Calculate weight-for-length centile
Why length, not age
Weight-for-length answers a different question from weight-for-age: is this child thin for their body size, regardless of how old they are? A tall 18-month-old and a short 18-month-old face different expectations, and comparing each against children of the same length removes the confusion that age introduces. That is why the WHO chose weight-for-length, not weight-for-age, as the defining indicator for wasting in children under two.
Wasting is the acute face of undernutrition: the body consuming its own tissues because intake has fallen short or illness has raised needs. Unlike stunting, which records months of deprivation in shortened bones, wasting can develop in weeks and can kill in weeks, which is why its cutoffs trigger urgent action. A child below -3 z-scores for weight-for-length has severe acute malnutrition until proven otherwise.
The length range of the reference, 45 to 110 cm, covers essentially all children from birth to two years. Beyond that age the indicator switches to BMI-for-age, because weight scales differently against standing height in older children. Using weight-for-length past age two, or against standing height, gives wrong answers.
Measuring for this indicator
Both measurements must be right, because the indicator divides one by the other and errors compound. Length is measured lying down on a length board: head against the fixed end, legs fully extended, feet at right angles, the sliding footboard pressed firmly. Weight is measured undressed on calibrated electronic scales to the nearest 10 grams. In field conditions, even experienced teams re-measure a sample to check digit preference and technique drift.
Oedema is the great confounder. A child with nutritional oedema can have a reassuring weight-for-length while being severely malnourished, because the scale weighs water as if it were tissue. Always press the top of both feet for ten seconds: pitting oedema means severe acute malnutrition whatever the centile says. Dehydration pulls the other way, understating weight in a child who is actually better nourished than the numbers suggest.
Reading the result
The WHO defines wasting as weight-for-length below -2 z-scores (about the 2.3rd centile) and severe wasting below -3 (about the 0.1st centile). Between -2 and +2 is normal; above +2 is high weight-for-length, which in infancy usually prompts feeding review rather than alarm.
As with every growth indicator, the trend amplifies the meaning. A child drifting down across the weight-for-length lines over two or three visits is faltering even before crossing -2, and deserves the same feeding and illness review as a child who has crossed it. Conversely, a child recovering from illness often shows rapid weight-for-length catch-up while length lags behind; that pattern is expected and reassuring.
Worked examples
Example 1. A boy with recumbent length 60.0 cm weighing 6.0 kg. The WHO median for 60 cm boys is about 6.0 kg: z-score near 0, 50th centile. Well proportioned.
Example 2. A girl with recumbent length 75.0 cm weighing 7.6 kg. The WHO median for 75 cm girls is about 9.2 kg; the z-score works out at -2.1, around the 1.6th centile: wasting by WHO definition. Assess feeding and recent illness promptly, and check for oedema.
Example 3. A boy with recumbent length 52.0 cm weighing 3.8 kg. The WHO median for 52 cm boys is about 3.8 kg: z-score near 0, about the 50th centile. Normal weight for length.
What to do when wasting is found
Below -2 z-scores, take a feeding history (what, how much, how often, breastfeeding status, difficulties), review recent and recurrent illness, examine for oedema, pallor and signs of chronic disease, and check immunisations and deworming status. Most programmes treat uncomplicated wasting between -3 and -2 with counselling and follow-up, and severe wasting or any complication with therapeutic feeding under clinical supervision.
Re-measure in one to two weeks, not months: wasting changes fast in both directions. And remember the appetite test used in malnutrition programmes: a child with severe wasting who cannot eat needs inpatient care whatever the numbers. The centile finds the children; clinical judgement manages them.
Prevention deserves equal emphasis. Exclusive breastfeeding for six months, timely and adequate complementary feeding, prompt treatment of diarrhoea and respiratory infections, and up-to-date immunisations each protect against the episodes of weight loss that accumulate into wasting. Growth monitoring only works when the measurements lead to action, so every low centile should leave the clinic with a plan and a date for re-measurement.
Key takeaways
- Weight-for-length is indexed by recumbent length, not age, and applies from birth to 2 years.
- Wasting is below -2 z-scores; severe wasting below -3 needs urgent assessment.
- Always check for oedema, which can hide severe malnutrition behind a normal weight.
- A falling trend across centile lines matters even before any cutoff is crossed.
- Measure length lying down and weight undressed; technique errors compound in this indicator.
Frequently asked questions
Why is age not asked for in this calculator?
Because weight-for-length compares the child's weight with the weight expected for their body length, and the WHO reference tables are indexed by length from 45 to 110 cm. Two children of the same age but different lengths correctly get different expectations. Age-based assessment is what weight-for-age does; this indicator deliberately removes age from the picture.
Can I use standing height instead of lying length?
No. The reference is built on recumbent length, and standing height reads about 0.7 cm less. More importantly, this indicator is only valid under 2 years, when all children are measured lying down. From age 2, use BMI-for-age instead.
My baby is long and lean. Is that wasting?
Not necessarily. Long, lean babies can sit at low weight-for-length centiles while thriving, especially with tall parents. The questions are whether the centile is stable, whether feeding is good, and whether development is normal. A stable low centile in a thriving baby is a different situation from a falling one.
What is the difference between wasting and stunting?
Wasting is low weight-for-length: thinness reflecting recent acute undernutrition or illness, which can develop and reverse in weeks. Stunting is low height-for-age: shortness reflecting chronic deprivation over months or years. A child can have either or both, and they are assessed with different indicators because they mean different things and need different responses.
The result says severe wasting but my child seems well. Could it be wrong?
Re-measure first: length boards and scales both produce errors, and a 1 cm length error shifts the z-score substantially. If the re-measurement confirms it, do not dismiss it because the child looks well to you; early wasting can be clinically subtle. Arrange prompt clinical assessment as the result advises.