How the percentile is computed
The WHO publishes LMS values (lambda, mu, sigma) for every month from 0 to 60. The calculator looks up the LMS row for your baby's age and sex, then applies the standard formula:
z = ((measurement / M)^L - 1) / (L * S) percentile = 100 * normalCDF(z)That's the same calculation behind every clinical growth-chart tool. We then map the percentile to a band (below 3rd, 3rd-15th, 15th-85th typical, 85th-97th, above 97th) with a short note about what each band typically means.
What the bands mean
- Below the 3rd percentile. Pediatricians monitor closely. A single reading isn't a diagnosis; trend across visits matters more.
- 3rd-15th percentile. Smaller side of the reference range. Many healthy babies live here long-term.
- 15th-85th percentile. The middle 70% - the typical range.
- 85th-97th percentile. Larger side of the reference range. Usually not a concern on its own.
- Above the 97th percentile. Pediatricians review the curve over time before drawing any conclusions.
Trend matters more than a single reading
A baby steadily tracking the 10th percentile is usually healthier than a baby dropping from the 50th to the 5th over three months. Pediatricians look at the slope of the curve, the head-to-weight ratio, feeding patterns, and developmental milestones - never a single number.
Pair this with
Breastfeeding calorie calculator for postpartum nutrition targets. Pregnancy week calculator if you're still tracking before birth.
Frequently asked questions
Which growth chart does this calculator use?
The WHO Child Growth Standards (2006), which are based on a multi-country study of breastfed babies raised in optimal conditions. WHO charts are the global reference for 0-24 months and are recommended by the CDC, AAP, and most pediatric societies for this age range.
What's a 'good' percentile?
There is no single 'good' percentile. WHO charts describe how a healthy reference population distributes - about 70% of babies fall between the 15th and 85th percentiles at any given age, but plenty of healthy babies sit below or above. What matters most is the trend across visits: a baby tracking steadily along the 10th percentile is usually healthier than one dropping from the 50th to the 5th over a few months.
How accurate is the percentile?
The calculator uses the official WHO LMS (lambda-mu-sigma) values and the standard z-score formula, so results match what your pediatrician's software outputs to within a fraction of a percentile. The only caveat is age: we interpolate between monthly LMS rows for ages like 8.5 months, which is the same approach clinical tools use.
Should I be worried if my baby is below the 3rd or above the 97th percentile?
A single reading at either extreme isn't a diagnosis. Pediatricians look at the growth trajectory over multiple visits, head-to-weight proportion, feeding, and developmental milestones. If your provider hasn't raised a concern, the curve is probably fine. If you have concerns, bring the calculator result to your next visit.
Why 0-24 months only?
WHO standards cover 0-60 months in total, but most parents check percentiles intensively in the first two years (when growth is fastest and clinic visits are most frequent). We may extend to 5 years in a future update.
Is my data private?
Yes - everything you enter stays in your browser. We don't store, log, or transmit any inputs. No signup required.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.