their belly feels full longer. 

3. Breastfed Babies Typically Eat More Often

As breast milk is digested and processed in your baby’s body quicker than formula, a baby will become hungrier quicker on breast milk and require feeds every 2-3 hours in the first few weeks.

For some mothers, it can seem like your little one is never full, but this will pass!

4. Breast Milk Has More Nutrients Per Ounce

Ounce for ounce, human milk contains more vitamins and nutrients than formula with around 60-80% alone accounting for whey protein.

Together with the remaining fats and calories, breast milk contains what’s needed to fight infection, enhance baby’s immune system, aid brain development, and promote healthy bacteria growth.

5. Formula Is More Filling

According to pediatric specialist Dr. Srikietr Dhana at Organic Life Start, formula tends to be more filling than breast milk due to “consistency, protein compositions, and the presence of starch in some formulas.”

6. It’s Easier To Overfeed Formula Than Breastmilk

When feeding from the breast, babies have more control over their intake, will stop feeding when they’re full, and can continue to suck for comfort while swallowing very little milk.

With formula, however, you can see how much is left in the bottle, so mothers tend to encourage babies to eat until the bottle is empty, increasing the likelihood of overfeeding.

7. Babies Typically Drink More From Bottle Than Breast

Milk flowing from the beast has a natural ebb and flow compared with bottled milk, which has a fast, consistent flow.

Because of this, it’s easier for babies to continue feeding from the bottle past the point of recognizing when they are full, so they’ll inevitably drink a lot more.

8. Formula Lacks Hormones Needed To Control Appetite

Formula is lacking in the hormones adiponectin and leptin – these are needed to help to regulate your baby’s appetite and metabolism.

The nutrients in formula are also used less efficiently by your baby’s body compared with breast milk, so more of it is required to meet their nutritional needs.

Can I Mix Breast Milk With Formula?

Yes, you can mix breast milk with formula in the same bottle to feed babies in need of a little extra nutrition.

Just note that it can change the consistency of the milk and you must be sure to first prepare the formula according to the instructions before adding this to breast milk.

“Never add undiluted powder formula or concentrated liquid formula directly into your breast milk” warns Nurse Donna Murray, BSN at Very Well Family, “as improper mixing can lead to an over-concentration of nutrients that may harm your baby.”

Signs Baby Is Hungry

From birth to 6 months, your little one will typically signal they are ready to feed in the following ways:

  • Waking up restless.
  • Moving their hand or clenched fists to their face.
  • Sucking their fists.
  • Smacking lips.
  • Turning toward your breast or the bottle after you stroke them on the cheek – this is a reflex called “rooting.” This usually passes by 4 months of age.
  • A specific “hunger cry” (typically short and low-pitched with rises and falls). This is a late sign of hunger.
A cute baby wearing a knit hat, smiling, and playing with the bottle nipple instead of drinking.

Signs Baby Is Full

Once baby has had their fill, they’ll often let you know with obvious signs such as:

  • Beginning to slow their sucking
  • Closing lips
  • Turning their head away from the breast or the bottle
  • Appearing distracted or disinterested in feeding, e.g., noticing their surroundings more and looking elsewhere
  • Starting to fall asleep
  • Relaxing their fingers and arms or legs

Signs Baby Is Not Getting Enough Milk

Some common signs that your baby isn’t getting as much milk as they need include things like:

  • They’re not gaining weight as they should
  • Baby is sleeping longer than usual or appears sluggish rather than alert at feeds – newborns need to eat every 2-4 hours.
  • They’re not producing stools – fairly dry diapers indicate they’re not getting enough fluid
  • Urine is dark/concentrated 
  • Latching is difficult or painful for you, or it feels as if baby hasn’t latched deeply enough

If you’re concerned that your baby may not be getting enough milk, please speak to your pediatrician as they’ll be able to offer breastfeeding support, guidance on bottle feeds, and assess your little one’s development.

Related Questions:

Is Formula More Filling Than Breast Milk?

Formula is considered to be more filling compared with breast milk due to how long it takes to digest.

Additionally, some non-organic formula types tend to have filler ingredients in them, contributing to longer digestion time and making baby feel fuller for longer.

Does Formula Help Babies Sleep Longer?

According to Baby Centre editor Polly Logan-Banks, breastfed babies tend to sleep in shorter bursts and may sleep less deeply throughout the night compared with formula-fed babies. However, breastfed infants have the benefit of melatonin, which helps regulate their sleep cycle.


To sum up, 4 ounces of breast milk and 4 ounces of formula may have the same volume, but the two are far from equal in terms of nutrients and how your little one metabolizes them.

Essentially, formula takes longer to digest than breast milk, making it more filling, which makes it easy for your little one to overfeed when drinking from a fast-flowing bottle.

If you’re concerned about making the switch to formula, consult your pediatrician to ensure your baby’s feeding schedule is on the right track for their age, weight, and other factors.