Find which centile your baby's weight falls on using UK-WHO growth chart references by age and sex
This calculator is for informational purposes only and does not constitute medical advice. It uses simplified UK-WHO growth chart data. Always consult a qualified healthcare professional or health visitor for accurate growth assessment.
Monitoring your baby's weight is one of the most important ways to check that they are growing well and getting enough nutrition. In the UK, the NHS uses the UK-WHO growth charts (found in your baby's Personal Child Health Record, commonly known as the "red book") to track weight, length, and head circumference from birth through to school age. These charts show a range of centile lines that represent the typical distribution of measurements across a large, healthy population. A baby's weight percentile tells you how their weight compares to other babies of the same age and sex. For example, a baby on the 50th centile weighs more than approximately 50% of babies their age and less than the other 50%. Importantly, there is no single "correct" centile -- healthy babies can be found across the entire range. What matters most is that your baby follows a consistent growth pattern along their centile over time, rather than suddenly dropping or rising across centile lines. The UK-WHO growth charts use nine centile lines: 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th, and 99.6th. These are based on the World Health Organisation's Child Growth Standards, which describe how healthy breastfed children grow under optimal conditions. The charts were adopted by the NHS in 2009 and are recommended by the Royal College of Paediatrics and Child Health (RCPCH).
To use the baby weight percentile calculator: 1. Enter your baby's weight in kilograms. If you know their weight in pounds, divide by 2.205 to convert (e.g. 15 lbs = 6.8 kg). Use the most recent weight from a health visitor weigh-in for best accuracy. 2. Enter your baby's age in months. For newborns, use 0. For babies between whole months, round to the nearest month. 3. Select your baby's sex. Boys and girls have different growth patterns, so the correct reference data is used for each. 4. View the results. The calculator shows your baby's approximate weight percentile, the centile band they fall in, a Z-score (the number of standard deviations from the median), and an interpretation of their growth position. 5. Compare with centile reference lines. The chart shows your baby's weight alongside the 25th, 50th, and 75th centile reference values for their age and sex, giving a visual comparison.
This calculator uses simplified UK-WHO growth chart reference data to estimate your baby's weight percentile. Here is how it works: For each age and sex, there is a median (50th centile) weight and a standard deviation (SD) that describes the spread of weights around the median. The calculator stores approximate reference values for key ages (birth, 1, 3, 6, 9, 12, 18, 24, 36, and 48 months) and interpolates between them for ages in between. A Z-score is calculated using the formula: Z = (baby's weight - median weight) / standard deviation. This tells you how many standard deviations your baby's weight is above or below the median for their age and sex. The Z-score is then converted to a percentile using the standard normal cumulative distribution function. For example, a Z-score of 0 corresponds to the 50th percentile (exactly average), a Z-score of +1 corresponds to approximately the 84th percentile, and a Z-score of -1 corresponds to approximately the 16th percentile. The percentile is then mapped to the nearest UK centile band (e.g. "Between 25th and 50th centile") to give a familiar reference point that matches the lines shown in the red book growth charts.
This calculator uses simplified reference data and provides an approximation only. For accurate growth assessment, your health visitor will plot your baby's measurements directly on the UK-WHO growth charts in your red book. The NHS recommends weighing your baby at birth, in the first two weeks, and then no more than once a month up to six months, once every two months from six to twelve months, and once every three months after one year. More frequent weighing can cause unnecessary worry due to normal day-to-day fluctuations. Key resources: NHS Start4Life (www.nhs.uk/start4life) provides guidance on infant feeding and growth. The RCPCH UK-WHO growth charts are available at www.rcpch.ac.uk/resources/uk-who-growth-charts. If you have concerns about your baby's growth, speak to your health visitor or GP.