Birth Weight Percentile Calculator — Newborn Growth Chart
Enter a newborn's birth weight in grams, gestational age in weeks and sex to estimate the weight percentile relative to published reference standards — and see whether the baby is small, appropriate or large for gestational age.
g
weeks
Sex
Appropriate for gestational age (AGA, P10–P90)
- 1
Reference median for gestational age & sex
3,460 gLinearly interpolated from Fenton 2013 / INTERGROWTH-21st reference table. - 2
Z-score
(3,460 − 3,460) ÷ 470 = 0 - 3
Birth weight percentile
Φ(0) × 100 = 50Φ is the standard normal CDF — the fraction of the reference population below this z-score.
How does this calculator work?
Birth weight percentile = Φ((weight − μ) / σ) × 100, where μ and σ are reference median and SD for the gestational age and sex (Fenton 2013 / INTERGROWTH-21st). <P3 = SGA; P10–P90 = AGA; >P97 = LGA. Values are approximate; certified growth chart software should be used for clinical decisions.
Formula
How this is calculated
The calculator looks up the reference median (μ) and standard deviation (σ) for the given gestational age and sex from a built-in table of values derived from the Fenton 2013 preterm growth chart (22–40 weeks) and INTERGROWTH-21st newborn standards. For gestational ages between the 2-week anchor points in the table, both μ and σ are linearly interpolated. The z-score is computed as (weight − μ) / σ, and the percentile is obtained from the standard normal cumulative distribution function Φ(z) implemented using the Abramowitz & Stegun polynomial approximation (maximum error < 1.5 × 10⁻⁷).
The standard classification thresholds used in neonatal care are: below P3 = small for gestational age (SGA), P3–P10 = low normal, P10–P90 = appropriate for gestational age (AGA), P90–P97 = high normal, above P97 = large for gestational age (LGA). SGA is associated with intrauterine growth restriction and increased risk of neonatal hypoglycaemia; LGA is associated with maternal diabetes and increased delivery complications. The distribution curve shows the bell-shaped reference curve with the baby's weight marked, and the shaded left-tail area represents the computed percentile.
Limitations: this calculator uses a Gaussian (normal) approximation; official charts use the LMS Box-Cox transformation to correct for mild right-skewness in birth weight distributions, so P3 and P97 estimates may differ slightly from certified chart software. Reference values in the internal table are approximate — derived from landmark published data — and should not replace certified tools (Fenton 2013 app, WHO Anthro) for clinical decision-making.
Frequently asked questions
A weight below P10 means the baby weighed less than 90% of infants born at the same gestational age and sex. Below P3 (small for gestational age, SGA) prompts neonatal observation for hypoglycaemia and feeding support. A single percentile figure is interpreted alongside gestational age, growth velocity, and clinical condition — a healthy small baby may simply have a small but normal constitution, while growth restriction implies placental insufficiency.
Fenton 2013 was constructed by pooling data from large national cohorts of preterm infants across multiple countries and bridges into the WHO child growth standards at 40 weeks. INTERGROWTH-21st is based on a prospective international cohort of "optimal" low-risk pregnancies across eight countries, designed to reflect healthy fetal growth potential. Both are widely used; this calculator uses approximate values calibrated to published landmark data from both sources.
For the middle of the distribution (roughly P5–P95) the Gaussian approximation is quite accurate. Birth weight distributions are mildly right-skewed, so the true extreme percentiles (P3, P97) differ somewhat from the Gaussian estimates. Certified growth chart tools use the LMS (lambda-mu-sigma) Box-Cox method to correct for skewness. This calculator uses the Gaussian approximation for simplicity; use dedicated chart software for precise extreme-percentile work.
Also known as
TG we-Calculate Editorial Team. (2026). Birth Weight Percentile Calculator — Newborn Growth Chart [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/birthweight-percentile-calculator
TG we-Calculate Editorial Team. "Birth Weight Percentile Calculator — Newborn Growth Chart." TG we-Calculate. 2026. https://we-calculate.com/calculator/birthweight-percentile-calculator.
TG we-Calculate Editorial Team, "Birth Weight Percentile Calculator — Newborn Growth Chart," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/birthweight-percentile-calculator
@misc{wecalculate_birthweight_percentile_calculator, title = {Birth Weight Percentile Calculator — Newborn Growth Chart}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/birthweight-percentile-calculator}}, year = {2026}, note = {TG we-Calculate} }
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