Birth Weight Percentile Calculator (by gestational age, 23–42 weeks)

Understanding the birth weight percentile calculator — concept and principle

This calculator compares a newborn's birth weight with babies of the same sex born at the same gestational age, from 23 to 42 weeks, using the UK90 reference revised for the UK-WHO growth charts. It gives the percentile, the z-score, whether the baby is small, appropriate or large for gestational age, and the median, 10th and 90th percentile weights. Parents and neonatal staff use it.

Weight compared at the same age — with the default inputs: 36.8th percentile.

How it is calculated

LMS values (L, M median, S) for each sex and completed week, interpolated between weeks
z = ((weight ÷ M)^L − 1) ÷ (L × S)
Percentile = share of babies below that z × 100
Below the 10th percentile: small for gestational age (SGA); above the 90th: large for gestational age (LGA)
10th and 90th percentile weights come from z = ±1.2816

The principle

Birth weight depends strongly on how long the pregnancy lasted, so a weight that is small at term may be large at 34 weeks. Comparing with babies of the same sex and gestational age separates growth from prematurity. The LMS method handles the skewed spread of birth weights. Different countries use different references, such as Fenton or INTERGROWTH-21st, which can shift the percentile by a few points.

Worked example

Defaults: boy, 39 weeks + 3 days, 3,300 g.

Median at this age = 3,453 g.
z-score = -0.34; percentile = 36.8.
10th percentile = 2,879 g; 90th percentile = 4,047 g.
3,300 g is between them: Appropriate (AGA).

Each point is the calculator re-run with only “Birth weight (g)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Birth weight (g)	Birth weight percentile
825	0
1,650	0
2,475	1.3
3,300	36.8
4,125	92.6
4,950	99.9
6,600	100

How to read the result

Where does your result fall?

Most healthy babies fall between the 10th and 90th percentiles. Being small for gestational age does not by itself mean a growth problem; many SGA babies are simply small and healthy. The clinical team considers the parents' size, the pregnancy and the baby's condition.

Common mistakes

Using weeks since the last period without days, or rounding up.
Comparing with the wrong sex.
Confusing SGA with prematurity.
Comparing percentiles from different references.

Keep in mind: Not a diagnosis. Discuss any concerns about your baby's size with your midwife or paediatrician.

Frequently asked questions

What is a normal birth weight percentile?

Between the 10th and 90th percentile is called appropriate for gestational age.

What does SGA mean?

Small for gestational age: below the 10th percentile for sex and gestational age.

Why do different charts give different percentiles?

They are based on different populations and methods.

Sources: Cole TJ, Freeman JV, Preece MA, Stat Med 1998;17:407–429 (British 1990 growth reference) · Cole TJ, Williams AF, Wright CM, "Revised birth centiles for weight, length and head circumference in the UK-WHO growth charts", Ann Hum Biol 2011;38(1):7–11

Checked 9 Nov 2026. British 1990 growth reference (UK90): Cole TJ, Freeman JV, Preece MA, Stat Med 1998;17:407–429; birth centiles revised in Cole TJ, Williams AF, Wright CM, “Revised birth centiles for weight, length and head circumference in the UK-WHO growth charts”, Ann Hum Biol 2011;38(1):7–11. LMS values (23–42 weeks) as distributed in the RCPCH rcpchgrowth package.

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