Baby Weight Percentile Calculator: WHO Growth Tracking

Use the Baby Weight Percentile Calculator

Enter age, sex, and weight to estimate WHO weight‑for‑age percentiles and track growth over time.

How to Use Baby Weight Percentile Calculator: WHO Growth Tracking

1. Step 1: Select sex

   Choose Male or Female to load the correct WHO curve.

2. Step 2: Enter age

   Type age in months; use decimals for weeks (e.g., 6.5).

3. Step 3: Add weight

   Pick kg or lb/oz and enter your baby’s weight accurately.

4. Step 4: Calculate percentile

   Tap Calculate to see the percentile, z‑score, and range.

5. Step 5: Save or export

   Add to history to track trends or download CSV for records.

Key Features

• Accurate WHO weight percentiles
• Age and sex-specific adjustments
• Growth trend visualization
• Exportable results and reports

Understanding Results

Baby Weight Percentile Calculator Formula

Percentiles are derived from the WHO method that uses L (Box‑Cox power), M (median), and S (coefficient of variation). The z‑score measures how far a measurement is from the median: when L ≠ 0, z = ((x/M)^L − 1)/(L·S); when L ≈ 0, z = ln(x/M)/S. The percentile is then the cumulative probability of the z‑score on a standard normal distribution.

Reference Ranges & Interpretation

In pediatrics, a range between about the 5th and 95th percentile is generally considered expected for healthy children of the same age and sex. Falling below the 5th does not by itself mean something is wrong, and being above the 95th is not automatically a problem. Growth is best understood as a trajectory across multiple visits rather than a single point. Consistent movement along a curve is usually more reassuring than a single high or low percentile.

Assumptions & Limitations

Percentile calculations assume accurate age and weight, and they reflect population distributions rather than individual health. Variations occur due to measurement technique, feeding patterns, and genetics. Babies born preterm may need age correction for the first 2 years; specialized charts may be used in clinical settings. Results here are informational and do not replace professional medical advice.

Complete Guide: Baby Weight Percentile Calculator: WHO Growth Tracking

Use the baby weight percentile calculator to see WHO weight‑for‑age percentiles. Enter age, sex, and weight to get your percentile, z‑score, and tips.

What baby weight percentiles mean

A percentile tells you how a measurement compares with a reference group of children the same age and sex. If your baby’s weight is at the 60th percentile, it means that—compared with the WHO reference sample—about 60% of babies in that group weigh less and about 40% weigh more. Percentiles are not grades or targets. They do not predict future height or weight by themselves, and they are not a pass/fail result.

Clinicians look for a steady trajectory. A baby who tracks around the same percentile curve over time is typically growing as expected. A sudden, persistent shift across multiple major percentile lines—up or down—can prompt a closer look, but even then context matters: recent illness, feeding changes, measurement technique, and genetics can all influence short‑term movement.

Think of percentiles as coordinates on a map. A coordinate by itself is informative, but it’s the path across multiple points that reveals direction. If your baby’s weight percentile is 35th at two months and 38th at four months, the path is essentially flat—steady growth. If it dips from the 50th to the 20th and remains there over several checks, your clinician may ask about feeding, illness, or measurement differences. None of this means there is a problem; it’s simply a cue to look a little closer.

How this calculator estimates WHO percentiles

The calculator uses the standard WHO LMS method (L for Box‑Cox power, M for median, and S for coefficient of variation) to compute a z‑score for the entered age, sex, and weight. The z‑score is then converted to a percentile using a normal distribution. For practical use at home, we provide smooth parameter approximations that closely follow WHO curves for 0–24 months. The output includes the percentile, z‑score, and the approximate 5th–95th range for your baby’s age.

If you’d like a broader growth context, consider reviewing weight alongside other measures: the baby length percentile calculator, the head circumference percentile calculator, and the growth chart calculator help you see how different curves relate.

Z‑scores are useful because they translate different distributions onto a common scale. A z‑score of 0 sits at the median, −1.64 aligns with roughly the 5th percentile, and +1.64 with the 95th. Values between −2 and +2 are common in healthy populations; outside that range can still be normal but may warrant attention when paired with symptoms or a rapid trajectory change.

Example: A 6‑month‑old boy weighs 7.9 kg. On our tool, the median (50th percentile) for that age/sex is about 7.9 kg, so the z‑score is near 0 and the percentile near 50. If another 6‑month‑old weighs 6.8 kg, the z‑score is negative, and the percentile may land around the 20s or 30s—still within an expected range if the trend has been steady.

How to measure weight accurately at home

Reliable inputs lead to reliable outputs. For babies, small differences in clothing or scale placement can change the number enough to nudge a percentile. For the best consistency: use the same scale when possible; place it on a solid, flat surface; weigh at a similar time of day; and minimize clothing or heavy diapers. If your scale allows it, tare (zero) with a light blanket or swaddle to keep the baby comfortable without adding weight.

Most home scales display pounds and ounces. Our calculator converts lb/oz to kilograms automatically—no separate math needed. If you weigh yourself holding the baby and then subtract your own weight, repeat the process twice and take the average.

