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Baby Growth Percentiles (0–24 mo) Calculator

The Baby Growth Percentiles calculator shows approximate length-for-age and weight-for-age percentiles for babies from birth to 24 months.

This calculator lets us enter age in months, sex, length (cm), and weight (kg) to estimate where growth sits on the curve and to generate simple notes we can share with family or save for our records. If we’d rather compute it manually, we outline the curve method and steps below.

Enter age in months (0–24), select sex, and add recumbent length (cm) and weight (kg). We estimate percentiles and provide curve notes for discussion with your pediatrician. This is educational, not a medical diagnosis.

0–24 months supported.

Recumbent length for infants.

Length-for-Age Percentile
—
Weight-for-Age Percentile
—
Age
6.0 mo (boy)
Length
66.0 cm
Weight
7.60 kg
Quick interpretation
Length is within the typical range for age. • Weight is within the typical range for age.

For clinical use, your pediatrician will use official growth standards and full charts. Our estimates are for education only.

Results interpretation

We estimate where length and weight fall relative to same-age peers. A percentile shows the share of peers that measure at or below your entry. For example, the 30th percentile means about 30% are at or below the value. What matters most over time is the trend—steady growth along a curve—rather than a single point.

How it works

To keep the calculator fast and simple, we approximate published growth curves with a small set of anchor points and linear interpolation. We then compute a z-score and map it to a percentile using a normal CDF.

Formulas, assumptions, limitations

Inputs. Age in months (0–24), sex (boy/girl), length (cm), weight (kg).

Interpolation. We interpolate median and SD between ages 0, 6, 12, 18, and 24 months for each sex.

Z-score. z = (value − median) ÷ SD. Percentile = Φ(z), where Φ is the normal CDF.

Output. Two results: length-for-age and weight-for-age percentiles, plus curve notes.

Limitations. This is an approximation for education, not a diagnostic instrument or a substitute for pediatric care.

Use cases & examples

6-month boy, 66 cm, 7.6 kg

Length and weight are close to typical medians at 6 months, so both percentiles sit near the middle of the curve.

12-month girl, 72 cm, 8.0 kg

Length slightly under the median, weight near the median → length percentile lower than weight, which is common and not necessarily concerning.

18-month boy, 84 cm, 12.0 kg

Both measurements above the medians at this age → percentiles skew higher; pediatricians will still focus on consistent trend.

Baby growth percentile FAQs

Which standards do pediatricians use?

Clinicians use official growth standards (e.g., WHO/CDC) with detailed age- and sex-specific tables and charts.

Is a low or high percentile a problem?

Not by itself. Pediatricians focus on steady tracking along a curve and overall health, not a single percentile value.

Why doesn’t this calculator show head circumference?

To keep things simple, we focus on length and weight. Head circumference percentiles are commonly tracked in clinics.

Can I use inches and pounds?

This version accepts cm and kg. You can convert with our unit tools and paste values here.

Understanding baby growth percentiles

Percentiles describe where a measurement sits compared with a reference group of the same age and sex. If a value is at the 60th percentile, it’s larger than about 60% of peers and smaller than about 40%. Percentiles are convenient because they compress complex distributions into a single, interpretable number.

Trends matter more than snapshots

Pediatric growth assessment emphasizes trajectory. Babies can temporarily jump or dip in percentiles during spurts, minor illnesses, or feeding changes. What clinicians look for is steady progress over time along a curve that’s appropriate for the child’s health history and family context.

Length and weight don’t always move together

It’s common for length and weight percentiles to differ. For instance, a baby may be above average in length and closer to average in weight. Clinicians integrate both measures and sometimes weight-for-length to understand proportionality and nutrition.

Why our calculator is an estimate

Official growth standards use extensive LMS parameters for every age in days, not just months. Our calculator provides a fast educational estimate by interpolating a handful of age anchors. It’s perfect for curiosity and learning, and it encourages healthy conversations with your pediatrician.

Collecting measurements well

  • Use a flat surface and a firm tape for recumbent length; measure twice and average.
  • Weigh at a consistent time of day, with minimal clothing.
  • Record age precisely in months; if you know age in days, divide by 30.44 for a close conversion.
  • Keep notes so your pediatrician can see the trend between visits.

When to check in with your pediatrician

Sudden changes in trend, persistently very low intake, or other health concerns are reasons to reach out. Pediatricians can review detailed charts, growth velocity, feeding, sleep, and development to provide tailored advice.