What percentile should my baby be in?

There's no 'should'. Healthy babies sit anywhere on the chart. What matters is steady growth along a similar curve over time and good feeding, wet diapers and development.

Which growth charts should I use for my baby?

The WHO Child Growth Standards for children under 2. The CDC recommends them for all US children from birth to 2 years.

Is the 50th percentile the average?

It's the median: half of healthy babies are below and half above. It isn't a target.

Should I use corrected age for a premature baby?

Yes, for babies born before 37 weeks, use corrected age when plotting growth until about age 2, unless your pediatrician advises otherwise.

Why did my baby's percentile drop?

Small changes can come from measurement error, illness or normal variation. A drop across two major lines, or weight falling behind length, is worth discussing with your pediatrician.

Baby Growth Percentile Calculator (WHO) / Tot

Medically reviewed by Safi Ullah Khan · Written by Muneeb Rana · Last updated 21 October 2026

Quick answer: A growth percentile calculator compares your baby's weight, length or head size with the WHO Child Growth Standards for age and sex. The 50th percentile is the median; the 25th means 25 of 100 healthy babies are smaller. Steady growth along a curve matters more than the number. CDC recommends WHO charts under age 2.

TL;DR

Uses the WHO LMS tables for weight, length and head circumference, 0-24 months.
Any percentile between roughly the 3rd and 97th can be normal; trends matter most.
Crossing two major percentile lines is worth discussing with your pediatrician.

This growth percentile calculator uses the official WHO Child Growth Standards, the same charts the CDC recommends for all US children under 2 and that UK and many other health services use. Enter a measurement and you get the percentile and z-score, plotted on the chart.

How to use the growth percentile calculator

Choose sex (the WHO standards are sex-specific).
Enter the birth date and the measurement date, or the age in months (use corrected age for preemies).
Enter weight (kg or lb/oz), length (cm or in) and/or head circumference.
Read the percentile and z-score for each, plotted on the WHO curve.
Add earlier measurements to see the trend.

How the growth percentile calculator works

The WHO publishes L, M and S values for each month. The calculator converts your measurement to a z-score and then to a percentile:

z = ((X / M) ** L - 1) / (L * S)      # when L != 0
z = ln(X / M) / S                    # when L == 0
percentile = normalCDF(z) * 100

LMS values are interpolated between months by age in days. Median (50th percentile) values from the WHO tables:

Age	Boys weight	Girls weight	Boys length	Girls length
Birth	3.3 kg	3.2 kg	49.9 cm	49.1 cm
3 months	6.4 kg	5.8 kg	61.4 cm	59.8 cm
6 months	7.9 kg	7.3 kg	67.6 cm	65.7 cm
9 months	8.9 kg	8.2 kg	72.0 cm	70.1 cm
12 months	9.6 kg	8.9 kg	75.7 cm	74.0 cm

Worked example

Lina is a 4-month-old girl who weighs 6.0 kg. The WHO median at 4 months is 6.4 kg (L −0.005, M 6.42, S 0.124). Her z-score is about −0.55, which is roughly the 29th percentile: 29 of 100 healthy girls her age weigh less. Her last two checks were also near the 30th, so she's growing steadily.

What percentiles mean (and don't)

A percentile isn't a score. A baby on the 10th percentile can be perfectly healthy if they're tracking along their curve and feeding well. Pediatricians look at the pattern over time and at weight compared with length. Read how to read a baby growth chart.

When to ask your pediatrician

Talk to your doctor if your baby's weight drops across two major percentile lines, measurements are below the 3rd or above the 97th percentile, weight is falling behind length, or you're worried about feeding. Most babies lose some weight in the first days and regain birth weight by about 2 weeks.

Measure accurately

Home scales and tape measures are less accurate than clinic equipment; small differences can move a percentile. Record clinic measurements in Tot's growth chart tracker to keep a clean history. See average baby weight by month.

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Sources

WHO: Weight-for-age standards: www.who.int (accessed 21 October 2026)
WHO: Length/height-for-age standards: www.who.int (accessed 21 October 2026)
WHO: Child Growth Standards: www.who.int (accessed 21 October 2026)
CDC: WHO Growth Charts for children under 2: www.cdc.gov (accessed 21 October 2026)
AAP HealthyChildren.org: How Often and How Much Should Your Baby Eat?: www.healthychildren.org (accessed 21 October 2026)

Medical disclaimer:

Tot and this website give general information only. Feeding, sleep and growth ranges vary; always follow your pediatrician's advice. If your baby is unwell, contact your doctor or emergency services.

Baby Growth Percentile Calculator (WHO Standards)