If measurements seem inconsistent, repeat them a few minutes apart. Large differences can signal a scale issue or placement problem. On slick surfaces, scale feet can slide slightly; placing a thin non‑compressible mat under the scale can improve stability.

Why trend over time matters more than one value

A single percentile offers a quick snapshot. Trend shows the story. Babies grow rapidly, and short‑term variation is normal. The goal is not to “chase” a specific number but to see steady progress over weeks and months. Use the “Add to history” feature to save measurements and watch the sparkline. If your baby consistently tracks around, say, the 40th–60th percentile band, that is typically reassuring—even if a single measurement dips or peaks.

If you’re comparing across tools, remember that rounding rules and chart vintages can differ slightly. Your pediatrician’s measurements remain the clinical reference. Our app is designed for clarity and planning between visits.

To make trends more meaningful, aim to measure in similar conditions: same time of day, before or after a feeding consistently, and on the same device. Over weeks, small day‑to‑day noise averages out and the underlying path becomes clear.

Preterm and corrected age

Babies born before 37 weeks often benefit from age correction when interpreting growth during the first two years. Corrected age subtracts the weeks early from chronological age, aligning growth comparisons with peers at a similar developmental stage. Many clinics use specialized charts for preterm infants. If your baby was born early, ask your clinician whether to apply corrected age for weight‑for‑age assessments and for how long.

As a simple example, a baby born at 33 weeks is about 7 weeks early. At a chronological age of 4 months (≈ 17 weeks), the corrected age is 10 weeks. When you enter age for percentile estimation, using the corrected age can give a more appropriate comparison during the first year or two, depending on your clinician’s advice.

Day‑to‑day fluctuations you can expect

Short‑term changes are normal. Feeding, stooling, hydration, and time since the last nap can shift a measurement by several ounces. Focus on consistent technique and similar measurement conditions rather than a single reading. If you notice a sustained pattern of unexpected loss or gain, especially with feeding difficulty or illness, contact your pediatrician promptly.

It’s also common to see growth “spurts” around developmental leaps, followed by quieter periods. Don’t be surprised if the percentile nudges up briefly and then settles. In most healthy babies, the longer‑term path is what matters.

Feeding transitions and growth spurts

Feeding evolves quickly in the first year. Around 4–6 months, many infants start solids while continuing breast milk or formula. Appetite can surge during growth spurts and settle afterward. Small dips or jumps in percentile may happen as routines change. The goal is steady, responsive feeding rather than hitting a specific percentile. For practical planning, tools like our baby food calculator and breastfeeding calculator can help you map intake patterns.

When solids begin, weights may become slightly more variable week to week. Offer a variety of textures and nutrient‑dense options while maintaining milk feeds as recommended. If your baby is learning to self‑feed, some meals will be exploratory—messy and lower in intake—which is developmentally appropriate.

Combine weight with length and head size

Weight is most helpful when viewed alongside other measurements. A baby tracking on the 30th percentile for weight and a similar range for length is often doing just fine. Consider checking length percentiles and head circumference percentiles to round out the picture. Later in childhood, BMI‑for‑age becomes a useful combined indicator—see the child BMI percentile calculator for that stage.

If you prefer a visual overview, the growth chart calculator lets you plot repeated measurements against percentile bands. Seeing weight and length together can make patterns more obvious at a glance.

When to discuss results with your clinician

Always use percentiles as a conversation starter, not a diagnosis. Contact your pediatrician if you observe a sustained drop in weight percentile, persistent feeding problems, signs of dehydration, or illness. Your clinician will consider exam findings, feeding history, birth details, and family growth patterns when interpreting measurements. Bringing a simple log or CSV export of recent weights can be helpful during appointments.

If you’re asked to track more closely, try to measure on a consistent cadence—perhaps once per week—rather than daily. Daily weighing can be noisy and stressful without adding much clinical value. A short, focused log over 2–4 weeks often provides clearer insights.

Quick answers to common questions

Is a higher percentile always better? No. Healthy babies thrive across a wide range of percentiles. What matters is growing at a steady pace over time.
Do breastfed and formula‑fed babies grow differently? Patterns can differ in the second half of the first year. Both can be healthy; clinicians look at the overall trajectory and clinical context.
Why do online tools sometimes disagree? Minor differences in reference data editions, rounding, and interpolation can lead to small discrepancies. Use the same method consistently when tracking.

What if my baby was sick recently? Short illnesses can affect appetite and hydration, which may temporarily nudge weight readings. As your baby recovers, weights and percentiles often return toward the previous trend.
Should I weigh before or after feeds? Be consistent either way. Many parents choose a morning time, before a feed and with a clean diaper, to reduce variability.
Can I share results with family or caregivers? Yes—use the CSV export to save measurements and send them securely, or take a screenshot for quick reference.

For additional context beyond infancy, try our weight percentile calculator for broader age ranges, or explore the full list on the calculators index.

Authoritative references: the WHO Child Growth Standards describe methodology and charts; the CDC growth charts provide tools and training materials for clinicians and public health.

Frequently Asked Questions