Newborn Weight Loss & Regain Calculator

Birth weight

Date of birth

Current weight

Date of this weigh-in

Leave blank to use today's date.

Compared to birth weight

days old

average g/day change

Most newborns lose some weight in the first few days after birth, then regain it by around 10–14 days old and go on to gain roughly 20–30g (about 0.7–1 oz) a day for the first few months. This is a general estimate, not a growth chart or diagnosis — your pediatrician tracks your baby's own pattern at every visit, which matters more than any single number here.

How this is calculated

The calculator takes the straightforward percent difference between your baby's current weight and their birth weight, and divides the total weight change by the number of days between the two weigh-ins to get an average grams-per-day rate. It's simple arithmetic, not a growth-chart lookup — it tells you where your baby sits relative to their own starting point, not relative to other babies.

Almost all newborns lose some weight in the first few days, mostly from fluid shifts and the time it takes feeding to fully establish, whether breastfeeding or formula feeding. A loss of up to about 7% of birth weight is generally considered typical, with many babies regaining their birth weight by 10–14 days old and then gaining steadily from there.

Common edge cases and things to know

Breastfed babies often lose a somewhat higher percentage in the first few days than formula-fed babies, since milk supply typically takes a few days to fully come in — this is usually normal and expected, not a sign of a problem on its own. Babies born by C-section, and babies whose mothers received IV fluids during labor, can also show a larger initial drop, partly reflecting extra fluid at birth rather than true tissue loss.

Preterm babies and babies with a low birth weight follow different, less predictable patterns and are typically monitored more closely by a neonatal team using their own reference ranges. Whatever the number here shows, a weight loss around or above 10% of birth weight, feeding difficulties, or a pediatrician's own concern always warrant a prompt follow-up — this tool is a general guide, not a diagnostic one.

The average gain rate this calculator reports can also look uneven in the very first days, since most of that window is still the initial weight-loss period rather than active gain — the g/day figure only becomes a meaningful "gain rate" once your baby has passed their lowest point and turned the corner toward regaining weight, typically somewhere around days 5 to 10. Weigh-ins on different scales, at different times of day, or with or without a diaper can also introduce small differences that matter less than the overall trend.

Sources

CDC · NICHD · Mayo Clinic.