Breast milk storage calculator

What the result tells you

Expressed breast milk is a food that spoils, and it spoils on a clock that depends on two things: whether it was ever frozen, and where it is kept now. The CDC summarizes this in one small table adapted from the Academy of Breastfeeding Medicine’s home-use protocol. This calculator turns the table into a date and time: you say which milk you have, where it is, and when the clock started, and it adds the allowed time. The clock starts at a different moment for each kind of milk, which is where most mistakes happen: for fresh milk it is the moment you pumped, for frozen milk that has been thawed it is the moment it was completely thawed, and for a bottle the baby has started it is the moment the baby finished feeding.

The CDC table the calculator uses

The CDC adds that milk can travel in an insulated cooler bag with frozen ice packs for up to 24 hours. The calculator offers that as a fourth place for fresh milk. For thawed milk in a cooler bag the CDC gives no time, so the calculator says so instead of guessing.

A worked example

You pump at Mon, Oct 5, 8:00 AM. On the countertop the clock ends at Mon, Oct 5, 12:00 PM, four hours later. In the refrigerator it ends at Fri, Oct 9, 8:00 AM, four days later, and the CDC’s advice is to freeze the milk before then if you will not use it. In the freezer the milk is at its best until April 5, 2027 (6 months) and still acceptable until October 5, 2027 (12 months). Now suppose you take one of these bags out of the freezer to thaw it overnight in the refrigerator, and it is completely thawed at Mon, Oct 12, 9:00 PM: the thawed milk must be used by Tue, Oct 13, 9:00 PM. Counting from the time you took the bag out of the freezer would have cut that window short, or, if the bag had started thawing hours earlier, would have made the milk look safe for longer than it is.

Common mistakes

Starting the thawed clock too early or too late. The CDC says the 24 hours run from the time the milk is completely thawed, “not from the time when you took it out of the freezer.” A bag that is still icy in the middle is not yet thawed, so the day has not started.

Refreezing. Once thawed, milk is never refrozen. If you are unsure how much the baby will take, thaw small bags (the CDC suggests 2 to 4 ounces) and thaw more only when needed.

Storing in the door. The CDC says to put milk at the back of the refrigerator or freezer, not the door, because the door is the warmest and most variable spot.

Heating in the microwave. It creates hot spots that can burn the baby’s mouth and, according to the CDC, destroys nutrients. Warm the sealed container in a bowl of warm water.

Limits of this method

The table is for healthy full-term babies at home. It is adapted from a protocol for home use for full-term infants; hospital units follow their own protocols, and you should follow the ones your care team gives you if your baby is premature, ill or immunocompromised.

The times assume the stated temperatures. A countertop at 77 °F or colder is not a car on a summer day. The calculator cannot know how warm your room is, how often your freezer is opened or how clean the container was, and the CDC gives its times as upper limits for best quality, so treat them as limits and not targets.

The clock is this device’s clock. The status line compares the deadline with the time on the phone or computer you are using. The deadlines themselves depend only on what you enter.

How long can breast milk sit out at room temperature?

The CDC says freshly expressed or pumped milk can stay on the countertop for up to 4 hours at 77 °F (25 °C) or colder. That is the room temperature the table is written for; the CDC gives no figure for a warmer room, so in a hot kitchen or car do not count on the full 4 hours. Milk that has been thawed from the freezer has a shorter window, 1 to 2 hours once it reaches room temperature or is warmed.

How long does breast milk last in the refrigerator?

Up to 4 days for freshly expressed milk, at 40 °F (4 °C) or colder. Put it at the back of the shelf, not in the door, where the temperature swings every time the door opens. If you do not expect to use the milk within 4 days, the CDC says to freeze it right away rather than waiting until day four. Milk that was frozen and then thawed in the refrigerator is good for 1 day (24 hours), counted from the moment it is completely thawed.

How long can I keep breast milk in the freezer?

The CDC gives two numbers for a freezer at 0 °F (−18 °C) or colder: within 6 months is best, and up to 12 months is acceptable. The calculator shows both dates. The CDC notes that the recommended times are important to follow for best quality, and it says to thaw the oldest milk first, so label every bag with the date it was expressed.

Can I refreeze thawed breast milk?

No. The CDC says never to refreeze human milk after it has been thawed. Thawed milk can be kept in the refrigerator for up to 24 hours from complete thawing, or left at room temperature for 1 to 2 hours. If the baby does not finish a bottle, the leftover milk is good for 2 hours after the baby is finished feeding and is then discarded; it does not go back into the refrigerator or freezer.

When does the 24-hour clock for thawed milk start?

From the time the milk is completely thawed, not from the time you took it out of the freezer. This is the single most common mistake in home use. If a bag comes out of the freezer on Monday night and is not fully thawed until Tuesday morning, Tuesday morning is the start. Set the first box of the calculator to “Thawed” and enter the time it was completely thawed. The CDC also says to thaw the oldest milk first, either overnight in the refrigerator or in a container of lukewarm water or under lukewarm running water.

Can I warm breast milk in the microwave or on the stove?

No. The CDC says never to thaw or heat milk in a microwave, because microwaving destroys nutrients and creates hot spots that can burn a baby’s mouth, and not to heat milk directly on the stove. Place the sealed container in a bowl of warm water or hold it under warm running water, swirl it to mix the fat that has separated, and test a few drops on your wrist: it should feel warm, not hot. Milk can also be served cold or at room temperature.

Do these times apply to premature or hospitalized babies?

No. The CDC brochure is adapted from the Academy of Breastfeeding Medicine protocol for home use for full-term infants. Premature, sick or hospitalized babies, and babies with weakened immune systems, are covered by instructions from the hospital or the baby’s doctor, so follow those instead of this table.

How should I store the milk to make it last?

Use breast milk storage bags or clean, food-grade glass or plastic containers with tight lids, not disposable bottle liners or ordinary plastic bags. Label each one with the date it was expressed (and the child’s name if it goes to child care). Freeze in small amounts of 2 to 4 ounces to avoid waste, and leave about an inch of space at the top, because milk expands as it freezes. In a cooler bag with frozen ice packs the milk keeps for up to 24 hours on a trip.

More for parents: the baby formula amount calculator, the diaper count calculator for newborn feeding checks, the sleep needs by age calculator, the food shelf life calculator and the baby growth percentile calculator.

Sources: Centers for Disease Control and Prevention, “Storage and Preparation of Breast Milk” (October 2024), cdc.gov/breastfeeding/pdf/preparation-of-breast-milk_h.pdf (the storage table, the thawing, warming and feeding rules and the cooler-bag time); the CDC brochure is adapted from Academy of Breastfeeding Medicine, Clinical Protocol #8: Human Milk Storage Information for Home Use for Full-Term Infants, revised 2017. This calculator is not endorsed by or affiliated with the CDC or the Academy of Breastfeeding Medicine.

For general information only — not medical advice. Ask your pediatrician or a lactation consultant about storing milk for a premature or sick baby. How we check our calculators.