Baby Weight Percentile
Track baby weight against percentiles
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Baby Weight Percentile helps you track baby weight against percentiles — free, instant, in your browser. Fill in Age (months) and Weight (kg) on Baby Weight Percentile and read your answer immediately. Baby Weight Percentile displays each intermediate figure as well. That makes Baby Weight Percentile handy for quick estimates anywhere. Nothing is uploaded while Baby Weight Percentile works, and the figures stay put. Looking for free online baby weight percentile calculator or a quick estimate? Baby Weight Percentile covers both. Baby Weight Percentile is designed for real people. Labels stay plain. Open Baby Weight Percentile and enter your numbers.
Baby weight percentile: is my baby on track?
This places your baby's weight on the WHO Child Growth Standards chart, using the same L, M and S reference values the published growth curves are drawn from: the weight is converted to a z-score and then to a percentile, for either sex, from birth to 24 months. A single reading is a snapshot rather than a verdict — growth is judged by the trend across months, and the 3rd to 97th percentile band is where most healthy babies sit.
Being a few per cent either side of an average is ordinary. A single weight is a point on a curve, and the curve is what paediatricians read.
How weekly gain is calculated
Average weekly gain = Total gain ÷ Weeks elapsed
Example: 14 kg gained over 28 completed weeks = 14 ÷ 28 = 0.5 kg per week.
| Total gain | Average weekly gain over 40 weeks |
|---|---|
| 9 kg | 0.225 kg |
| 11 kg | 0.275 kg |
| 12 kg | 0.30 kg |
| 14 kg | 0.35 kg |
| 16 kg | 0.40 kg |
The rate is rarely even: most clinicians expect little gain in the first trimester and faster gain later. Recommended ranges depend on the pre-pregnancy BMI and are set by a midwife or clinician, so treat this page as arithmetic only. Related: BMI calculator and health guides.
How the comparison works
The page holds a reference weight for each age in months and reports the difference, both in kilograms and as a percentage of the expected figure. At six months: expected 7.9 kg, entered 7.5 kg, difference −0.4 kg, or about −5%.
That reference is a population average, not a target. Healthy babies sit across a wide band around it: roughly the 3rd to the 97th percentile covers the normal range, and that band is far wider than ±5%.
Worked example
Six months, 7.5 kg → −5% against a 7.9 kg reference. Still comfortably inside the usual range.
Six months, 6.5 kg → about −18%, which is below the band and worth raising at the next visit.
Six months, 9.5 kg → about +20%, also outside the band, and usually less concerning than the low side but still worth a conversation.
What actually matters: the curve, not the point
- Trend. A baby tracking along the 25th percentile is doing better than one who has fallen from the 75th to the 10th, even though the second baby is heavier.
- Crossing lines. Dropping two percentile bands is the signal clinicians act on, not a single reading below average.
- Feeding and output. Wet nappies, feeding well, alertness and development carry more information than a number on a scale.
- Measurement conditions. A nappy, a full feed or a different scale can move a small baby’s reading by 100 g or more.
What this page cannot do
It cannot assess a baby. It does not know birth weight, gestational age, feeding method, length, head circumference or the shape of the growth curve, and it is not a percentile chart. If a weight is worrying you, the baby’s own growth chart and a clinician’s judgement are the tools that answer the question.
Input notes
Age is entered in months; convert weeks carefully in the first months, because the average moves fast in that period. Weight is in kilograms — 7.5 kg, not 16.5 lb.
Frequently Asked Questions
Is 5% below average a problem?
On its own, no. A few per cent either side of an average is well inside the normal range, which runs from roughly the 3rd to the 97th percentile. What matters is whether the baby is following their own curve.
Why is my baby compared with an average at all?
Because an average is a quick sanity check. It is not a target and not a diagnosis — growth charts use percentiles precisely because a wide spread is normal.
What weight should a six-month-old be?
The page uses about 7.9 kg as the reference for six months. Real babies of that age commonly range from about 6.5 kg to 9.5 kg, and both ends can be entirely healthy.
When should I be concerned?
When the trend changes — a baby dropping across two percentile bands, or losing weight — or when feeding, alertness or output changes. A single reading slightly below average is not that signal.
Does this use the WHO or CDC charts?
It uses a single reference average per age, not a published percentile chart. For a real assessment, use the WHO or CDC growth chart your clinic uses, plotted over several visits.
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