Baby Weight Gain Calculator: Grams Per Day From Two Weigh-Ins

You weighed your baby twice and now you are trying to work out how much that is per day. Enter the date of birth and both weigh-ins above and the calculator gives you the daily gain, plus how it compares to the guide for your baby's age. The arithmetic itself is simple: divide the weight gained by the days that passed. The number is easy to misread, though, so this guide also covers how to read it properly.

The Formula

Item	Detail
Formula	(latest weight − previous weight) ÷ days between the two dates
Example	Sept 1: 5,200g → Sept 15: 5,600g
Result	400g ÷ 14 days = about 29g per day

The part people get wrong is counting the days. September 1 to September 15 is 14 days, not 15. Enter the dates above and the calculator handles it for you.

Typical Gain by Age

Growth naturally slows down over the first year. A six-month-old not gaining like a newborn is not a problem — that is the expected path.

Age	Per Day	Per Month	Milestone
0-3 months	25-30g	About 750-900g	Birth weight regained by about 2 weeks
3-6 months	15-20g	About 450-600g	Birth weight doubles around 4-6 months
6-12 months	10-15g	About 300-450g	Birth weight roughly triples by age one

Three Ways This Number Gets Misread

1. Weighing again after only a couple of days A wet diaper or a recent feed alone can move the reading by more than 100g. Weigh two days apart, find 200g, and the math says 100g a day — but that is measurement conditions, not growth. Weigh at least a week apart for the daily figure to mean anything.

2. Taking a newborn's number at face value Babies lose weight in the first days after birth. Around days 3-5 they shed 5-10% of their birth weight (physiological weight loss), and most are back to birth weight by days 10-14. A zero or negative result in that window is not a broken calculation — it is simply that stage. If more than 10% is lost, or the weight is not back by two weeks, that does need a doctor.

3. Judging on a single number This is the most common mistake. Weight should be read as a trend on the growth chart, not as one reading. Following the same percentile band is usually fine even if the weight stalls for a few weeks, and the signal worth checking is a drop of two or more percentile bands. Our guide on a baby not gaining weight covers that in detail.

💡 This calculator is a reference for gauging whether growth is on track. It is not a diagnosis, and if you are worried, reviewing the growth chart with your pediatrician is the most accurate route.

When to See a Doctor

• The weight is still lower on a repeat weighing
• A gain of 15g a day or less continues for several days before three months
• The percentile drops two or more bands on the growth chart
• Height and head circumference have stalled as well
• Feeding drops noticeably, or wet diapers fall below six a day

If the feeding volume itself is the question, start with the formula calculator to check the amount for your baby's weight.

Tracking Growth With BebeSnap

• Log each weigh-in and the daily gain is calculated for you
• The percentile is plotted on the growth chart so the trend is visible at a glance
• Feeding and diaper logs build up alongside it, making causes easier to spot

Frequently Asked Questions (FAQ)

Q: How do I calculate my baby's daily weight gain?
A: Subtract the previous weight from the latest weight and divide by the number of days between the two dates. If a baby was 5,200g on September 1 and 5,600g on September 15, that is 400g over 14 days, or about 29g a day. Note that September 1 to September 15 is 14 days, not 15.

Q: How many grams a day should a baby gain?
A: It depends on age. From 0-3 months the pace is fastest at 25-30g a day, then 15-20g at 3-6 months and 10-15g at 6-12 months. Per month that is about 750-900g, 450-600g and 300-450g. The drop after six months reflects solids and increased activity, so it is expected.

Q: How often should I weigh my baby?
A: At least a week apart if you want a meaningful daily figure. Diaper state and whether a feed just happened can shift the reading by over 100g, and weighing a few days apart amplifies that error straight into the daily number. Weekly weighing under the same conditions beats daily weighing.

Q: My baby lost weight. Should I see a doctor?
A: Within the first two weeks this may be physiological weight loss. Babies shed fluid in the days after birth and most regain their birth weight by about two weeks. Past that window, if a repeat weighing is still lower or wet diapers fall below six a day, see your pediatrician.

References

• AAP HealthyChildren - Growth Charts
• Nemours KidsHealth - Failure to Thrive
• AboutKidsHealth - Poor Weight Gain in Infants and Young Children
• Boston Children's Hospital - Slow Weight Gain in Infants and Children
• WHO - Child Growth Standards