How the growth percentile calculator works
Choose boy or girl, enter your baby’s date of birth and one or more measurements. The calculator works out your baby’s exact age in days, looks up the World Health Organization (WHO) Child Growth Standards for that age and sex, and calculates a percentile and z-score for each measurement. It also shows the median (50th percentile) for your baby’s age, so you can see how the measurement compares with an average baby.
The WHO standards were built from healthy, breastfed babies raised in good conditions in six countries, and describe how babies should grow when their needs are met. They are recommended for children from birth to 2 years in many countries, including by the US CDC.
Understanding percentiles
A percentile shows how a measurement compares with other babies of the same age and sex. If your baby’s weight is on the 25th percentile, about 25 in every 100 babies weigh less and about 75 weigh more. That is not “below average” in any worrying sense — it simply describes your baby’s size.
| Percentile | What it usually means |
|---|---|
| Below the 3rd | Smaller than most babies — worth discussing with a health professional, who will look at the whole picture |
| 3rd–97th | The range where most healthy babies sit |
| Above the 97th | Larger than most babies — also worth mentioning at a check-up |
The trend matters more than one result. Healthy babies tend to follow roughly the same percentile curve over time. Many lose some weight in the first days after birth and regain it by about two weeks, so early percentiles can jump around.
How to measure accurately
- Weight: use the same scale each time, with the baby undressed or in a clean nappy.
- Length: for babies under 2, length is measured lying down, from the top of the head to the heels with legs gently straightened — hard to do at home without help.
- Head circumference: measure around the widest part of the head, just above the eyebrows and ears, with a soft tape.
Measurements taken at home are usually less precise than those taken at a clinic, so small differences are normal. For the most reliable picture, use the measurements in your baby’s health record.
Weight, length and head growth
Weight is the measurement checked most often in the first year. Length shows long-term growth and changes more slowly. Head circumference reflects brain growth and is usually checked at routine reviews. Your baby’s doctor or health visitor looks at all three together, along with feeding, development and family size — tall parents often have long babies.
For babies born early, use their corrected age. To check your baby’s exact age, use the Baby Age Calculator. If you have any concerns about your baby’s growth or feeding, speak to your doctor, health visitor or nurse.