NHS UK-WHO Baby Growth Standards · Free · No Sign-Up
Baby Weight Percentile Calculator UK
Track your baby's growth using official NHS UK-WHO baby growth chart standards. Calculate weight and length percentiles for boys and girls aged 0–24 months — instantly and for free.
0–24
Months Covered
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UK-WHO Standards
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Baby Weight Percentile Calculator UK — How It Works
Our baby weight percentile calculator UK uses the NHS UK-WHO growth chart standards to calculate where your baby's weight and length sit compared to other babies of the same age and sex in the United Kingdom. The result is expressed as a percentile (or centile) — a number from 0 to 100 that tells you what percentage of babies weigh less than your baby.
For example, a baby on the 75th weight centile is heavier than 75% of babies their age. A baby on the 25th centile weighs more than 25% of babies their age. Crucially, any centile between the 0.4th and 99.6th is within the normal healthy range on NHS growth charts.
This baby growth percentile calculator is designed to give parents a reliable, evidence-based reference between scheduled health visitor appointments. It mirrors the same calculations used on the NHS Personal Child Health Record (Red Book) growth pages.
NHS UK-WHO Baby Growth Chart Standard
From birth to 4 years, the NHS uses the UK-WHO hybrid growth standard, adopted in May 2009 following joint recommendations from SACN and RCPCH. This standard combines:
WHO 2006 Child Growth Standards (based on healthy breastfed infants across six countries under optimal conditions)
UK1990 (UK90) birth reference data for weight and length at birth
The result is an evidence-based reference that represents how babies should grow under ideal conditions, rather than simply how they do grow in a mixed-feeding, mixed-socioeconomic UK population. This is why the NHS moved away from older UK growth charts — the new standard is a true growth standard, not merely a reference.
👶 Tip for parents: For babies born before 37 weeks, always use corrected age (chronological age minus weeks premature) when using this baby weight centile calculator. Continue using corrected age until at least 2 years of age for very preterm babies.
How to Measure Your Baby Correctly
Accurate measurements are the foundation of meaningful centile results from the NHS baby percentile calculator:
Weight: Weigh your baby undressed (without nappy) using calibrated baby scales. Use the same scales for consecutive measurements to ensure consistency.
Length (0–2 years): Measure lying flat using a firm, flat surface — a neonatometer or measuring mat. One person holds the baby's head in position (Frankfort plane) while another extends the legs fully and reads the measurement at the heel. Do not use a tape measure alone as results are inaccurate.
Head circumference: Measured at the widest point above the eyebrows. Plotted on separate NHS head circumference centile charts (not included in this calculator).
Average Baby Weight and Length by Age — NHS Reference
Age
Avg Weight — Boys
Avg Weight — Girls
Avg Length — Boys
Avg Length — Girls
Birth
3.35 kg
3.23 kg
49.9 cm
49.2 cm
1 month
4.47 kg
4.19 kg
54.7 cm
53.7 cm
3 months
6.40 kg
5.85 kg
61.4 cm
59.8 cm
6 months
8.19 kg
7.43 kg
67.6 cm
65.7 cm
9 months
9.41 kg
8.56 kg
71.9 cm
70.1 cm
12 months
10.46 kg
9.53 kg
75.7 cm
74.0 cm
18 months
12.00 kg
11.10 kg
82.3 cm
80.7 cm
24 months
13.18 kg
12.37 kg
87.8 cm
86.4 cm
50th centile (median) values from UK-WHO reference. Your baby does not need to match these averages — all centiles between 0.4th and 99.6th are healthy.
How to Interpret Your Baby's Growth Percentile
Once you have calculated your baby's weight and length centile using this baby growth chart calculator, here is how to make sense of the results:
Following the Centile Line
The most important thing about a baby's centile is consistency over time, not the absolute number. A healthy baby who is born on the 10th centile and stays on the 10th centile is growing perfectly normally. What concerns health professionals is when a baby's centile drops across two or more centile lines between measurements — this is called growth faltering and warrants further assessment.
