Baby Weight Percentile Calculator
Calculate your baby’s weight percentile based on UK-WHO growth charts used by the NHS and health visitors across the United Kingdom. This calculator uses RCPCH standards for children aged 0-4 years.
Your Baby’s Results
How to Use This Calculator
- Select your baby’s gender (boys and girls have different growth patterns)
- Choose the age unit and enter your baby’s current age
- Select your preferred weight unit and enter the weight
- Click Calculate to see which percentile your baby falls into
What Are Percentiles?
Percentiles show how your baby’s weight compares to other babies of the same age and gender. The RCPCH-WHO growth charts use the following centile lines:
If your baby is at the 50th percentile, it means half of babies weigh more and half weigh less. A baby at the 75th percentile weighs more than 75% of babies of the same age and gender.
Growth Chart Principles
The UK uses WHO Child Growth Standards combined with UK birth data, developed by the Royal College of Paediatrics and Child Health. These charts reflect optimal growth for healthy, breastfed babies.
- Birth to 2 weeks: Weight loss of up to 10% is normal. Most babies regain birthweight by 3 weeks.
- 0-6 months: Rapid weight gain period. Babies typically gain 140-200g per week.
- 6-12 months: Growth rate slows as babies become more active and start solids.
- 1-4 years: Steady, gradual weight gain as toddlers develop.
When to Seek Advice
Contact your health visitor or GP if:
- Your baby loses more than 10% of their birthweight
- Your baby hasn’t regained birthweight by 3 weeks
- Your baby’s weight crosses down through 2 or more centile lines
- Your baby’s weight is below the 2nd centile
- You have any concerns about feeding or your baby’s health
Factors Affecting Baby Weight
- Genetics: Parents’ size influences baby’s natural growth pattern
- Feeding method: Breastfed and formula-fed babies may gain weight at slightly different rates
- Gestational age: Premature babies often follow adjusted growth curves
- Birth weight: Initial weight affects early growth trajectory
- Activity level: More active babies may have slower weight gain
- Illness: Temporary illness can slow weight gain for 2-3 weeks
Frequently Asked Questions
NHS guidance recommends: no more than monthly until 6 months, no more than every 2 months from 6-12 months, and no more than every 3 months after age 1. Frequent weighing can cause unnecessary worry due to natural daily fluctuations.
Yes, it’s completely normal. Many babies have different percentiles for weight, length, and head circumference. The two measurements are usually fairly similar, but variations are common and not a concern unless there are other issues.
Moving up or down by one centile line is common. Crossing two or more centile lines downwards may need assessment by a health visitor, especially if it happens quickly. Crossing upwards is usually less concerning but should still be monitored.
The WHO growth standards are based on breastfed babies, representing optimal growth. Some exclusively breastfed babies may gain weight more slowly in the first few months compared to formula-fed babies, but this is normal and healthy growth.
The Personal Child Health Record (PCHR), commonly called the red book, is given to all parents at birth. It contains growth charts where health professionals record your baby’s weight, length, and head circumference measurements.
For premature babies (born before 37 weeks), health visitors typically use corrected gestational age until around 2 years. The RCPCH charts include guidance for babies born at 32-36 weeks gestation. Discuss with your health visitor for babies born earlier.
Comparison: Boys vs Girls Growth Patterns
The RCPCH-WHO charts provide separate standards for boys and girls because their growth patterns differ:
- Boys: Typically slightly heavier at birth (average 3.5kg) and gain weight slightly faster in the first year
- Girls: Average birth weight around 3.4kg, with more gradual weight gain patterns
- Length differences: Boys are generally 1-2cm longer at birth and throughout infancy
- Puberty impact: Growth patterns diverge more significantly during adolescent years
Weight Measurement Tips
- Weigh your baby naked or in a clean, dry nappy only
- Use the same scales each time for consistency
- Weigh at the same time of day, ideally before a feed
- Ensure scales are on a flat, hard surface
- Zero/tare the scales before weighing
- Record the weight immediately in your red book
References
- Royal College of Paediatrics and Child Health. UK-WHO Growth Charts 0-4 years. Available at: https://www.rcpch.ac.uk/resources/uk-who-growth-charts-0-4-years
- World Health Organization. WHO Child Growth Standards: Methods and development. Geneva: WHO, 2006.
- NHS. Your baby’s weight and height. Available at: https://www.nhs.uk/baby/babys-development/height-weight-and-reviews/baby-height-and-weight/
- Wright CM, Williams AF, Elliman D, et al. Using the new UK-WHO growth charts. BMJ. 2010;340:c1140.
- National Institute for Health and Care Excellence. Faltering growth: recognition and management of faltering growth in children. NICE guideline NG75, 2017.
- Royal College of Paediatrics and Child Health. UK-WHO growth charts – neonatal and infant close monitoring (NICM). Available at: https://www.rcpch.ac.uk/resources/growth-charts