A growth percentile indicates how your baby's measurement (weight, length, or head circumference) compares to a reference population of healthy children of the same age and sex. For example, if your baby is at the 75th percentile for weight, they weigh more than 75% of babies their age and less than 25%. Percentiles between 5 and 95 are generally considered within the normal range. The WHO standards used here represent optimal growth patterns from a diverse, international sample of healthy, breastfed infants.

The WHO charts (used here for 0–24 months) are based on a multinational study of healthy, exclusively or predominantly breastfed infants living in optimal conditions. They represent a prescriptive standard — how babies should grow. The CDC charts are based on a U.S. reference population and represent a descriptive reference — how babies do grow. The CDC and American Academy of Pediatrics recommend WHO charts for children under 24 months. This tool uses WHO data primarily, extended with CDC-compatible data for ages 24–36 months.

There is no single "ideal" percentile. Healthy babies naturally come in different sizes. Generally:
• 5th to 95th percentile: Considered within the normal range.
• 3rd to 5th / 95th to 97th: Borderline — may warrant monitoring.
• Below 3rd / Above 97th: Should be discussed with a pediatrician.
What matters most is consistent growth over time rather than a single measurement. A baby who tracks steadily along the 10th percentile is often perfectly healthy, while a sudden drop from the 80th to the 20th percentile may need investigation.

Weight-for-Length (WFL) assesses whether a baby's weight is proportionate to their length, independent of age. It helps identify:
• Wasting (low WFL): May indicate undernutrition or illness.
• Overweight/Obesity (high WFL): May indicate overfeeding or metabolic concerns.
A WFL between the 5th and 95th percentiles is generally considered healthy. This metric is especially useful for babies under 2 years, where BMI is not typically used.

This tool uses the official WHO LMS (Lambda-Mu-Sigma) methodology and data tables to calculate Z-scores and percentiles. The interpolation between data points provides estimates accurate to within 0.5–1 percentile points for most measurements. For clinical decision-making, always consult a pediatrician who can plot your baby's measurements on official growth charts and interpret them in the context of your child's overall health, genetic background, and growth trajectory.

Head circumference is a proxy for brain growth and development. In the first year of life, a baby's brain grows rapidly, and measuring head circumference helps pediatricians monitor neurological development. An abnormally small head (microcephaly) or large head (macrocephaly) can indicate underlying conditions that require further evaluation. Head circumference is typically measured at every well-child visit from birth through age 2–3.

Weight: Weigh yourself holding your baby, then subtract your weight. Use a digital scale on a hard, flat surface. For infants, a baby scale is ideal.
Length: Lay baby on a flat surface with head against a wall/board. Gently straighten legs and mark heel position. Measure with a tape measure. Two people make this easier.
Head circumference: Use a flexible tape measure around the widest part of the head — just above the eyebrows and ears, around the back of the head. Measure 2–3 times and use the largest reading.
For clinical accuracy, measurements should be taken by a healthcare professional during well-child visits.

For premature infants (born before 37 weeks), growth should be assessed using corrected age rather than chronological age, at least until age 2. Corrected age = chronological age minus the number of weeks the baby was born early. For example, a 4-month-old born 6 weeks early has a corrected age of 2.5 months. This tool accepts decimal month values, so you can enter the corrected age. Always consult your pediatrician for specialized preterm growth charts.

Some fluctuation is normal. Babies may cross percentile lines during growth spurts or when transitioning to solid foods. However, significant shifts — such as dropping more than two major percentile lines (e.g., from P75 to below P25) over a few months — should be evaluated by a pediatrician. The key is the overall trend. A single measurement gives a snapshot; multiple measurements over time tell the real story of your baby's growth.