Weight Percentile vs Length Percentile
When using our baby weight percentile calculator UK, pay attention to the relationship between weight and length centiles, not just weight alone:
If weight centile is significantly higher than length centile, your baby may be gaining weight faster than expected for their size
If weight centile is significantly lower than length centile, this may suggest inadequate weight gain relative to growth
If both centiles are similar, this indicates proportionate growth — the ideal scenario
When to Contact Your Health Visitor or GP
This nhs baby percentile calculator is a screening tool only. Always consult your health visitor or GP if:
Your baby's weight centile drops across two or more centile lines between measurements
Your baby's weight falls below the 0.4th centile at any age
Your newborn has not regained birth weight by 3 weeks of age
You are concerned about feeding, wet nappy output, or energy levels
Your baby's weight gain slows or stops entirely between visits
NHS Guidance: Don't weigh your baby too frequently. The NHS recommends monthly weighing up to 6 months, every 2 months from 6–12 months, and every 3 months after that. Over-frequent weighing can cause unnecessary worry as day-to-day weight fluctuations are completely normal.
The Role of Feeding in Baby Growth
Feeding method significantly influences early growth patterns on a baby growth percentile calculator:
Breastfed babies typically gain weight quickly in the first 3–4 months, then grow more slowly. The UK-WHO growth standard was derived from breastfed infants, so it perfectly represents expected growth for breastfed babies.
Formula-fed babies may grow along a slightly different trajectory, particularly after 3–4 months, and may appear to be above the 50th centile on UK-WHO charts. This is not necessarily concerning.
Mixed-fed babies typically show patterns between the two groups.
Normal Birth Weight Loss
A widely misunderstood aspect of early baby growth is the normal weight loss that occurs after birth. Nearly all newborns lose weight in the first 3–5 days as they lose excess fluid and feeding becomes established:
A weight loss of up to 7% of birth weight is entirely normal
Losses of 7–10% should be monitored closely by a midwife
Losses of more than 10% require immediate midwifery assessment
Most babies regain their birth weight by 10–14 days
When using this baby weight centile calculator for newborns in the first 2 weeks, bear in mind that temporary weight below birth weight is normal and does not necessarily indicate a low centile.
Frequently Asked Questions — Baby Weight Percentile Calculator UK
A baby weight percentile shows how your baby's weight compares to other babies of the same age and sex in the UK. For example, a baby on the 75th weight centile is heavier than 75% of babies their age. Any centile between the 0.4th and 99.6th is within the normal range. What matters most is consistent tracking along a centile line over time.
NHS baby growth charts are based on the UK-WHO hybrid standard, adopted in May 2009. It combines WHO 2006 Child Growth Standards (from healthy breastfed babies across six countries) with UK birth data. This makes it a growth standard reflecting optimal growth, not just an average of how UK babies grow in practice.
The NHS recommends: weighing at birth, around day 5 and 10–14; then no more than once monthly up to 6 months; every 2 months from 6–12 months; and every 3 months after that. Over-frequent weighing can cause unnecessary anxiety as normal weight gain is not perfectly smooth day-to-day. Use this baby growth percentile calculator between appointments for reassurance.
Yes. Most newborns lose up to 7–10% of birth weight in the first few days as they lose excess fluid. This is entirely normal. Most babies regain birth weight by 10–14 days. Losses over 10% require midwifery assessment. Do not be alarmed if a temporary weight dip shows a low centile in the first two weeks.
Being on the 9th centile means your baby weighs more than approximately 9% of babies their age — in the lower quarter of the normal range. This is entirely healthy on NHS growth charts, especially if parents are small-framed. A concern arises only if the centile drops significantly between measurements, crossing two or more centile lines downward.
Length is measured lying flat (supine) and is used for babies and toddlers under 2 years. Height is measured standing, used from age 2 onwards. Babies measured standing are typically 0.5–1.0 cm shorter than measured lying. Always use lying length when entering measurements in this baby weight percentile calculator UK for accurate results.
Consult your health visitor or GP if: the weight centile drops across two or more lines between measurements; baby falls below the 0.4th centile; baby has not regained birth weight by 3 weeks; you are concerned about feeding or wet nappy output. This baby growth chart calculator is a guide only and does not replace professional clinical assessment.
Yes, but you must use corrected age. Subtract the number of weeks premature from your baby's actual age to find corrected age. For example, a 3-month-old born 6 weeks early has a corrected age of about 6 weeks. Enter corrected age in this baby weight centile calculator until your baby reaches at least 2 years corrected age.