Mixing Formula With Breast Milk – Is It Safe?

By Lynn •  Updated: 08/08/23 •  6 min read
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Mixing formula with breastmilk– Is it a good idea? With so much information out there about what to do and what not to do, it can be difficult to tell heads from tails when it comes to feeding our babies. 

Still, whether or not mixing formula with breastmilk is a good idea is a question many parents find themselves asking, and for good reason. Thus, we’ve set out to provide you with the answers you need to this relevant and important subject. 

So, let’s get to it. 

Can I Mix Formula With Breastmilk?

Yes, you can mix formula with breastmilk. In fact, it may be preferable. 

Because breastmilk contains so many wonderful nutrients for babies and typically does not cause upset tummies, combining breastmilk with formula can be a great tactic for getting your baby the nutrients they need while also supplementing your breastmilk with formula. 

There are a variety of ways to mix breastmilk with formula. These include:

  • Combining formula with breastmilk in a single bottle.
  • Feed your baby a breastmilk bottle and a separate formula bottle during one feeding. 
  • Feeding your baby breastmilk in a bottle for one feeding and a formula bottle for the next.
  • Feeding your baby breastmilk during the day and formula from a bottle at night (or vice versa).

No matter which of the above methods you choose, know that combining breastmilk and formula won’t cause your baby issues in and of itself. 

Is It Safe to Mix Breastmilk and Formula?

Is It OK to Mix Formula With Breastmilk? Is it Good For the Baby?

The answer to this question is a resounding yes. Because breastmilk is best for babies, combining breastmilk with formula will offer more benefits than if you were to simply offer formula. 

Thus, it is absolutely okay to mix formula with breastmilk– but you need to make sure you mix the two in the right way. 

when to stop sterilizing baby bottles

Mixing Breastmilk and Formula Together

How to Mix Breastmilk With Formula

Although it is safe to mix breastmilk and formula, you will need to know how to do so in a way that nourishes your baby without overloading them with too many vitamins. 

This can happen when you combine unprepared baby formula with breastmilk. Because the formula is undiluted, combining it with breastmilk will result in an overload of nutrients which, believe it or not, is not good for your baby.

grayscale photo of baby feeding

How Do You Combine Breastmilk and Formula?

So, how to mix breastmilk and formula together? Consider the following steps for preparing your next baby bottle with both breastmilk and formula: 

  • Ready-Made Formula: Simply combine your ready-made pre-mixed formula with breastmilk at a 1:1 ratio. Gently warm the bottle using your method of choice–preferably not the microwave to reduce the chances of scorching your baby and the ingestion of unwanted chemicals–and shake the bottle to mix. Ensure that the milk is at the right temperature by dabbing a little on your wrist before feeding your baby. 
  • Unprepared Formula: Prepare the formula according to package directions. Remember to prepare only half of the amount you normally would for a full bottle, or prepare a full bottle and refrigerate any leftovers for later. Add to the formula to the same amount of fresh breastmilk at a 1:1 ratio. Shake to combine. Warm the bottle if necessary or serve at room temperature or chilled. Is a bottle warmer necessary? Not always, but if your baby likes warm milk, it might be worth it to invest in one.

Important: Remember never to combine breastmilk with an unprepared formula that hasn’t been diluted. 

Mixing Breastmilk and Formula in Same Bottle

Can I Mix Formula and Breastmilk in the Same Bottle?

As mentioned before, it is completely acceptable to mix formula and breastmilk in the same bottle. 

But be sure to do so the right way. 

Unprepared formulas will require dilution, so be sure to prepare the formula according to package directions before adding it to breastmilk at a 1:1 ratio. The formula that is already pre-mixed and ready to consume can be immediately mixed with breastmilk without dilution.

Is It Safe to Mix Breastmilk and Formula in the Same Bottle?

Yes. There is nothing harmful about mixing breastmilk and formula in the same bottle provided that the two are mixed correctly and are at the right temperature (not dangerously hot).

Can You Mix Cold Breast Milk With Formula?

Yes, you can mix cold breast milk with formula. However, if your baby doesn’t like their milk chilled, it may be best to try to gently warm your baby’s bottle of milk using a bottle warmer or placing your baby’s bottle in a pot of warm water so that they’ll be more likely to drink it. 

Does Mixing Breastmilk and Formula Cause Constipation?

No. Mixing breastmilk and formula does not typically cause constipation. If you do notice that your baby is constipated after mixing formula and breastmilk, it is more likely to be the formula or another ailment that is causing your baby issues. Rarely, if ever, will it be breast milk.

Having said that, your baby can react to foods that you are eating that may end up in your breastmilk. Foods like broccoli, beans, and cabbage can upset your baby’s tummy if you eat too much of it before breastfeeding. 

If your little one experiences gastric issues after drinking a bottle of mixed breastmilk and formula, check your diet, try tummy soothing techniques, and contact a pediatrician for advice on whether or not you should try a different type of formula.

Baby feeding from a bottle

Mixing Formula and Breastmilk Can Be Safe!

Mixing formula with breastmilk can be a wonderful way to supplement your breast milk supply while still giving your baby what they need in terms of nutritional value. Just remember to always follow the package directions when it comes to readying your baby’s formula and be sure to always combine the formula and breastmilk at a 1:1 ratio for best results. 


Lynn is a freelance writer, a wife, and a mother of two beautiful kids. Lynn started Infant Empire with the aim of making parenting easier for fellow mums and dads. She believes the parenting tips provided here will be of great help to all parents.

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