How long does breast milk keep?

Per the CDC's breast-milk storage guidance, freshly expressed or pumped milk keeps for up to 4 hours at room temperature (77°F / 25°C or colder), up to 24 hours in an insulated cooler with frozen ice packs, up to 4 days in the refrigerator (40°F / 4°C), and in the freezer (0°F / -18°C or colder) for about 6 months at best and up to 12 months at the outside. Those are the safety windows, not targets — sooner is always better for milk quality. If you know you won't use refrigerated milk within about four days, freeze it right after pumping rather than waiting. Store milk toward the back of the fridge or freezer, not in the door, where the temperature holds steadiest.

Once breast milk has been thawed, the CDC says you should never refreeze it. Use previously frozen milk within 24 hours if it has stayed in the refrigerator (counting from when it finished thawing, not from when you pumped it), or within 1–2 hours if it has already reached room temperature. Thawed milk simply doesn't keep as long as fresh — the safety clock is shorter and there's no restarting it by refreezing. If you thawed more than your baby will take and it's still cold, use it within that 24-hour fridge window; if you can't, it's safest to discard it. When in doubt, throw it out.

Thaw frozen milk slowly — in the refrigerator overnight, by holding the container under lukewarm running water, or by setting it in a bowl of warm (not hot) water. Never thaw or heat breast milk in the microwave: microwaving destroys some of the milk's protective components and creates uneven hot spots that can badly scald your baby's mouth. Breast milk doesn't have to be warmed at all — some babies take it cool or at room temperature — but if you do warm it, keep it lukewarm and test a few drops on your wrist first. Swirl the container gently to mix the layers rather than shaking it hard. Once milk has been warmed or reached room temperature, use it within a couple of hours and don't put it back in the fridge to reheat later.

Usually not. Breast milk naturally separates as it sits: the fattier cream rises to the top and the thinner milk settles below. A gentle swirl blends it back together — you don't need to shake it. Color varies too, tinted by what you've eaten, and frozen-then-thawed milk can smell soapy or metallic (often from an enzyme called lipase) while still being perfectly safe to feed. Milk that has genuinely spoiled tends to smell sour or rotten in a way that doesn't swirl away, and it may stay clumpy. If a bottle has been out well past its keep-time or truly smells off, the safe choice is to discard it rather than risk it — when in doubt, throw it out.

You can combine milk, but cool the fresh milk first. Adding freshly pumped, still-warm milk directly onto already-frozen milk can partly thaw the frozen layer, and warming chilled milk with warm milk isn't ideal either. The safer approach is to refrigerate the freshly expressed milk until it's cold, then add it to other cold milk from the same day. Leave room at the top of any container you're freezing, because milk expands as it freezes. When you do combine batches, the whole container follows the storage clock of the oldest milk in it, so label it with that earliest date.

These keep-times are the CDC's general guidance for healthy, full-term infants. For a premature baby, a baby in the NICU, or an infant with a medical condition, hospitals often use stricter storage and handling rules, and your care team may give you specific instructions that differ from the numbers here — follow theirs. On privacy: this calculator runs entirely in your browser. The choices and any time you enter are used for the math on your own device and are never sent to a server, saved to an account, or shared — close the tab and they're gone. It's general educational information based on published CDC guidance, not medical advice, and it can't replace a lactation consultant, your pediatrician, or your hospital's instructions.