Babies born small or large for their gestational age need extra monitoring for low blood sugar, temperature and feeding problems. This calculator uses the Hadlock fetal growth reference, which is close to birth-weight standards at term; neonatal units may use Fenton or INTERGROWTH-21st charts, especially for preterm babies.
How to use the Birth Weight Percentile Calculator
- Gestational age at birth
- Weight
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the gestational age at birth, weight and sex.
Tips
- Measure lying length for babies under 2 and standing height from age 2.
- Use the exact age in months for babies – a few weeks matters.
- Follow the trend across several visits; crossing two major percentile lines is more important than one value.
- Discuss concerns with your paediatrician.
Common mistakes to avoid
- Comparing a baby with the wrong sex chart.
- Reading the 50th percentile as “normal” and everything else as abnormal.
- Using corrected age incorrectly for premature babies.
Inputs explained
| Field | What to enter |
|---|---|
| Gestational age at birth | In weeks |
| Weight | Units: kg, g, lb |
What each result means
- Average weight at this gestation = exp(0.578 + 0.332×[Gestational age at birth] – 0.00354×[Gestational age at birth]^2)÷1000
- Birth weight percentile = ncdf(([Weight] – [Average weight at this gestation])÷(0.127×[Average weight at this gestation]))×100
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Mean weight for GA (Hadlock): ln W = 0.578 + 0.332 × GA − 0.00354 × GA²; SD ≈ 12.7% of the mean
Percentile = Φ((birth weight − mean) ÷ SD)
SGA < 10th · AGA 10th–90th · LGA > 90th
Worked example
Inputs — Gestational age at birth: 39 weeks; Weight: 3.4 kg.
Results — Average weight at this gestation = 3.43 kg; Birth weight percentile = 47 %; Category = Appropriate for gestational age (AGA); Birth weight category = Normal birth weight.
Key terms
- AGA
- Appropriate for gestational age.
Frequently asked questions
What is a normal birth weight?
About 2.5–4.0 kg at term.
What is low birth weight?
Under 2,500 g, regardless of gestational age.
Why compare with gestational age?
A 2 kg baby at 33 weeks is normal, but at 40 weeks is small.