Calculate weight-for-age centile
What weight-for-age tells you, and what it cannot
Weight-for-age is the oldest and simplest growth indicator: how heavy is this child compared with healthy children of the same age and sex? A low weight-for-age, called underweight, is the composite result of everything that can slow growth, which is both its strength and its weakness. It will catch a problem, but it cannot tell you what kind of problem it caught.
A child can be light for their age for two quite different reasons. They may be stunted: short for their age because of long-standing undernutrition or chronic disease, but normally proportioned. Or they may be wasted: thin for their length because of recent acute undernutrition or illness. Weight-for-age blends the two together. That is why the WHO insists it should never be the only indicator used: length-for-age separates the stunted, weight-for-length separates the wasted, and weight-for-age is the first-pass screen that says whether to look closer.
In practice this means a low weight-for-age centile is the start of an assessment, not its conclusion. The next steps are always to measure length, compute the other two indicators, review feeding and illness history, and look at the trend. A child tracking steadily on the 5th centile with a normal examination is a very different case from a child falling from the 50th to the 5th.
The WHO standards behind the numbers
The reference used here is the WHO Child Growth Standards, published in 2006 from the Multicentre Growth Reference Study. Unlike older references that described how children did grow, the WHO standards describe how children should grow: the study enrolled only breastfed children of non-smoking mothers living in environments without economic or environmental constraints on growth, across Brazil, Ghana, India, Norway, Oman and the United States.
For each completed month from 0 to 60, and for each sex, the standard publishes LMS parameters: L for skew, M for the median weight, S for variability. The z-score is ((weight / M)^L - 1) / (L × S), and the centile is the corresponding point of the normal distribution. The calculator matches the child's age to the nearest completed month, exactly as the WHO tables are intended to be used.
Reading the result
The WHO defines underweight as weight-for-age below -2 z-scores (about the 2.3rd centile) and severe underweight below -3 z-scores (about the 0.1st centile). Between -2 and +2 is the normal range; above +2 is high weight-for-age, which in young children usually prompts a review of feeding rather than alarm.
Remember that a single centile is a snapshot. Growth faltering is defined by change: crossing downwards across centile lines, or a z-score falling by a meaningful amount between visits. A child born on the 40th centile who is on the 15th at six months needs assessment even though the 15th is technically inside the normal range.
Worked examples
Example 1. A 12-month-old boy weighing 9.65 kg. The WHO median for 12-month boys is 9.65 kg, so the z-score is 0.00 and the centile is the 50th: perfectly average, reassuring.
Example 2. A 24-month-old girl weighing 9.5 kg. The WHO median for 24-month girls is about 11.5 kg; the z-score works out at -2.16, around the 1.6th centile: underweight by WHO definition. This needs assessment of feeding, recent illness and the growth trend, plus a length measurement to separate wasting from stunting.
Example 3. A newborn girl weighing 3.23 kg. The WHO median at birth for girls is 3.23 kg: z-score 0, 50th centile. Birth weight centiles are also used, with different cutoffs, to identify small-for-gestational-age babies, but that is a separate assessment against gestational-age charts.
When to worry, and what to do next
Worry more when the centile is falling than when it is merely low, when feeding is poor, when there are recurrent infections or chronic symptoms such as persistent diarrhoea, and when the child looks wasted or has oedema. Any child below -3 z-scores, or with visible wasting or nutritional oedema, needs urgent assessment for severe acute malnutrition.
The next measurements are length and mid-upper-arm circumference, then a feeding history: what, how much, how often, and any difficulties. Most growth faltering in the first two years traces back to feeding problems, recurrent infection or an unrecognised chronic condition, and most of it is reversible when the cause is found early. Plot every measurement on the chart; the lines tell the story that single numbers cannot.
Key takeaways
- Weight-for-age is a first-pass screen; it cannot distinguish stunting from wasting on its own.
- Underweight is below -2 z-scores (about the 2.3rd centile); severe underweight is below -3.
- The WHO 2006 standards describe optimal growth in breastfed children from six countries.
- A falling centile matters more than a low but stable one; always read the trend.
- A low result should trigger length measurement, feeding review and timely re-weighing.
Frequently asked questions
What is the difference between underweight, stunting and wasting?
Underweight is low weight-for-age and is a composite indicator. Stunting is low length-for-age, reflecting chronic undernutrition or long-standing disease. Wasting is low weight-for-length, reflecting recent acute undernutrition or illness. A child can be stunted but not wasted, wasted but not stunted, or both. Weight-for-age alone cannot separate them, which is why guidelines require all three indicators.
My baby dropped from the 60th to the 30th centile. Should I worry?
A downward crossing of centile lines deserves review even when the new centile is inside the normal range, because it represents a real change in growth velocity. Common benign explanations include a change in feeding pattern or measurement error, but persistent faltering needs assessment of feeding, illness and development. Re-measure carefully first, then discuss the trend with your clinician.
Why does the calculator only go to 5 years?
The WHO Child Growth Standards cover birth to 5 years. From 5 to 19 the WHO 2007 reference applies, which uses BMI-for-age rather than weight-for-age as its main indicator. This calculator implements the 0 to 5 standards; use the BMI percentile calculator for older children.
Does breastfeeding affect the centiles?
The WHO standards were built on breastfed infants as the normative model, so breastfed babies track them well. Formula-fed babies tend to gain weight faster after the first months and may sit slightly higher on the weight charts; that difference is expected and is one reason the standards were created.
How accurate must the weight be?
Weigh undressed on calibrated electronic scales, to the nearest 10 grams for infants. A 200 gram error in a 6 kg infant shifts the z-score by about 0.3, which can move a borderline child across a cutoff. Always re-weigh before acting on an unexpected result.