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How to read a baby percentile
A percentile tells you where your baby sits compared with 100 healthy babies of the same age and sex. At the 25th percentile for weight, about 25 babies weigh less and 75 weigh more. It is not a grade. A baby on the 10th percentile who stays there, feeding well and meeting milestones, is usually growing exactly as they should.
What pediatricians watch most is the trend: does your baby roughly follow their own curve over time? Crossing two or more of the main percentile lines up or down, or weight and length heading in very different directions, is a reason for a closer look.
Which growth chart does this use?
This calculator uses the World Health Organization Child Growth Standards, from the official LMS data files published by the CDC. The CDC and the American Academy of Pediatrics recommend the WHO charts for all children from birth to 2 years in the United States. They are based on healthy, mostly breastfed babies from six countries, growing in good conditions, so they show how babies should grow rather than just how they do grow on average.
From age 2, US pediatricians switch to the CDC growth charts. The UK uses UK-WHO charts built on the same WHO standards for this age.
Measuring at home
- Weight: weigh yourself holding your baby, then weigh yourself alone, and subtract. Home scales are less precise than a clinic's baby scale, so small differences mean little.
- Length: hard to measure accurately at home. Lay your baby flat with legs gently straightened and measure from the top of the head to the heel. Clinic measurements are more reliable.
- Head: wrap a soft tape around the widest part of the head, just above the eyebrows and ears.
This tool is for curiosity and conversation, not diagnosis. Growth is assessed by your pediatrician using clinic measurements over time. If you are worried about your baby's weight, feeding or growth, call them.