Baby Growth Calculator

Every time your baby is weighed, your health visitor plots the result on the growth chart in their red book. Our Baby Growth Calculator shows where your baby's weight and length sit on the same kind of chart.

How to use the calculator (Baby Growth Calculator)

1. Choose girl or boy.
2. Enter your baby's age in weeks or months.
3. Enter their weight, in kilograms or pounds and ounces.
4. Optionally, enter their length.

You'll see the weight-for-age centile, which centile lines it falls between, and, if you enter the length, the length-for-age and weight-for-length centiles.

Why babies don't use BMI

BMI isn't used for babies under 2. Instead, health professionals look at weight, length and head circumference over time. The calculator shows a BMI figure for reference, but the centiles are what matter.

Understanding centiles

The UK-WHO growth charts have 9 centile lines: the 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th and 99.6th. A baby on the 25th centile for weight is heavier than 25 out of 100 babies the same age and lighter than 75.

There's no "best" centile. Healthy babies can be on any line. What matters is that your baby grows steadily, roughly following the same line.

A worked example

With the calculator's default settings, a 6-month-old girl weighing 7.3kg and measuring 65.7cm is on the 50th centile for weight and close to the 50th for length.

When to speak to your health visitor

• Your baby's weight drops across two or more centile spaces.
• Your baby isn't back to their birth weight by 3 weeks.
• You're worried about feeding, wet or dirty nappies, or how alert your baby is.

Premature babies

If your baby was born before 37 weeks, use their corrected age: their actual age minus the number of weeks they were born early. This is usually done until age 1, or age 2 for babies born before 32 weeks.