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 61.3rd percentile, so he is heavier than about 61 of every 100 boys his age. The number comes from the CDC growth charts, 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.

How the percentile is calculated

Weights at a given age are strongly skewed: the heaviest children sit much farther from the median than the lightest ones do, so a plain mean and standard deviation would misplace almost everybody. The CDC therefore publishes three parameters for every half-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, so you can see not only where a child sits but what the 5th, 50th and 95th percentile weights are at that exact age. Because the CDC parameters are metric, 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 months, weighing 70 lb. Convert first: 70 × 0.45359237 = 31.7515 kg. At 114 months the CDC boys' weight-for-age parameters are L = −1.289872, M = 30.1769 kg and S = 0.172213, interpolated between the 113.5-month and 114.5-month rows of the CDC file.

Now apply the formula. The ratio X ÷ M is 31.7515 ÷ 30.1769 = 1.052179. Raised to the power L that becomes 1.052179^−1.289872 = 0.936499. Subtract 1 to get −0.063501, then divide by L × S = −1.289872 × 0.172213 = −0.222133. The two negatives cancel and the result is a z-score of +0.286. Feeding that into the standard normal distribution gives 61.25%, so this boy is at the 61st percentile: heavier than roughly 61 of every 100 boys aged 9 years 6 months, lighter than about 39 of them. The median for his age is 30.18 kg, or 66.5 lb, so he is 3.5 lb above the middle of the chart. The 3rd percentile at that age is 50.8 lb and the 97th is 101.2 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 separate CDC infant file, so switch the calculator to the baby chart. A girl of 6 months weighing 16 lb 8 oz is 16.5 lb, or 7.4843 kg. The CDC girls' parameters at 6 months are L = 0.408278, M = 7.2114 kg and S = 0.116030. The ratio is 1.037846, raised to the power L it is 1.015282, and L × S is 0.047373, so z = 0.015282 ÷ 0.047373 = +0.323. That is the 62.6th percentile. The median 6-month-old girl on the CDC chart is 15 lb 14 oz, the 3rd percentile is 12.6 lb and the 97th is 19.6 lb.

Ounces matter far more at this age than pounds do later. The same girl at 16 lb 0 oz is at the 52.2nd percentile and at 17 lb 0 oz the 72.1st, 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 83.1st percentile at 5 months, the 62.6th at 6 months and the 41.1st at 7 months. And sex matters: the same 16 lb 8 oz for a 6-month-old boy is only the 32.4th percentile.

CDC baby weight percentiles, birth to 36 months

All figures below are computed from the CDC infant weight-for-age LMS parameters at exactly the month shown, in pounds.

Two patterns stand out. The spread widens fast: from the 5th to the 95th percentile is 4.0 lb at birth for boys, 6.8 lb at 6 months, 8.3 lb at a year and 11.8 lb at three years. And growth slows just as fast: the median boy gains 2.0 lb in his first month, 1.2 lb in the month from 5 to 6, and 0.7 lb in the month from 11 to 12. The median boy doubles his birth weight by about 5 months and triples it by about 13 months; the median girl doubles hers by about 5.5 months and triples it by about 14.5 months.

CDC weight percentiles for boys, ages 2 to 20

Computed from the CDC child and teen weight-for-age parameters at exactly the birthday shown, in pounds.

The chart fans out sharply with age: the distance from the 5th to the 95th percentile is 10.0 lb at age 2, 46.4 lb at 10, 75.2 lb at 14 and 85.5 lb at 18. That is why a percentile, not a pound figure, is the only sensible way to compare two children.

CDC weight percentiles for girls, ages 2 to 20

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

The two charts cross over. Girls are lighter than boys through early childhood, take the lead at 11 and 12 (medians of 82.0 lb against 79.1 lb, then 91.8 lb against 89.2 lb) while their growth spurt runs first, and fall behind from the mid teens: at 16 the medians are 118.8 lb for girls and 134.3 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, 8.7 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 2.8 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 at different ages, from the same LMS 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: a baby boy at 22 lb is at the 38.3rd percentile at 12 months but would be far above the 97th 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 0 to 36 months from the CDC infant file; child or teen covers 2 to 20 years. Between 24 and 36 months either works and both give the same answer.
2. Pick the sex. Boys and girls have separate CDC curves, and the difference is large in infancy and again in the teens.
3. Enter the age. Months matter: 70 lb is the 72.4th percentile for a boy of 9 years, the 61.3rd at 9 years 6 months and the 48.7th at 10 years. Count from the birth date to the day of the weigh-in.
4. Choose your units. Pounds and ounces is the default; switch to kilograms if that is what the clinic recorded.
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 3rd through 97th 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.
• Weight-for-age: the CDC chart this page uses. It ranks weight against age alone and takes no account of height.
• BMI-for-age: the chart that does account for height, and the only one used to classify a child as underweight, overweight or obese.
• LMS parameters: the three numbers (skew, median, variation) the CDC publishes 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. The steeper the curve, the more a few weeks matter. A 6-month-old girl at 16 lb 8 oz drops from the 83.1st percentile at 5 months to the 41.1st 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 three points. Diapers, clothes and shoes are the biggest source of error.
• Sex. The same 16 lb 8 oz at 6 months is the 62.6th percentile for a girl and the 32.4th for a boy.
• Which chart you picked. The CDC infant reference and the WHO growth standards give different percentiles for the same baby. 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 in months 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 a doctor's visit

A percentile is a screening number, and a low or high one is not a diagnosis. The situations clinicians generally want to see are a weight persistently below the 3rd percentile, a fall across two or more percentile channels over a few months in a baby or a year in an older child, a newborn who has not regained birth weight by about two weeks, weight that is far out of step with the child's height percentile in either direction, or a sudden jump or drop with no obvious explanation. 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 U.S. children measured in national surveys, mostly between 1963 and 1994. They are a reference for how American children grew, not a standard for how any child ought to grow.
• 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 child and teen chart ends at 240 months. Past 20 years there is no percentile here; adults are assessed on BMI instead.
• For children under 24 months, U.S. practice follows the WHO growth standards, so a CDC infant percentile may differ from the one plotted at the pediatric office.
• The chart is not corrected for prematurity. For a preterm baby, clinicians plot corrected age or use a preterm growth reference.
• L, M and S are interpolated linearly between CDC half-month rows, which is standard practice and differs negligibly from the published values.
• Children with certain genetic or chronic conditions are followed on condition-specific charts, not the general CDC chart.

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 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.