Weight Percentile Calculator

A weight percentile calculator answers one question: out of 100 children of the same age and sex, how many weigh less than this one? A boy of 9 years 6 months at 70 lb lands at the 60.2nd percentile, so he is heavier than about 60 of every 100 boys his age. The number comes from the official growth charts, the WHO standards for babies under 2 and the CDC charts from age 2, not from a rule of thumb about what a child "should" weigh.

Two sister tools cover the neighboring questions. Use the BMI Calculator for Kids when the real question is whether a child is underweight or overweight, because that verdict needs height as well as weight. Use the Height Percentile Calculator when you want to rank the child's height on the matching CDC stature chart. This page is the right one when you have an age, a sex and a number off the scale and you want to know where that number sits.

Which chart applies: WHO under 2, CDC from 2

CDC recommends the WHO growth standards for US children from birth to 2 years and its own 2000 growth charts from age 2, and this calculator follows that rule. The WHO standards describe how healthy children should grow under optimal environmental and health conditions, which is why CDC prefers them for the first two years. The CDC charts, built mainly from five national US surveys collected between 1963 and 1994, run continuously to age 20. On the WHO charts, CDC recommends the 2.3rd and 97.7th percentiles (2 standard deviations below and above the median) as the screening cutoffs, rather than the 5th and 95th.

How the percentile is calculated

Weights at a given age are skewed: the heaviest children sit much farther from the median than the lightest ones do, so a plain mean and standard deviation would misplace many children. Both charts therefore publish three parameters for each month of age and each sex, the LMS method: L is a Box-Cox power that removes the skew, M is the median weight in kilograms, and S is the coefficient of variation. The measurement becomes a z-score with:

where X is the weight in kilograms. If L happens to be zero the formula collapses to z = ln(X ÷ M) ÷ S. The z-score is then read off the standard normal distribution: z = 0 is the 50th percentile, z = +1 the 84.1st, z = +2 the 97.7th, z = −1 the 15.9th and z = −2 the 2.3rd. Running the formula backwards gives the weight at any chosen percentile:

That second formula fills the reference table in the calculator. Two details differ between the charts. The WHO tables give one row per completed month, so the calculator interpolates L, M and S to the baby's exact age in months and weeks, and for weights more than 3 standard deviations from the median it applies the WHO rule for weight, which measures the excess in units of the distance between the 2 and 3 standard deviation weights. The CDC file lists each month of age at its half-month point (the 114.5-month row covers 114.0 up to 115.0 months), so a child entered in completed years and months uses that row directly. Pounds and ounces are converted to kilograms first, using the exact factor 1 lb = 0.45359237 kg.

Worked example: a boy of 9 years 6 months at 70 lb

Take a boy aged 9 years 6 months, which is 114 completed months and therefore the CDC row for 114.5 months, weighing 70 lb. Convert first: 70 × 0.45359237 = 31.7515 kg. The CDC boys' weight-for-age parameters for that row are L = −1.284375, M = 30.3177 kg and S = 0.172729.

Now apply the formula. The ratio X ÷ M is 31.7515 ÷ 30.3177 = 1.047291. Raised to the power L that becomes 1.047291^−1.284375 = 0.942380. Subtract 1 to get −0.057620, then divide by L × S = −1.284375 × 0.172729 = −0.221848. The two negatives cancel and the result is a z-score of +0.260. Feeding that into the standard normal distribution gives 60.25%, so this boy is at the 60.2nd percentile: heavier than about 60 of every 100 boys aged 9 years 6 months, lighter than about 40 of them. The median for his age is 30.32 kg, or 66.8 lb, so he is 3.2 lb above the middle of the chart. The 3rd percentile at that age is 50.9 lb and the 97th is 101.8 lb, which shows how wide the normal range already is at nine.

Second example: a baby girl of 6 months at 16 lb 8 oz

Babies use the WHO standards, so switch the calculator to the baby chart. A girl of 6 months 0 weeks weighing 16 lb 8 oz is 16.5 lb, or 7.4843 kg. The WHO girls' parameters at 6 months are L = −0.0756, M = 7.297 kg and S = 0.12204. The ratio is 1.025665, raised to the power L it is 0.998086, and L × S is −0.009226, so z = −0.001914 ÷ −0.009226 = +0.207. That is the 58.2nd percentile. The median 6-month-old girl on the WHO chart weighs 16 lb 1 oz, the 2.3rd percentile is 12.6 lb and the 97.7th is 20.6 lb.

