How much weight has my newborn lost?

It is normal for babies to lose some weight in the first few days of life — most of it is water weight they were born with, lost while your milk is coming in. Up to about 7 to 10% of birth weight is commonly seen, weight usually reaches its lowest point around day 3 to 4, and most newborns are back to their birth weight by about 2 weeks (almost all by 3 weeks). This calculator applies the plain percentage — birth weight minus current weight, divided by birth weight — and reads it against those commonly cited ranges. It is a screening guide, not a diagnosis: what confirms feeding is working is the whole picture, including wet and dirty diapers and steady weigh-ins, which is why the tool always points you back to them.

Contact your baby's pediatrician or a lactation/nurse line promptly for a weigh-in and a feeding check — over about 10% is the point clinicians look closely and rule out dehydration. It doesn't automatically mean something is wrong: measurement differences between scales, the exact timing, and how feeding is going all matter, and many babies just need feeding support to turn the corner. But it isn't something to sit on. The Academy of Breastfeeding Medicine flags loss of roughly 8–10% or more as a signal for careful assessment and possible breastfeeding help — that mark is anchored to day 5 (120 hours) or later, so earlier in the first days the same loss is faster than expected and read more closely. Bring the number, your baby's age in days, and how their diapers are going to the conversation — those are exactly what your provider will want to know.

Because a single weight is one data point, and diaper output is a running signal of whether milk is actually going in and being used. Once your milk comes in, a well-fed newborn generally has at least 6 wet diapers and 3 or more dirty diapers a day, feeds roughly 8 to 12 times in 24 hours, and seems settled after most feeds. If the percentage looks fine but wet diapers have dropped off, that still deserves a call; if the percentage is on the high side but diapers, feeding, and alertness are all good, that's reassuring context your provider will weigh. Neither the number nor the diapers is the whole answer alone — together they're far more informative, which is why this tool reports the percentage but never in isolation.

The ones to act on are: fewer wet diapers than expected (under about 6 a day once your milk is in), a baby who is very sleepy or hard to wake for feeds or is feeding poorly, a dry mouth or lips or no tears when crying, a sunken soft spot (fontanelle) on the top of the head, and far fewer dirty diapers or dark, scanty urine. Any of these means contact your baby's doctor or a nurse line promptly — today, not at a routine visit — whatever a weight calculator says. If your baby is extremely difficult to wake, not feeding at all, breathing fast, or you are seriously worried, seek urgent care. Dehydration in a newborn is checked without delay, so when in doubt, call: this tool can't examine your baby, and it never overrides what you're seeing in front of you.

The day matters for reading the number, not for the math. Weight normally dips to its lowest point around day 3 to 4 and then climbs back, so the same percentage means something different on day 2 than on day 12 — by about 2 weeks a baby is usually back to birth weight, and still being below it then is a reason to check in. That's why the tool asks for your baby's age in days: it's optional, but it lets the read flag a baby who hasn't recovered by the two-week mark. The unit, on the other hand, doesn't change anything: percent loss is a ratio, so grams, kilograms, and pounds-and-ounces all give the identical percentage. Just enter the birth weight and current weight in the same unit — the tool keeps them matched for you.

Yes. This calculator runs entirely in your browser: the weights and answers you enter are compared on your own device and are never sent to a server, stored, or shared — close the tab and they're gone. There's no account and nothing to delete. What you get back is general educational information based on the plain percentage math and published, commonly cited ranges from the AAP, the Academy of Breastfeeding Medicine, and MedlinePlus; it isn't medical advice, a diagnosis, or a feeding assessment, and it can't replace your pediatrician, a lactation consultant, or the scale at your baby's weigh-in.