Baby Weight Gain Calculator
Enter birth weight, today's weight and your baby's age to see how much they have gained per day and per week, how that compares with typical ranges for their age, and, in the first two weeks, how much of the early weight loss has been regained.
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How to use the Baby Weight Gain Calculator
- Choose units and enter the birth weight.
- Enter today's weight and your baby's age in days, weeks or months.
- Optionally add a previous weigh-in and how many days ago it was, to see the recent rate.
- Press Calculate to see the weekly gain and how it compares with typical ranges.
- Share the trend with your pediatrician, who will plot it on a growth chart.
Typical baby weight gain by age
| Age | Grams per week | Ounces per week | About per day |
|---|---|---|---|
| First 3–5 days | Weight loss of up to about 7% is common | ||
| By 10–14 days | Back to birth weight | ||
| 2 weeks – 4 months | 155–240 g | 5.5–8.5 oz | 22–34 g (0.8–1.2 oz) |
| 4–6 months | 92–147 g | 3.25–5.25 oz | 13–21 g (0.5–0.75 oz) |
| 6–12 months | 50–85 g | 1.75–3 oz | 7–12 g (0.25–0.4 oz) |
Ranges are drawn from WHO Child Growth Standards and describe healthy breastfed babies; they are a guide, not a target.
How the calculation works
- Change from birth = current weight − birth weight; percent = change ÷ birth weight × 100.
- First 14 days: the result is the percentage below or above birth weight, compared with the 7% and 10% thresholds.
- After 14 days: average weekly gain = (current − birth weight) ÷ (age in days − 12) × 7, counting from when most babies are back to birth weight.
- Typical range = birth weight plus the low weekly rate from day 14 and the high weekly rate from day 10, for each age band.
- Recent rate (optional) = (today − previous weight) ÷ days between × 7.
Example: a baby born at 7 lb 8 oz (3,402 g) who weighs 9 lb 12 oz (4,423 g) at 6 weeks (42 days) has gained 1,021 g. Over the 30 days since regaining birth weight that is about 238 g (8.4 oz) a week, within the typical range of 4,022–4,499 g for that age.
Tips for accurate weigh-ins
- Use the same scale each time, ideally the one at your pediatrician's office.
- Weigh without clothes and with a clean diaper, at a similar time relative to feeds.
- Look at the trend over 2–4 weeks rather than a single reading.
- Count wet and dirty diapers too: after the first week, 6 or more wet diapers a day is a good sign of enough milk.
Frequently asked questions
How much weight should a baby gain per week?
Typical ranges based on WHO growth data are about 155–240 g (5.5–8.5 oz) a week from birth weight regain to 4 months, 92–147 g (3.25–5.25 oz) a week from 4 to 6 months, and 50–85 g (1.75–3 oz) a week from 6 to 12 months.
How much weight loss is normal for a newborn?
Most newborns lose some weight in the first 3–5 days. A loss of up to about 7% of birth weight is common, 7–10% usually means feeding is checked more closely, and more than 10% needs prompt medical advice.
When should a baby be back to birth weight?
Most babies regain their birth weight by 10 to 14 days of age. If your baby hasn't by two weeks, your pediatrician will usually want to review feeding.
When do babies double their birth weight?
On average babies double their birth weight by about 4–5 months and roughly triple it by their first birthday, though healthy babies vary.
Why does the calculator measure gain from about day 12?
Weekly gain guidelines are counted from the point a baby gets back to birth weight, not from birth, because nearly all babies lose weight first. Measuring from birth would make early weekly gains look too low.
Do breastfed and formula-fed babies gain differently?
In the first months they gain at similar rates; after about 3–4 months breastfed babies often gain a little more slowly. The WHO growth standards are based on breastfed babies.
Is my baby's weight gain a problem?
One reading outside a range isn't a diagnosis: scales, clothing and feeds all change weight. Talk to your pediatrician if gain is consistently low or high, if your baby has fewer wet diapers, or if you are worried.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.