About the Newborn Weight Loss Calculator
This newborn weight loss calculator works out how much of their birth weight your baby has lost, in grams, ounces and as a percentage, and how much they need to gain to get back to birth weight. Enter weights in grams or in pounds and ounces, plus the baby’s age in days, and the result is checked against the thresholds clinicians commonly use.
Almost every newborn loses weight in the first few days as they pass meconium and shed extra fluid, especially breastfed babies while milk supply builds. Nursing guidance summarised in the medical literature is that loss should not exceed about 7–10% of birth weight and that most babies are back to birth weight by 10 to 14 days. Loss above 7% is a common prompt to check how feeding is going, and loss of more than 10% has been associated with jaundice (hyperbilirubinemia) and dehydration with high sodium levels.
Percentages alone do not tell the whole story: hour-by-hour charts from more than 100,000 exclusively breastfed babies (the Newborn Weight Loss Tool, NEWT) show that babies born by caesarean usually lose more than those born vaginally. Use this as a quick check between visits — your midwife, lactation consultant or pediatrician should interpret the result alongside feeding, diapers and how your baby looks.
With the default inputs, the weight lost since birth is 6.47%. Change any value above to recalculate instantly.
How to use the newborn weight loss calculator
- 1Choose grams or pounds and ounces.
- 2Enter the birth weight from the hospital record.
- 3Enter the most recent weight, ideally measured naked on the same type of scale.
- 4Enter your baby’s age in days.
- 5Read the percent lost and status, and talk to your provider if it is above 7%.
Formula and method
The percentage compares today’s weight with the birth weight, so a 3,400 g baby who now weighs 3,180 g has lost 220 g, or 6.47%. Pounds and ounces are converted to grams first (1 lb = 453.59 g, 1 oz = 28.35 g) so mixed units work. The average loss per day simply divides the percentage by the baby’s age in days; real weight loss is usually fastest in the first 48–72 hours and then levels off.
The status bands follow commonly used clinical thresholds: under 7% is typical, more than 7% is a reason to have breastfeeding or feeding assessed, and 10% or more is considered excessive and associated with jaundice and dehydration. A baby who has not regained birth weight by about 2 weeks should also be reviewed. The 7% and 10% threshold weights show the weight at which each flag would apply.
- Birth weight
- Weight recorded at birth
- Current weight
- Most recent weight, ideally on the same scale, undressed
Worked examples
Day 3, 3,400 g baby now 3,180 g
The baby has lost 3,400 − 3,180 = 220 g, which is 220 ÷ 3,400 = 6.47% of birth weight (about 7.8 oz). That averages 2.16% per day over 3 days and is within the usual range, though close enough to 7% that feeding should be watched.
Day 4, 11% loss
A drop from 3,500 g to 3,115 g is 385 g, or exactly 11% of birth weight. That is above the 10% threshold usually regarded as excessive, so the baby should be seen the same day to check feeding, hydration and jaundice.
7 lb 8 oz baby now 6 lb 13 oz (day 2)
7 lb 8 oz is 120 oz and 6 lb 13 oz is 109 oz, so the baby has lost 11 oz (about 312 g), or 11 ÷ 120 = 9.17%. That is above 7% on day 2, a signal to get feeding checked by a nurse, midwife or lactation consultant.
Frequently asked questions
How much weight is normal for a newborn to lose?+
Most newborns lose some weight in the first days of life. Loss of up to about 7% of birth weight is generally considered typical, more than 7% is a reason to check feeding, and 10% or more is considered excessive. Breastfed babies and babies born by caesarean tend to lose a little more.
When should a baby be back to birth weight?+
Most babies regain their birth weight by 10 to 14 days of age. Not regaining birth weight by 10–14 days is listed among the signs of inadequate intake, so if your baby is not back to birth weight by about 2 weeks your pediatrician will want to review feeding.
Why do newborns lose weight after birth?+
Babies are born with extra fluid that they shed in the first days, they pass meconium, and they take in only small volumes while colostrum transitions to mature milk around days 2–5. Weight usually starts climbing once milk volume increases.
Is 10% weight loss in a newborn dangerous?+
Weight loss of 10% or more is a warning sign associated with dehydration, high sodium levels and jaundice. It does not always mean something is seriously wrong, but the baby should be assessed promptly and a feeding plan — sometimes including supplementation — agreed with a clinician.
What is the NEWT newborn weight loss tool?+
NEWT (newbornweight.org) uses hour-by-hour weight-loss percentiles from more than 100,000 exclusively breastfed newborns in California, separately for vaginal and caesarean births. Clinicians use it to spot babies on a steeper-than-usual trajectory earlier than a fixed percentage would.
Can different scales change the result?+
Yes. A few dozen grams of difference between hospital and home or clinic scales, or weighing with a diaper or clothes on, can shift the percentage by about 1%. For tracking, weigh undressed on the same scale each time.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.
Sources
- Flaherman VJ et al. Early weight loss nomograms for exclusively breastfed newborns (Pediatrics 2015)
- Guidelines for primary pediatric care professionals to help patients establish and protect milk supply (PMC)
- Recent advancements and best practices in supporting newborn feeding: a narrative review (PMC)
- ABM Clinical Protocol #3: Supplementary feedings in the healthy term breastfed neonate (2017)