Baby · Growth

Baby Growth Percentile Calculator

What weight, length, and head-circumference percentile is your baby on? WHO 2006 chart, 0-24 months. Plus how to read it properly — the trend matters more than any single number.

Last reviewed 28 May 2026

WHO growth percentile (0–24 months)

Where is my baby on the growth chart?

Sex

Enter your baby’s age in months (0–24), then fill in any of the measurements to see WHO percentiles.

What percentile is my baby on?

Enter your baby’s age in months (0-24), sex, and whichever measurements you have. The calculator returns the WHO percentile for each — plus a plain-language interpretation band.

What does a percentile actually mean — in plain English?

A percentile compares your baby to a group of 100 healthy babies the same age and sex. 30th percentile means about 30 of those 100 weigh the same or less, and 70 weigh more. 50th = exact median. Below 3rd or above 97th = outside the typical range (only 5% of healthy babies fall here). Anywhere between the 3rd and 97th is statistically normal. Percentiles are not grades. A baby on the 20th is exactly as healthy as a baby on the 80th if they have grown consistently along that line.

Should I worry if my baby drops percentiles?

A single point on the chart almost never tells the story. What matters is the trend across visits. Two patterns warrant a clinical conversation:

  • Crossing two or more major percentile lines downwards over a few months (e.g. 50th → below 15th).
  • Consistently below 3rd or above 97th over multiple visits.

A child who has always tracked at the 10th percentile is almost certainly healthy as long as they keep tracking there. Conversely, a 50th-percentile baby who drops fast is a different conversation.

What is faltering growth (failure to thrive)?

When growth doesn’t keep up with the expected curve. UK NICE and RCPCH define it as: a fall across one or more major centile lines (depending on birth-weight centile), or weight below the 2nd centile, or persistent low-centile tracking. About 5% of children meet criteria at some point. Around 80% have an organic cause — feeding difficulty, reflux, cow’s milk protein allergy, recurrent illness, malabsorption, congenital condition — and 20% are non-organic (feeding environment, parent-child interaction, social factors). The first response is a careful feeding history and a 6-week reassessment, not a panic.

Different scenarios — how the numbers actually play out

Scenario 1: 6-month-old breastfed boy, weight at 20th, length 25th, head 30th

All consistent with each other, well within normal range. If he’s been tracking these centiles since birth, this is a healthy small baby. Reassuring picture.

Scenario 2: 4-month-old, started at 50th, now at 15th

One major centile line crossed. Common around 4-6 months in breastfed babies (WHO chart largely accounts for this). Confirm feeding is going well, plenty of wet nappies, behaviourally well. Reassess in 4-6 weeks rather than panic. Cross two more lines or new symptoms → GP.

Scenario 3: 8-month-old, weight 99th, length 70th

Big baby for length. Weight-for-LENGTH is a better measure of relative fatness under 2 — that’s what your paediatrician will look at. Confirm growth has been steady (not rapidly accelerated), feeding appropriate, and developmental milestones on track.

Scenario 4: 12-week-old, weight 2nd centile, struggling to feed, reflux history

Faltering growth picture with a likely cause. GP / paediatric review. Feeding plan from lactation consultant, possible cow’s milk protein allergy workup, reflux assessment (see /calculators/baby-reflux), weekly weighing for a period.

Scenario 5: 2-year-old preterm baby born at 30 weeks

Use corrected age until age 2: at 2 years chronological, that’s 22.5 months corrected. Plot at 22.5 months on the WHO chart. After age 2, no adjustment. Very preterm babies may use the Fenton chart (see /calculators/fenton-growth).

How does premature birth affect percentiles?

For babies born before 37 weeks: adjust the chronological age for prematurity until age 2. Born at 32 weeks (8 weeks early)? At 6 months chronological age, plot them at 4 months corrected age on the chart. Standard UK and US practice. After age 2, drop the correction. For very preterm babies (under 32 weeks), the Fenton 2013 preterm chart is more appropriate until term-equivalent age.

What measurements matter at each age?

  • Birth to 2 weeks: birth weight, weight loss percentile (NEWT), regain by day 10-14.
  • 2-12 weeks: weight at each immunisation visit + head circumference.
  • 3-6 months: weight + length + head circumference at well-baby visits.
  • 6-12 months: weight + length + head; weight-for-length matters.
  • 12-24 months: weight + length (or height if can stand) + head; BMI from age 2.

How is your baby measured at health checks?

  • Weight: calibrated digital baby scales, nappy off (or clean dry nappy with tare). UK red book / US Bright Futures growth chart.
  • Length: baby flat on a length board, head touching one rigid end, legs straight, heels touching the sliding end. Needs two people. Approximate at home.
  • Head circumference: soft tape measure around widest part — above eyebrows and the bumpiest part at the back.