Ounces matter far more at this age than pounds do later. The same girl at 16 lb 0 oz is at the 48.2nd percentile and at 17 lb 0 oz the 67.4th, so half a pound moves the answer by roughly ten points. A month of age moves it further still: 16 lb 8 oz is the 74.6th percentile at 5 months, the 58.2nd at 6 months and the 43.1st at 7 months. And sex matters: the same 16 lb 8 oz for a 6-month-old boy is only the 29.7th percentile.

Baby weight percentiles, birth to 24 months (WHO)

All figures below are computed from the WHO weight-for-age parameters at exactly the month shown, in pounds. The 2.3rd and 97.7th percentiles are the screening cutoffs CDC recommends for the WHO charts.

Two patterns stand out. The spread widens fast: from the 2.3rd to the 97.7th percentile is 4.3 lb at birth for boys, 7.7 lb at 6 months, 9.4 lb at a year and 12.4 lb at two years. And growth slows just as fast: the median boy gains 2.5 lb in his first month, 0.9 lb in the month from 5 to 6 and 0.5 lb in the month from 11 to 12. The median boy doubles his birth weight at about 3.5 months and triples it at about 14 months; the median girl doubles hers at about 4 months and triples it at about 15.5 months.

Weight percentile by age: boys 2 to 19 (CDC)

Computed from the CDC child and teen weight-for-age parameters for a boy who has just had the birthday shown, in pounds.

The chart fans out sharply with age: the distance from the 5th to the 95th percentile is 10.1 lb at age 2, 46.8 lb at 10, 75.4 lb at 14 and 85.6 lb at 18. That is why a percentile, not a pound figure, is the only sensible way to compare two children.

Weight percentile by age: girls 2 to 19 (CDC)

Same method, the girls' curves. Note how the chart flattens out several years earlier than the boys' chart does.

The two charts cross over twice. Boys are a little heavier until about age 8, girls are heavier from about 8 to 13 while their growth spurt runs first (medians of 82.4 lb against 79.5 lb at 11, then 92.2 lb against 89.7 lb at 12), and boys pull well ahead from the mid teens: at 16 the medians are 118.9 lb for girls and 134.7 lb for boys. Median gain per year tells the same story. Boys add about 3.7 lb between ages 2 and 3, 5.3 lb between 6 and 7, 8.8 lb between 10 and 11 and a peak of 11.9 lb between 13 and 14. Girls add 4.0 lb between 2 and 3, 5.6 lb between 6 and 7, a peak of 9.8 lb between 11 and 12, then only 2.7 lb between 16 and 17.

Why the same weight slides down the chart

This is the part that confuses most parents, so it deserves its own table. Below is the percentile of a child weighing exactly 70 lb on different birthdays, from the same CDC parameters.

A child who simply stops gaining does not hold a percentile, they fall through the chart. The same effect runs the other way in the first year: on the WHO chart a baby boy at 22 lb is at the 62.1st percentile at 12 months but above the 97.7th at 6 months. Growth charts measure a moving target, which is exactly why pediatricians read the slope of the line rather than any single dot.

How to use this weight percentile calculator

1. Pick the chart. Baby covers birth to 24 months on the WHO standards; child or teen covers 2 years 0 months through 19 years 11 months on the CDC chart.
2. Pick the sex. Boys and girls have separate curves, and the difference is large in infancy and again in the teens.
3. Enter the age. For a baby, enter completed months plus weeks; for an older child, completed years plus months. Months matter: 70 lb is the 71.5th percentile for a boy of 9 years 0 months, the 60.2nd at 9 years 6 months and the 47.6th at 10 years.
4. Choose your units. Pounds and ounces is the default; switch to kilograms if that is what the clinic recorded. The weight you entered is converted.
5. Enter the weight. Babies undressed on a baby scale to the nearest ounce, older children in light clothing with shoes off.
6. Read the percentile and the z-score. The headline number is the percentile; the z-score below it is the same result in standard deviations, which is what most clinical notes record.
7. Scan the reference table. It lists the reference percentile weights for that exact age and sex, so you can see how far an ounce or a birthday actually moves things.

Who this calculator is for

• Parents of babies who came home from a weigh-in with a number and want to plot it between appointments.
• Parents of school-age children checking a number written on a well-child form or a sports physical.
• Anyone tracking a growth line over several months, where the direction of travel matters more than any single reading.
• Coaches and school nurses who need age-adjusted comparisons rather than raw pounds.
• Students learning how LMS parameters and z-scores turn a measurement into a percentile.

