Breast Milk to Formula Conversion – 8 Influencing Factors

| Reviewed By Sarah Schulze, MSN, APRN, CPNP

4 ounces of breast milk is the same amount as 4 ounces of formula when measured by volume. However, this does not mean they have the same value, as there are other factors to consider:

  • Breast milk is easy to digest, leading to more frequent feedings.
  • Formula is harder to digest and can cause constipation.
  • Formula is more filling and takes longer to digest.
  • Breast milk contains more nutrients and changes to meet baby’s needs.
  • Overfeeding is much more likely with formula.
  • Formula lacks hormones needed to control appetite.

The American Pregnancy Association shares this key insight:

“Something important to note is that we do not fully know all of the contents of breast milk or fully understand all the changes it goes through as the baby grows.

This means that formula companies can never accurately mimic the contents of breast milk.”

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, taking into account the considerations and insights above, you can see that a truly accurate conversion rate is an impossibility.

According to

“On average, your baby should take in about 2½ ounces (75 mL) of infant formula a day for every pound (453 g) of body weight.”

When giving formula instead of breast milk, let your baby decide when they have had enough.

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 the baby’s immune system, aid brain development, and promote healthy bacteria growth.

5. Formula Is More Filling

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

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.

While formula might help some babies sleep longer at night, Parenting Science shares this amazing fact: 

“Breast milk pumped at night has more melatonin — the hormone of drowsiness — and higher levels of nucleotides thought to promote sleep.”

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.

Breast MilkFormula
Tailored to baby’s needs; changes over timeFixed composition; may not match breast milk
Contains antibodies and immune cellsFormula may have added immune-boosting substances
Easily digested; less chance of constipationMay be harder to digest; can cause constipation
Always available; no need for preparationRequires preparation; may not be readily available
Economical (free)Costly; ongoing expense
Promotes bonding during breastfeedingLess opportunity for direct bonding
Flavors change based on maternal dietConsistent flavor
Consistency can vary; changes over timeConsistent composition
Provides protection against allergiesSome infants may be allergic to components in formula
Supports the development of a healthy microbiomeMay not provide the same microbiome benefits

Common Concerns

A cute baby wearing a knit hat, smiling, and playing with the bottle nipple instead of drinking.

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.

Nurse Donna Murray warns:

“Never add undiluted powdered infant formula or concentrated liquid formula directly into your breast milk, and never use your breast milk in place of water to mix concentrated or powdered infant formula.”

Benefits of Supplementing With Formula

Supplementing with baby formula can have several benefits for both the baby and the parents.

While breastfeeding is a valuable and natural way to nourish an infant, formula feeding provides flexibility and options that can be helpful in certain situations.

  1. Flexibility: Formula feeding allows both parents to share caregiving duties and provides breaks for the mother.
  2. Monitoring Intake: Formula feeding makes it easy to monitor the baby’s milk intake, especially for tracking weight gain.
  3. Situational Convenience: Formula is convenient in situations like returning to work, travel, or health-related challenges.
  4. Shared Responsibilities: Others can participate in feeding, fostering bonding and shared responsibilities.
  5. Predictable Nutrient Composition: Formula offers a consistent nutrient composition, which is beneficial for those wanting precise nutritional control.
  6. Maternal Well-Being: Formula provides relief for mothers facing physical or emotional challenges associated with breastfeeding.
  7. Access to Specialized Formulas: Specialized formulas cater to specific nutritional needs or allergies.
  8. Extended Family Involvement: Formula feeding allows extended family or friends to participate in feeding, enhancing social interactions.
  9. Integration into Lifestyle: Formula feeding is more easily integrated into a busy lifestyle, providing convenience.

Possible Side Effects of Supplementing With Formula 

Supplementing with formula is generally considered safe and can be a necessary choice for many families.

However, some babies may be allergic or sensitive to components in formula, such as cow’s milk protein or soy.

This can lead to symptoms like fussiness, gas, or digestive issues.

Formula may be slightly harder for some babies to digest compared to breast milk.

This can lead to differences in bowel habits, such as firmer stools or less frequent bowel movements. 

While some formulas include prebiotics and probiotics to support gut health, they may not replicate the exact composition of breast milk.

The long-term implications of these differences are still an area of ongoing research.

Additionally, some babies might learn to prefer formula over breast milk, thus bringing your breastfeeding journey to a premature end.

A father in a purple shirt feeding his baby a bottle.

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.

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 your 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.