How much weight should my baby gain?
This baby weight gain calculator takes your baby’s birth weight and a later weighing and works out the average gain since birth — in grams a day and per week, or ounces a week if you use pounds. It then places both weights on the CDC weight-for-age chart to show the percentile at birth and now, how far the weight has moved between centile lines, and what your baby would weigh today if they had stayed exactly on their birth centile.
Health visitors and paediatricians look at the pattern of growth rather than one number, so the calculator also quotes the thresholds the UK’s NICE guideline on faltering growth uses for asking for a review. It never diagnoses anything and it does not replace your baby’s red book, clinic chart or check-ups.
How to use the baby weight gain calculator
- Choose boy or girl and the units: kg, or lb and oz.
- Enter the date of birth and the birth weight from the hospital record.
- Enter the date and weight from the latest weighing — ideally on the same clinic scales, without clothes or nappy.
- Read the average gain, the percentiles and the centile change; the monthly chart below shows typical weights for each month.
Baby Weight Gain Calculator features
Gain per day and week
Grams a day, grams a week, or ounces a week, from any two dates in the first two years.
Percentile at birth and now
CDC 2000 LMS method, the same data as the growth percentile calculator.
Centile spaces
Movement between centile lines in the units growth charts use.
Same-centile weight
What your baby would weigh today on their birth centile.
Baby weight chart by month
3rd to 97th percentile weights, birth to 12 months, in kg or lb and oz.
How the calculation works
Average gain = (latest weight − birth weight) ÷ days between them. Percentiles use the LMS method: z = ((weight ÷ M)L − 1) ÷ (L × S), with L, M and S for the exact age from the CDC table. A centile space is the gap between neighbouring lines on UK-WHO charts — two-thirds of a standard deviation — so centile spaces moved = (z at birth − z now) ÷ 0.67.
Worked example: a boy born at 3.4 kg weighs 5.6 kg at 70 days. He gained 2.2 kg, which is 2,200 ÷ 70 = 31 g a day or about 220 g a week. The CDC median boy gains about 28 g a day over the same weeks; this baby moves from the 41st to the 55th percentile — half a centile space up — and on his birth centile he would weigh about 5.34 kg.
What NICE says about early weight loss and faltering growth
- It is common for babies to lose some weight in the first days; this usually stops after 3–4 days, and most are back to birth weight by 3 weeks.
- A loss of more than 10% of birth weight in the early days calls for a clinical assessment of feeding — the newborn weight loss calculator works out the percentage.
- After the early days, NICE suggests concern when weight falls across 1 or more centile spaces (birth weight below the 9th centile), 2 or more (between the 9th and 91st) or 3 or more (above the 91st), or when weight is below the 2nd centile.
- Feeding questions? See the baby feeding chart or the breast milk calculator.
Limits of this calculator
- It uses the CDC 2000 charts. Under 2 years, US, UK and Indian guidance recommends the WHO growth standards (UK-WHO charts in the UK); breastfed babies in particular can track a little differently on CDC charts.
- It compares two points only. Plot every weighing on the child growth percentile calculator to see the curve.
- For premature babies, ask your clinic how to use corrected age.
- Scales, clothes, a full feed or a dirty nappy can change one reading by 100 g or more.
Baby Weight Gain Calculator FAQ
How much weight should a baby gain per week?
It varies with age, size and feeding. The calculator shows how much the CDC median baby gains over the same period, and whether your baby is following their own centile — which matters more than any single figure.
Can I use the baby weight gain calculator based on birth weight only?
You need a second, later weight and its date. The calculator then works out the average daily gain from the birth weight.
Is it normal for my baby to cross percentile lines?
Small movements are common, especially in the first weeks. NICE uses falls of 1–3 centile spaces, depending on birth weight, as thresholds for a review — the calculator shows where your numbers sit.
Why does my clinic’s chart give a different percentile?
Clinics in the UK and India use WHO-based charts for babies; this tool uses the CDC tables, so percentiles can differ by a few points.
Does it work in kg and in pounds?
Yes. Switch between kg and lb & oz; the gain is shown in grams a day or ounces a week.
Is this medical advice?
No. It is information only, as of October 2026. Talk to your health visitor, GP or paediatrician about any concern with feeding or growth.