What does the calculator show?
- Weight-for-age: is your child’s weight right for their age?
- Length/height-for-age: is their length right for their age?
- BMI-for-age: is their weight right for their length? The best single measure of thinness or overweight.
- Head circumference-for-age: if you measured it; especially important in the first two years.
What is a percentile?
A percentile compares your child with 100 healthy children of the same age. A weight on the 30th percentile means heavier than 30% of children that age and lighter than 70%. By WHO criteria, anything between −2 and +2 standard deviations (roughly the 2nd to 98th percentile) is within the normal range; your child doesn’t need to be on the 50th percentile to be healthy.
Normal weight and length by age
The median for each age from the WHO standards, with the normal range underneath (−2 to +2 standard deviations, roughly the 2nd to 98th percentile). Figures are rounded to one decimal, so check a measurement right at a limit in the calculator.
| Age | Boy weight (kg) | Girl weight (kg) | Boy length (cm) | Girl length (cm) |
|---|---|---|---|---|
| Birth | 3.32.5–4.4 | 3.22.4–4.2 | 49.946.1–53.7 | 49.145.4–52.9 |
| 1 month | 4.53.4–5.8 | 4.23.2–5.5 | 54.750.8–58.6 | 53.749.8–57.6 |
| 2 months | 5.64.3–7.1 | 5.13.9–6.6 | 58.454.4–62.4 | 57.153–61.1 |
| 3 months | 6.45–8 | 5.84.5–7.5 | 61.457.3–65.5 | 59.855.6–64 |
| 4 months | 75.6–8.7 | 6.45–8.2 | 63.959.7–68 | 62.157.8–66.4 |
| 6 months | 7.96.4–9.8 | 7.35.7–9.3 | 67.663.3–71.9 | 65.761.2–70.3 |
| 9 months | 8.97.1–11 | 8.26.5–10.5 | 7267.5–76.5 | 70.165.3–75 |
| 1 year | 9.67.7–12 | 8.97–11.5 | 75.771–80.5 | 7468.9–79.2 |
| 18 months | 10.98.8–13.7 | 10.28.1–13.2 | 82.376.9–87.7 | 80.774.9–86.5 |
| 2 years | 12.29.7–15.3 | 11.59–14.8 | 87.881.7–93.9 | 86.480–92.9 |
| 3 years | 14.311.3–18.3 | 13.910.8–18.1 | 96.188.7–103.5 | 95.187.4–102.7 |
| 4 years | 16.312.7–21.2 | 16.112.3–21.5 | 103.394.9–111.7 | 102.794.1–111.3 |
| 5 years | 18.314.1–24.2 | 18.213.7–24.9 | 110100.7–119.2 | 109.499.9–118.9 |
How to measure correctly
- Weight: on a baby scale, without heavy clothes or a full nappy.
- Length: under 2 years, measured lying down (length); from 2 years, standing (height). If it was measured the other way, the calculator adjusts it by 0.7 cm, as the WHO does.
- Head circumference: with a tape around the widest part of the head, above the eyebrows and ears.
Frequently Asked Questions
My baby is below average. Is something wrong?
No. The average isn’t the only normal; roughly anywhere between the 2nd and 98th percentile is normal. What matters is that your child stays roughly on the same curve over time. If weight drops across percentile lines or stops increasing, see your doctor.
Why do you ask whether the baby was premature?
A baby born early is compared by corrected age, counted from the due date (40 weeks), up to 2 years. Without correction, they would look small for their age.
Do these standards apply to formula-fed babies?
Yes. The WHO standards were built from breastfed children in six countries and are the reference for all children, however they are fed. Formula-fed babies sometimes gain weight a little faster after the first few months.
My child is older than 5.
The WHO standards for young children end at 5 years. For children aged 5 to 19, the WHO has BMI-for-age charts, which your paediatrician can assess. Do not use adult categories (such as our BMI calculator) for children.
The result says to check the measurement.
The number is so far from the usual range that there is probably a mistake: pounds instead of kilograms, inches instead of centimetres, or a wrong date of birth. Please check the details.