Key terms explained

• Percentile: the share of the reference population that weighs less. The 30th percentile means 30 of 100 same-age, same-sex children are lighter and 70 are heavier.
• Z-score: the same result in standard deviations. Zero is the median, +1.64 is the 95th percentile, −1.64 is the 5th, and ±2 are the 97.7th and 2.3rd.
• Weight-for-age: the chart this page uses. It ranks weight against age alone and takes no account of height or length.
• WHO growth standards: the charts CDC recommends from birth to 2 years. They describe how healthy babies grow under good conditions.
• BMI-for-age: the chart that does account for height, and the one used from age 2 to classify a child as underweight, overweight or obese.
• LMS parameters: the three numbers (skew, median, variation) published for every age and sex, which is what makes an exact percentile possible instead of a squint between printed curves.
• Percentile channel: the band between two printed curves. Healthy growth normally tracks along one channel rather than crossing several.

What changes the result the most

• Age in months and weeks. The steeper the curve, the more a few weeks matter. A girl at 16 lb 8 oz drops from the 74.6th percentile at 5 months to the 43.1st at 7 months on the same weight.
• Measurement precision. For a baby, 8 oz is worth around ten percentile points. For a 9-year-old, half a pound moves the answer about one and a half points. Diapers, clothes and shoes are the biggest source of error.
• Sex. The same 16 lb 8 oz at 6 months is the 58.2nd percentile for a girl and the 29.7th for a boy.
• Which chart you use. The WHO standards and the CDC charts give different percentiles for the same child; at the age-2 switch a 26 lb boy is the 39.6th on WHO and the 23.5th on CDC. Compare like with like.
• Timing of puberty. An early or late growth spurt moves a teenager several percentile channels with nothing wrong; median gain peaks at 11.9 lb per year for boys around 13 to 14 and 9.8 lb per year for girls around 11 to 12.

Tips for an accurate weigh-in

• Use the same scale every time. Two household scales can disagree by a pound, which is enough to invent a trend that is not there.
• Put the scale on a hard floor, never carpet, and let it settle before reading.
• Weigh babies undressed and without a diaper, on an infant scale that reads in ounces.
• Weigh older children in light clothing with shoes off, ideally at the same time of day and before a meal.
• Record the exact date. Age is half of the calculation, and a birthday changes the answer.
• Weigh twice. If the two readings differ by more than a few ounces for a baby or half a pound for a child, take a third and use the middle value.

When a percentile is worth raising with your pediatrician

A percentile is a screening number, and a low or high one is not a diagnosis. CDC notes that growth charts are not meant to be the sole basis for a diagnosis; they flag a measurement for a closer look. Readings outside the screening cutoffs (below the 2.3rd or above the 97.7th percentile on the WHO chart for babies, outside the 3rd to 97th on the CDC chart), a growth line that crosses several percentile curves, weight that is far out of step with length or height, or a sudden change with no obvious explanation are all worth raising at the next visit. Bring the whole growth record rather than one reading, and note any illness, feeding change or medication that lines up with the change.

Limitations and assumptions

• The CDC 2000 charts describe US children measured in national surveys between 1963 and 1994. They are a reference for how American children grew, not a standard for how any child ought to grow.
• The WHO standards used under age 2 describe healthy babies growing under good conditions; CDC recommends them for US babies of any feeding type.
• Weight-for-age ignores height. It cannot tell a tall child from a heavy one, and it is not the chart used to classify overweight or obesity.
• The calculator covers birth through 19 years 11 months. Adults are assessed on BMI instead.
• The chart is not corrected for prematurity. For a preterm baby, clinicians plot corrected age or use a preterm growth reference.
• For babies, the WHO monthly values are interpolated linearly to the exact age; WHO's own software works with daily values, so results can differ very slightly.
• Children with certain genetic or chronic conditions are followed on condition-specific charts, not the general charts.

How it compares to related calculators

This page ranks a weight that has already been measured. If your question is different, a sister tool fits better:

• To decide whether a child aged 2 to 19 is underweight or overweight, use the BMI Calculator for Kids, which scores weight against height on the CDC BMI-for-age chart.
• To rank the same child's height, use the Height Percentile Calculator and read the two percentiles side by side.
• Once a person is 20 or older, switch to the adult BMI Calculator with its fixed 18.5, 25 and 30 cutoffs.
• For a healthy adult weight range at a given height, use the Ideal Weight Calculator.
• For body composition rather than size, use the Body Fat Calculator.