Birthweight Percentile Calculator

Q: How is the percentile calculated?

This calculator uses the LMS (Box-Cox) method from the INTERGROWTH-21st newborn size standards: the birthweight is converted to a z-score with z = ((weight/M)^L − 1) / (L×S), using sex- and gestational-age-specific L, M and S parameters fitted to the published standard, then the percentile is the standard normal cumulative probability of that z-score times 100.

Q: What do SGA, AGA and LGA mean?

SGA (small for gestational age) means birthweight below the 10th percentile; AGA (appropriate for gestational age) means between the 10th and 90th percentiles; LGA (large for gestational age) means above the 90th percentile. These are statistical groupings used for screening, not diagnoses on their own.

Q: Which gestational ages are supported?

From 33 weeks + 0 days to 42 weeks + 6 days, the validated range of the INTERGROWTH-21st newborn size standards. Babies born before 33 weeks fall under the separate INTERGROWTH-21st very-preterm size-at-birth reference and are not covered by this calculator.

Q: How is this different from a baby growth percentile calculator?

This calculator answers 'how big was the baby AT BIRTH for its gestational age' using the INTERGROWTH-21st newborn size standards. Once the baby is born, ongoing growth is tracked by postnatal age (days/months old) with the WHO Child Growth Standards — a different tool, our baby percentile calculator, covers that.

Q: What is the INTERGROWTH-21st standard?

INTERGROWTH-21st is a multicountry research project (Brazil, China, India, Italy, Kenya, Oman, the UK and the USA) that measured more than 20,000 newborns of healthy, well-nourished mothers to build international, prescriptive newborn size standards by gestational age and sex, published by Villar et al. in 2014.

How does a newborn's weight compare for its gestational age? Enter weeks+days, birthweight and sex — get the INTERGROWTH-21st percentile, z-score and SGA / AGA / LGA classification. Free, private, runs entirely in your browser.

Medical information — not medical advice. This tool is for education and screening only. A percentile outside the typical range is a flag to discuss with a pediatrician, midwife or obstetrician — never a diagnosis, and never a basis for clinical decisions on its own.

Calculate birthweight percentile

Standard: INTERGROWTH-21st Newborn Size Standards · gestational age 33+0 to 42+6 · nothing leaves your browser.

Gestational age — weeks 33 to 42 completed weeks

Gestational age — extra days 0 to 6 days

Birthweight

Unit

Sex at birth

Boy Girl

–th percentile

AGA

0 (SGA < 10)50100 (LGA > 90)

INTERGROWTH-21st reference table

Published smoothed birthweight centiles (grams) by completed gestational week — Villar et al., Lancet Diabetes & Endocrinology 2014, Table 2.

Week	10th	50th	90th
33	1,430	1,950	2,520
34	1,710	2,220	2,790
35	1,950	2,470	3,030
36	2,180	2,690	3,250
37	2,380	2,890	3,450
38	2,570	3,070	3,630
39	2,730	3,240	3,790
40	2,880	3,380	3,940
41	3,010	3,510	4,060
42	3,120	3,620	4,170

Week	10th	50th	90th
33	1,410	1,860	2,350
34	1,680	2,130	2,640
35	1,920	2,380	2,890
36	2,140	2,600	3,120
37	2,330	2,800	3,320
38	2,500	2,970	3,510
39	2,650	3,130	3,660
40	2,780	3,260	3,800
41	2,890	3,370	3,920
42	2,980	3,460	4,010

Below the 10th percentile = SGA range · 10th–90th = AGA range · above the 90th = LGA range. The calculator above interpolates between exact weeks for day-level precision.

Worked examples

Example 1 — AGA. Boy born at 40+0 weighing 3,500 g. Median for 40-week boys is 3,380 g, so 3,500 g lands near the 61st percentile (z ≈ +0.27): AGA, within the typical range.

Example 2 — SGA flag. Girl born at 38+2 weighing 2,400 g. The 10th percentile for 38-week girls is 2,500 g, so 2,400 g is around the 5th percentile: SGA range — a screening flag to discuss with the care team, not a diagnosis.