Care guidance — supporting healthy growth

  • Feed on demand in the first 6 months. Don’t worry about a strict schedule.
  • Breastmilk or appropriate formula as the main calorie source through 6 months.
  • Start solids around 6 months with iron-rich foods early (red meat, lentils, fortified cereal). See /calculators/weaning-readiness.
  • Don’t restrict fat in the first 2 years — full-fat dairy, butter, healthy oils. Brain needs fat.
  • No juice or sugary drinks in the first year.
  • Vitamin D drops for breastfed babies (NHS: 8.5-10 μg/day; AAP: 400 IU/day).
  • Plenty of skin-to-skin in the first weeks supports breastfeeding establishment.

The simplified LMS calculation used here

WHO publishes Child Growth Standards in LMS form: three parameters (L = skewness, M = median, S = coefficient of variation) tabulated at each age. The full LMS z-score formula is:

z = ((X / M)^L − 1) / (L · S) when L ≠ 0, or z = ln(X / M) / S when L = 0.

We use the real WHO M values at monthly intervals 0-24 months for boys and girls, with fixed L and S per measurement: weight (L=0, S=0.13), length (L=1, S=0.040), head (L=1, S=0.035). Percentile is computed from the z-score using the standard normal CDF (Abramowitz & Stegun erf approximation). Accuracy: roughly ±5 percentile points centrally; noisier at the tails.

Limitations of this tool

  • Covers 0-24 months only. From age 2, AAP / CDC use the CDC 2000 charts.
  • Fixed-L, fixed-S simplification makes the tails (under 3rd, over 97th) less precise than full LMS. Don’t use this to confirm an extreme reading — see your paediatrician.
  • WHO standards describe healthy breastfed infants under optimal conditions. Formula-fed babies often track higher; WHO chart is still the recommended reference under 2.
  • Educational; doesn’t replace the proper plotted chart from your health visitor / paediatrician with your baby’s growth history.

Sources

  • World Health Organization. WHO Child Growth Standards: Length/height-for-age, weight-for-age, weight-for-length, weight-for-height and body mass index-for-age — Methods and development. 2006.
  • de Onis M, et al. Development of a WHO growth reference for school-aged children and adolescents. Bull WHO 2007;85:660-7.
  • Grummer-Strawn LM, et al. Use of WHO and CDC Growth Charts for Children 0-59 Months in the US. MMWR 2010;59(RR-9):1-15.
  • Cole TJ, Green PJ. Smoothing reference centile curves: the LMS method. Stat Med 1992;11:1305-19.
  • NICE NG75. Faltering growth: recognition and management of faltering growth in children. 2017.

