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Enter your values, then click Calculate result.How this calculator helps
Estimate WHO baby weight-for-age percentile at exact monthly ages from birth to 24 months using sex-specific published LMS reference values. Weight-for-age percentile compares an entered weight with a sex-specific WHO reference at the stated age. A result near the 50th percentile is close to the reference median, not a score or a growth goal. This tool uses the monthly LMS parameters published in the WHO tables. It does not assess body composition, weight-for-length, feeding adequacy or overall health. A clinician interprets growth in context and over time.
How to use it
- 1
Choose the stated method or shape and read the labels before replacing the example values.
- 2
Enter the measurements in the exact units shown. Use the inputs described in the methodology below.
- 3
Click Calculate result. Editing inputs keeps the previous submitted result visible until you calculate again.
- 4
Compare the supporting figures with the formula and worked example, then review the method’s specific limitations.
Formula and methodology
For LMS reference values at the selected monthly age: z = ((weight/M)^L − 1)/(L×S), or ln(weight/M)/S when L=0. Percentile = standard-normal cumulative probability of z × 100.
The calculator applies the displayed arithmetic to the values entered on this device. It does not silently load a local tax rate, currency conversion or commercial assumption.
Worked calculation example
For the WHO boys reference at exactly six months, 7.9340 kg is the median and produces approximately the 50th weight-for-age percentile.
How to interpret your result
This method applies only within the stated scope. WHO weight-for-age at exact monthly ages 0–24 only and |z|≤3. No gestational-age percentile, prematurity correction, diagnosis, daily-age interpolation or feeding guidance. The worked scenarios below identify the measurements and conventions needed to interpret the displayed output.
For different inputs or formulas, use Height Calculator; Body Surface Area Calculator; Weighted Mean Calculator.
Related questions this calculator covers
- baby percentile calculator
Scenario comparison
| Scenario | What it shows |
|---|---|
| Boys reference at exact 6 months, 7.9340 kg | approximately 50th percentile. |
| Girls reference at exact 6 months, 7.2970 kg | approximately 50th percentile. |
| Age between monthly points | this simplified table does not produce an exact-age percentile. |
Common mistakes to avoid
- Rounding age to fit a monthly reference point.
- Using weight-for-age as gestational-age birthweight percentile.
- Treating a percentile as a feeding target or a diagnosis.
Reproduce the equation shown on this page using the same entered values. For the WHO boys reference at exactly six months, 7.9340 kg is the median and produces approximately the 50th weight-for-age percentile. Keep input units and the selected method with any recorded result. Check unusual measurements rather than relying on extra displayed decimal places.
Authoritative reference. WHO weight-for-age at exact monthly ages 0–24 only and |z|≤3. No gestational-age percentile, prematurity correction, diagnosis, daily-age interpolation or feeding guidance.What can affect the result?
Exact monthly ages only
The reference bundled here covers exact monthly points from birth through twenty-four months. Enter a whole-number age only when the measurement corresponds to that exact monthly point. Do not round a baby’s age up or down to use the tool. Days and weeks are important during early growth, and this simplified version does not interpolate daily reference values. For an age between monthly points, use a clinical growth system that supports exact dates and the appropriate reference.
Weight and reference selection
Use measured kilograms and choose the relevant WHO boys or girls reference. Pounds must first be converted to kilograms. A percentile depends on age and reference selection, so comparing results from different charts or ages without that information can mislead. The displayed median is the M parameter at the selected age and helps verify the data lookup. The calculator stores no identifying baby information and does not need a name or birth date.
Birthweight, prematurity and extremes
This is weight-for-age, not birthweight-for-gestational-age. It does not use gestational age, correct for prematurity or classify small or large for gestational age. The simplified LMS output is limited to z-scores from minus three to plus three; more extreme values are rejected for professional interpretation rather than extrapolated. A single percentile should not determine a feeding or treatment change. Concerns about growth, dehydration or illness should be discussed with a clinician.
Privacy and browser processing
Values entered on this page are processed in the current browser session. SolvePilot does not require an account and does not receive the values entered into the calculator. Refreshing or closing the page clears the working values unless the browser itself restores a previous session. Avoid entering identifying or account information because the calculation needs summary values only.
Accuracy and verification
Accuracy depends first on input quality. Confirm definitions, scales, dates and source information before entering a value. Keep an independent record of any result used for planning because this page does not create an official statement or retain a calculation history.
Limits of this estimate
WHO weight-for-age at exact monthly ages 0–24 only and |z|≤3. No gestational-age percentile, prematurity correction, diagnosis, daily-age interpolation or feeding guidance.
Sources and review information
Frequently asked questions
Is this a birthweight percentile calculator?＋
No. Birthweight-for-gestational-age requires gestational age and a suitable newborn reference. This tool compares weight with the WHO age reference at monthly points. Choose a gestational-age chart with professional guidance when that is the question you need to answer.
Why can I only enter whole months?＋
The bundled official table contains monthly LMS parameters. This implementation deliberately avoids pretending that rounding or an unspecified interpolation gives an exact daily-age percentile. Use it at the stated monthly points or use a validated clinical chart for dates between them.
What does the 50th percentile mean?＋
It corresponds to the reference median at the selected age and sex. It is not a recommended weight for every child and is not a pass mark. Growth trajectory, measurement quality and clinical context matter more than trying to reach a particular displayed percentile.
Why is an extreme weight rejected?＋
This simplified implementation is restricted to z-scores between minus three and plus three. Extreme-value interpretation needs appropriate reference handling and clinical assessment. Returning a highly precise percentile outside the supported method would give a misleading impression of validity.
Can I use it for a premature baby?＋
It does not correct age or use a preterm reference. Ask the baby’s clinician which growth chart and age convention should be used. Do not apply a correction yourself based only on this calculator, and do not change feeding from a single percentile result.