Example 3 — LGA flag. Boy born at 39+0 weighing 4,100 g. The 90th percentile for 39-week boys is 3,790 g, so 4,100 g is near the 97th percentile: LGA range — worth mentioning to the pediatrician.

Scope boundary. This tool answers “how big was the baby at birth for its gestational age” (INTERGROWTH-21st newborn size standards). Once born, growth is tracked by postnatal age with the WHO Child Growth Standards — see our baby percentile calculator. For estimating a delivery date from gestational age, see our pregnancy due date calculator.

Frequently asked questions

What is a birthweight percentile?

A birthweight percentile tells you how a newborn's weight compares with other babies born at the same gestational age and of the same sex. For example, a 40th percentile means the baby is heavier than about 40 out of 100 comparable newborns. It is always relative to gestational age — a 2,500 g baby is small at 40 weeks but large at 33 weeks.

How is the percentile calculated?

With the LMS (Box-Cox) method from the INTERGROWTH-21st newborn size standards. The birthweight is converted to a z-score via z = ((weight/M)^L − 1) / (L×S) using sex- and gestational-age-specific L, M and S parameters fitted to the published standard; the percentile is the standard normal cumulative probability of that z-score, times 100. Values between exact weeks are interpolated day by day.

What do SGA, AGA and LGA mean?

SGA (small for gestational age) = birthweight below the 10th percentile. AGA (appropriate for gestational age) = between the 10th and 90th percentiles. LGA (large for gestational age) = above the 90th percentile. These are statistical groupings used for screening, not diagnoses by themselves.

Which gestational ages are supported?

From 33 weeks + 0 days to 42 weeks + 6 days — the validated range of the INTERGROWTH-21st newborn size standards. Babies born before 33 weeks fall under the separate INTERGROWTH-21st very-preterm size-at-birth reference (Villar et al., Lancet 2016) and are not covered here.

Why are there separate calculations for boys and girls?

Boys are on average heavier than girls at every gestational age. At 40 weeks the median (50th percentile) is about 3,380 g for boys versus 3,260 g for girls in the INTERGROWTH-21st standard, so sex-specific curves keep the percentile fair for each baby.

Is this a medical diagnosis?

No. This tool is for education and screening only. A percentile outside the typical range is a flag to discuss with a pediatrician, midwife or obstetrician — it is not a diagnosis of any condition, and clinical decisions should never be based on this calculator alone.

How is this different from a baby growth percentile calculator?

This calculator answers “how big was the baby at birth for its gestational age” using the INTERGROWTH-21st newborn size standards. After birth, growth is tracked by postnatal age (days/months old) against the WHO Child Growth Standards — our baby percentile calculator covers that.

What is the INTERGROWTH-21st standard?

INTERGROWTH-21st is a multicountry research project — Brazil, China, India, Italy, Kenya, Oman, the UK and the USA — that measured more than 20,000 newborns of healthy, well-nourished mothers to build international, prescriptive newborn size standards by gestational age and sex (Villar et al., 2014). It complements the WHO Child Growth Standards, which start at birth but do not differentiate by gestational age.

Method & sources

LMS parameters per gestational week were fitted (weighted least squares, 3× weight on the 10th/50th/90th centiles) to the published smoothed birthweight centiles of Villar J et al., International standards for newborn weight, length, and head circumference by gestational age and sex: the Newborn Cross-Sectional Study of the INTERGROWTH-21st Project, Lancet Diabetes & Endocrinology 2014; 2:857–68 (Table 2). Maximum deviation from the published centiles is under 19 g (≈0.9 percentile points); day-level values between exact weeks are linearly interpolated from the weekly curves. Cross-checked against the official INTERGROWTH-21st daily reference tables and against independently published 40-week values (boys 50th = 3.38 kg, girls 50th = 3.26 kg). Known limitation: beyond ±2 SD the LMS approximation diverges slightly from the paper's original skewed-t model — the SGA/AGA/LGA classification range (10th–90th) is well inside the accurate region. Your inputs never leave this page: all computation happens locally in your browser.

Free tool by NRTechStudio

Birthweight Percentile Calculator

Calculate birthweight percentile

INTERGROWTH-21st reference table

Worked examples

Frequently asked questions

Method & sources

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