Frequently asked questions

What does a baby growth percentile actually mean?
A percentile compares your baby's weight, length, or head size to a reference group of healthy children of the same age and sex. If your 6-month-old is at the 30th percentile for weight, it means about 30 of every 100 healthy babies the same age weigh the same or less, and 70 weigh more. Percentiles are NOT grades — being on the 25th is exactly as healthy as being on the 75th if your baby has tracked there consistently.
What's a normal weight percentile for a baby?
Anywhere from the 3rd to the 97th percentile is considered the normal range — that captures 95% of healthy babies. The 50th is the exact median. Below 3rd or above 97th is outside the typical range and worth a clinical conversation. But the SINGLE most important thing isn't the actual percentile — it's the TREND. A baby who tracks at the 15th from birth is fine; a baby who drops from the 50th to the 15th over 3 months needs evaluation.
What's the difference between WHO and CDC growth charts?
The WHO 2006 charts are based on a multi-country sample of healthy, EXCLUSIVELY BREASTFED children raised under optimal conditions — they describe how children SHOULD grow. The CDC 2000 charts are based on a US reference sample of mostly formula-fed children — they describe how children DID grow in that cohort. CDC and AAP both now recommend WHO charts under 2 years, CDC from age 2 onwards. This calculator uses WHO 0-24 months.
Why is my baby tracking lower than the 50th percentile?
Most babies are NOT at the 50th — by definition, half are below and half are above. If your baby has always been around the 25th and continues to grow steadily on that line, this is completely normal. Genetic factors (parents' size), birth size, gestational age, and feeding patterns all contribute. The percentile that matters is your baby's OWN curve over time.
Should I worry if my baby drops percentiles?
Look at how much and how fast. Crossing one major percentile line down (e.g. 50th to 25th) over several months can be normal — especially for breastfed babies who often track lower than the formula-fed reference around 4-6 months. Crossing TWO major lines downwards (50th to 10th, say) in a few months — particularly if combined with feeding issues, illness, or reduced wet nappies — needs a paediatric review. Single-point readings during illness or growth spurts can be misleading; the trend across 2-3 visits is what matters.
What is failure to thrive (FTT) or faltering growth?
It's the term used when a child's growth doesn't keep up with the expected curve. UK NICE / RCPCH definitions: a fall across one or more major percentile lines (depending on birth weight percentile), weight under the 2nd percentile, or persistent weight tracking near the bottom. Roughly 5% of children meet criteria at some point. About 80% have organic causes (feeding difficulty, reflux, food intolerance, recurrent illness, malabsorption) and 20% have non-organic causes (feeding-environment, social). Management starts with a feeding history and a careful 6-week reassessment, not a panic.
How often is my baby weighed?
UK NHS routine: birth, 5 days, 10-14 days, then before each immunisation (8, 12, 16 weeks), and at the 9-12 month and 24-month checks. More often (weekly or fortnightly) if there's concern. NICE specifically says babies shouldn't be weighed more often than every 2 weeks in the first 6 months unless there's a clinical concern — over-weighing creates anxiety and noisy data. US AAP schedule: 1-2 weeks, 2 months, 4 months, 6 months, 9 months, 12 months, 15 months, 18 months, 24 months.
Do percentiles predict adult size?
Loosely. By age 2, a child's weight and length percentiles correlate moderately with adult size — but a lot still changes between 2 and 18 years. A toddler at the 10th percentile can become a very tall adult and vice versa. Genetic mid-parental height is a better predictor of adult height than the percentile in the first year. See /calculators/child-height-predictor for a parent-height-based estimate.
Why are breastfed babies often smaller after 6 months?
The WHO charts (based on breastfed babies) actually account for this — but compared to the OLDER CDC charts, breastfed babies often dropped percentiles around 4-6 months because formula-fed babies in the CDC sample grew faster after this point. WHO charts fixed this anomaly. If you're using WHO charts (this calculator does), the apparent 'drop' largely disappears. Worry only if there are other signs (poor feeding, reduced output, illness, listlessness).
Is head circumference percentile important?
Yes — it tracks brain growth. Cause for concern if: head circumference is < 3rd percentile or > 97th percentile AND falls outside the family pattern (parents' head sizes are relevant); or if there's rapid acceleration (crossing percentile lines up — risk of hydrocephalus) or deceleration (crossing percentile lines down — risk of microcephaly or developmental concern). Mild macrocephaly or microcephaly that matches a parent's head size is usually benign. Bring up at the next health-visitor / GP visit.
What's a healthy weight gain pattern for a newborn?
Newborns lose up to 10% of birth weight in the first 5 days (more than 10% needs investigation). Back to birth weight by day 10-14. Then typical gains: weeks 1-3 = 20-30 g/day, months 1-4 = 25-30 g/day, months 4-6 = 15-20 g/day, months 6-12 = 10-15 g/day. By 5 months, most babies have doubled birth weight; by 12 months, tripled. Our /calculators/newt-weight-loss tool tracks the newborn weight-loss percentile specifically.
My baby is in the 95th+ percentile — should I worry about obesity?
Not on its own. A high-percentile baby is usually a big baby — same as the height percentile, this is normal variation. Concern grows if there's RAPID upward crossing of percentile lines (e.g. 50th to 95th in 3 months), if weight-for-LENGTH is also high (the better fatness measure for under-2s), or if there's an underlying medical condition. AAP doesn't diagnose obesity under 2 years on the basis of weight-for-age alone — they use weight-for-length and trajectory.
How do I measure my baby at home?
WEIGHT: digital baby scales available cheaply, or weigh yourself with and without baby (less accurate). Always measure in a clean nappy, ideally same time of day. LENGTH: hard to do accurately at home before walking — you need two people, baby flat on a measuring mat with crown and heel against rigid surfaces. HEAD CIRCUMFERENCE: soft tape measure around the widest part (above eyebrows, around the bumpiest part at the back). All of these are better done at health visitor / GP visits with calibrated equipment for any reading that will affect a decision.
What's the 'red book' and how does it work?
UK NHS Personal Child Health Record — given to every parent at birth. Contains growth charts (UK-WHO version), immunisation records, developmental milestone check pages. Bring it to every health visit. Now also available as a digital version (eRedbook) in many UK areas. US equivalent: AAP Bright Futures-aligned well-child visit summaries.
My baby was premature — how do I correct the percentile?
Adjust their age for prematurity until age 2. If born at 32 weeks (8 weeks early), at 6 months chronological age, plot them at 4 months 'corrected age' on the chart. Standard practice in UK and US. After age 2, no adjustment needed. Some clinicians use prematurity-specific charts (Fenton 2013 for very preterm) until age 2. Our /calculators/fenton-growth covers the preterm-specific chart.
How does this relate to other calculators on BumpBites?
Companion: /calculators/newt-weight-loss for newborn weight-loss percentile; /calculators/child-height-predictor for adult-height prediction; /calculators/fenton-growth for preterm-specific growth; /calculators/baby-size-by-week for in-utero growth tracking; /calculators/toddler-feeding for 12-36 month nutrition targets; /calculators/breastfeeding-latch if feeding mechanics matter.

Continue with BumpBites

Newborn Weight Loss (NEWT)

Birth weight loss percentile.

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Child Height Predictor

Adult height estimate.

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Fenton Preterm Growth

Preterm-specific growth chart.

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