Birth Weight Percentile Calculator
This calculator works out your newborn's birth weight percentile for their exact gestational age and sex, using the INTERGROWTH-21st international newborn size standards from research published in The Lancet. A birth weight in grams alone tells you little, because a baby born at 28 weeks and one born at 40 weeks need very different reference points, so comparing weight against babies of the same gestational age and sex gives a far more meaningful picture of how your baby is growing. Enter your baby's sex, their gestational age in completed weeks from 24 to 42, and their birth weight in grams. The calculator returns the percentile, the z-score showing how many standard deviations the weight sits from the median, and a classification of small, appropriate, or large for gestational age (SGA, AGA or LGA). You also get a full set of reference percentiles from the 3rd to the 97th for your baby's gestational age and sex, a visual percentile position bar, and a reference table of median and threshold weights across common gestational ages. A percentile below the 10th is flagged as SGA and above the 90th as LGA, though many babies in these ranges are perfectly healthy. This gives a reference figure only, not a diagnosis, and any concerns about your baby's growth should be discussed with your midwife, LMC or doctor, who may also use locally customised growth charts.
1. Baby Details
2. Reference Percentiles (your gestational age)
Percentile Position
Measurement Details
Weight in Kilograms
INTERGROWTH-21st Reference Table (selected gestational ages)
| Gestational Age | 3rd %ile | 10th %ile | 50th %ile (median) | 90th %ile | 97th %ile |
|---|
* Selected row matches your current gestational age and sex inputs. Females weigh slightly less at the median.
How Birth Weight Percentile Is Calculated
A birth weight percentile places a newborn's weight in context relative to all other babies born at the same gestational age and of the same sex. The calculation uses the INTERGROWTH-21st newborn size standards (Villar et al. 2014), the most widely accepted international reference for this purpose.
The method uses a Box-Cox power transformation (LMS method). For each sex and gestational age in completed weeks from 24 to 42, the reference provides three parameters:
- Mu (M): the median birth weight in grams
- Sigma (S): the coefficient of variation (standard deviation divided by mean)
- Lambda (L): the Box-Cox power that normalises the skewed distribution
The z-score is computed as: Z = [(weight / M)^L - 1] / (L x S)
This z-score is then converted to a percentile using the cumulative distribution function of the standard normal distribution.
Worked Example
A male baby born at 40 weeks weighing 3,435 g (the default):
- INTERGROWTH-21st male 40-week median (M): 3,435 g
- Coefficient of variation (S): 0.1230
- Box-Cox power (L): 0.35
- Z = [(3435 / 3435)^0.35 - 1] / (0.35 x 0.1230) = [1 - 1] / 0.04305 = 0.00
- Percentile = phi(0.00) = 50.0th percentile
- Classification: AGA (appropriate for gestational age)
This matches the calculator's default output of the 50th percentile for a 3,435 g male at 40 weeks.
Classification Thresholds
| Classification | Percentile Range | Meaning |
|---|---|---|
| Very small for gestational age (VSGA) | Below 3rd | Weight below the 3rd centile; clinical review recommended |
| Small for gestational age (SGA) | 3rd to below 10th | Weight below the 10th centile; may warrant monitoring |
| Appropriate for gestational age (AGA) | 10th to 90th | Normal range for gestational age |
| Large for gestational age (LGA) | Above 90th to 97th | Weight above the 90th centile; may warrant monitoring |
| Very large for gestational age (VLGA) | Above 97th | Weight above the 97th centile; clinical review recommended |
These thresholds are used by clinicians and midwives to identify babies who may benefit from additional monitoring. SGA does not always indicate a problem; some babies are small because their parents are small (constitutionally small). Clinical assessment always considers growth trends, Doppler findings, and family history.
Limitations
This calculator provides a reference centile based on international population data. It is not a diagnostic tool and does not replace clinical assessment by a midwife, obstetrician, or neonatologist. Actual clinical classifications in New Zealand may use locally customised growth charts (such as the GROW customised centile system), which adjust for maternal height, weight, parity, and ethnicity. If you have concerns about your baby's weight or growth, speak with your LMC, midwife, or GP.
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Sources and method: Villar J, Cheikh Ismail L, Victora CG, et al. (2014). International standards for newborn weight, length, and head circumference by gestational age and sex: the INTERGROWTH-21st Newborn Cross-Sectional Study. The Lancet 384(9946):857-868. INTERGROWTH-21st project (intergrowth21.tghn.org). Ministry of Health New Zealand, Well Child Tamariki Ora programme.
This calculator provides reference centiles for informational purposes only. It is not a substitute for professional clinical assessment. Centiles use INTERGROWTH-21st international standards, which may differ from locally customised growth charts used in clinical practice in New Zealand. Always discuss your baby's growth with your midwife, LMC, or doctor.