What does the 50th percentile mean?

It is the middle. A baby on the 50th percentile for weight is heavier than half of the healthy babies of the same age and sex in the WHO study, and lighter than the other half. It is not a target or a pass mark.

Is a low or high percentile a problem?

Not on its own. Healthy children sit on every line of the chart, often close to where their parents were. Doctors look more closely below the 2nd or above the 98th percentile, or when a child moves across two or more lines between measurements.

Why did my baby's percentile change?

Small changes are normal and often come from measuring, especially length. In the first days most babies lose some weight and regain their birth weight by about 2 weeks, and many settle onto their own line during the first months. A steady fall or rise over several measurements is worth discussing at a check-up.

Should I use corrected age for a premature baby?

Yes. For a baby born before 37 weeks, use the age counted from the due date until about the second birthday. Using the real age would make a premature baby look small for no reason.

Why is this different from the chart in my child's health record?

Many countries print their own national charts, and some measure length and weigh babies in slightly different ways. The percentile can differ by a few points between charts. Follow the one your doctor or health visitor uses, and use this tool as a second view.

Baby growth percentile calculator (WHO, 0 to 5)

Where the numbers come from

The calculator compares your child with the WHO Child Growth Standards, published by the World Health Organization in 2006. Between 1997 and 2003, the WHO followed about 8,500 healthy children in Brazil, Ghana, India, Norway, Oman and the United States. They were breastfed, their mothers did not smoke, and they had good food and care. The result is a picture of how children grow when nothing holds them back, which is why many countries use these charts for babies and toddlers.

The standards are separate for boys and girls and run from birth to 5 years. The calculator covers the three measurements most often written in a child's health record: weight, length or height, and head circumference.

How the percentile is calculated

For every age, the WHO gives three numbers for each measurement, called L, M and S. M is the median, the middle value. S describes how spread out children are around it. L corrects for the fact that weight is not spread evenly: a few children are much heavier than average, fewer are much lighter.

Your child's measurement X becomes a z-score with the formula z = ((X / M)^L − 1) / (L × S). The z-score says how many standard deviations the measurement is above or below the median. The calculator then turns it into a percentile using the normal distribution: z = 0 is the 50th percentile, z = 1 is about the 84th, z = 2 is about the 98th.

The WHO publishes these numbers by day, week and month of age. The calculator uses these tables and fills in the days in between, which gives the same answer as the WHO's daily tables to within a fraction of a percentile point. For very high or very low weights, beyond 3 standard deviations, it follows the WHO's own adjustment so that the extremes are not understated.

A worked example

A boy aged exactly 6 months weighs 8.5 kg. For boys of that age, the WHO gives L = 0.1257, M = 7.934 kg and S = 0.10958.

8.5 divided by 7.934 is 1.0713.
Raised to the power 0.1257, that is 1.0087. Minus 1 leaves 0.0087.
Divided by 0.1257 × 0.10958 (which is 0.0138), the z-score is about 0.63.

A z-score of 0.63 is the 74th percentile: he is heavier than about 74 out of 100 boys of the same age. Most boys of 6 months, those between 2 standard deviations below and above the median, weigh between about 6.4 and 9.8 kg.

Length under 2, height from 2

Babies are measured lying down, as length, and older children standing up, as height. A child is a little shorter standing, because the spine settles, so the WHO uses length up to the second birthday and height after it, with a difference of about 0.7 cm. The calculator switches tables at 2 years. If your toddler of 20 months was measured standing, the length percentile will look slightly low.

Head circumference is measured with a soft tape around the widest part of the head, just above the eyebrows and ears.

What the percentile tells you

A percentile is a position, not a score. The 25th percentile is as healthy as the 75th. Small parents often have small babies, and a baby who is long and lean can be on a high line for length and a lower one for weight.

What doctors watch is the pattern over time. Most children roughly follow one line, with some wobbling in the first months. Signs worth a closer look are a measurement below the 2nd or above the 98th percentile, or a child who moves across two or more of the main lines on the chart between check-ups. Weight and length that move together are usually less worrying than weight dropping while length keeps going.

Premature babies

The WHO standards describe babies born at term. If your baby was born before 37 weeks, use their corrected age, counted from the due date, until about age 2. The baby age calculator works it out for you. With the real age, a premature baby would be compared with older babies and look smaller than they are.

Limits of this calculator

This tool reads one measurement against the WHO tables. It cannot tell you why a child is on a certain line, and a single number can be off: a nappy on the scale, a wriggly baby on the measuring mat or a tape in the wrong place can move the result by several percentile points. Measure in the same way each time, ideally with the same scale.

Your child's health record may use a national chart rather than the WHO one, so the numbers can differ a little. If a result worries you, or a child seems to be moving away from their line, bring the measurements to your doctor, health visitor or child health clinic. They will look at the whole curve, at how your child is eating and developing, and at the family's heights.