Baby Weight Percentile Calculator

Updated Aug 15, 2026 By Jehan Wadia

Introduction

The Baby Weight Percentile Calculator shows how your baby's size compares to other babies of the same age and sex. Enter your baby's birth date, sex, and measurements, and the tool gives you a percentile for weight, length, and head circumference.

A percentile is just a way to rank size. If your baby's weight is at the 60th percentile, it means your baby weighs more than about 60 out of 100 babies the same age. There is no "best" number. Babies come in many healthy sizes. If you want to see how ranking works in general statistics, our Percentile Calculator explains the same idea with any set of numbers.

This calculator uses the WHO Child Growth Standards for babies from birth to 24 months. If your baby was born early, check the premature box and enter the weeks of pregnancy at birth. The tool will then use corrected age, which is fairer for preemies. You can confirm the weeks of pregnancy at birth with our Gestational Age Calculator.

You can add up to 24 measurements, so you can track growth over time. The growth chart plots each point against the WHO percentile curves. Steady growth along a curve matters more than one single number. Always talk to your child's doctor about growth concerns. For a broader view of length and head growth together, try the Baby Percentile Calculator.

How to use our Baby Weight Percentile Calculator

Enter your baby's sex, birth date, and measurements. The calculator shows your baby's weight, length, and head size percentiles based on WHO growth charts, plus a growth chart and step-by-step math.

Sex: Pick Boy or Girl. Boys and girls grow at different rates, so they use different WHO growth charts.

Baby's Date of Birth: Enter the day your baby was born. This is used to work out your baby's age at each measurement. You can double-check the exact days between two dates with our Date Duration Calculator or the Age Calculator.

Was your baby born premature? Tick this box if your baby was born before 37 weeks.

Gestational Age at Birth (weeks): If your baby was premature, type how many weeks along you were at birth (22 to 36). The calculator then uses corrected age for fairer percentiles.

Show weight in: Choose lb + oz, kg, or g for your baby weight results and chart. Our Pounds To Kilograms Calculator and Kg To Lbs Calculator can help if your clinic uses a different unit than your home scale.

Show length in: Choose inches or centimeters for length results. See the Cm To Inches Calculator if you need to switch units by hand.

Show head circumference in: Choose inches or centimeters for head size results.

Date of Measurement: Enter the day you weighed or measured your baby. It must be between the birth date and today, and within 24 months of birth.

Weight: Type your baby's weight and pick the unit. With lb + oz, fill in both boxes.

Length / Height: Type your baby's length, measured lying down, and pick inches or cm.

Head Circumference: Type the measurement around the widest part of your baby's head, then pick inches or cm.

Add Another Measurement: Click this to add up to 24 entries. More entries let you track your baby's growth over time on the chart.

Calculate: Click to see percentiles, the growth chart, and the full table. Use Reset / Clear All to start over.

What Is a Baby Weight Percentile?

A percentile compares your baby to other babies of the same age and sex. If your baby's weight is at the 60th percentile, it means your baby weighs more than 60 out of 100 babies that age. The other 40 weigh more. A percentile is not a grade. It does not mean good or bad. It just shows where your baby sits on a growth chart. Behind the scenes the tool converts each measurement into a z-score and then reads it off the normal distribution.

The Three Growth Measurements

Weight for age – shows how heavy your baby is compared to other babies.
Length for age – shows how long (tall) your baby is. Babies under 2 are measured lying down. Later on, the Height Percentile Calculator covers older children and adults.
Head circumference for age – the distance around the widest part of the head. It tells doctors how the brain and skull are growing.

WHO Growth Standards

These percentiles come from the World Health Organization (WHO) Child Growth Standards for babies from birth to 24 months. WHO studied healthy, breastfed babies from many countries to build these charts. Doctors around the world use them as the normal growth guide for the first two years. After age 2, weight-for-height is usually tracked with a Child BMI Calculator instead.

What Counts as Normal Growth?

Most healthy babies fall between the 5th and 95th percentile. But a baby at the 3rd percentile can be just as healthy as one at the 90th. Some babies are simply small or large, often because their parents are. What matters most is the trend. A baby who stays near the same curve month after month is usually growing well. A big jump up or a big drop across several curves is what a doctor wants to look at. If you want to measure the size of a change between two weigh-ins, the Percent Weight Change Calculator is handy.

Corrected Age for Premature Babies

Babies born before 37 weeks need their age adjusted. If a baby was born 6 weeks early, you subtract 6 weeks from their age before checking the chart. This is called corrected age. Without it, a preemie will look far behind when they are really growing right on track. Doctors usually use corrected age until about age 2. Our Weeks To Months Calculator makes it easy to translate corrected weeks into months, and the Due Date Calculator shows the date your baby was originally expected.

Things That Change the Numbers

Small details can shift a percentile. Weigh your baby with no clothes or diaper. Measure length with the legs gently straightened. Use a soft tape for the head, just above the ears and eyebrows. Babies also lose up to 10% of their birth weight in the first days and usually gain it back by 2 weeks. To see how much that drop or regain is in percent terms, use the Percent Change Calculator.

When to Call the Doctor

Talk to your pediatrician if your baby drops across two or more percentile curves, is not back to birth weight by 2 weeks, has fewer wet diapers, seems weak or very sleepy, or if the head is growing much faster or slower than expected. A percentile is one clue, not a diagnosis. Your doctor looks at feeding, energy, milestones, and family size together. Developmental screening tools such as the ASQ Calculator and dosing tools like the BSA Calculator are other pieces of the same picture.

Formulas used

Chronological age in days

\text{age}_{\text{days}} = \text{date}_{\text{measurement}} - \text{date}_{\text{birth}}

Corrected age for prematurity (days)

\text{age}_{\text{corrected}} = \text{age}_{\text{days}} - (40 - \text{GA}_{\text{weeks}}) \times 7

Age in months for WHO table lookup

t = \frac{\text{age}_{\text{corrected}}}{30.4375}

Linear interpolation of WHO L, M, S values between whole months

X = X_{i} + (X_{i+1} - X_{i}) \cdot (t - i), \qquad i = \lfloor t \rfloor,\; X \in \{L, M, S\}

LMS z-score (general case, L \ne 0)

z = \frac{\left(\dfrac{y}{M}\right)^{L} - 1}{L \times S}

LMS z-score when L = 0

z = \frac{\ln\left(\dfrac{y}{M}\right)}{S}

Percentile from z-score

P = \Phi(z) \times 100

Measurement value at a given percentile curve (inverse LMS)

y = M\left(1 + L S z\right)^{1/L}, \qquad y = M e^{S z} \;\text{ if } L = 0

Frequently asked questions

How often should I weigh and measure my baby?

Most families measure at each well-baby visit. That is usually at birth, 1, 2, 4, 6, 9, 12, 15, 18, and 24 months.

Weighing at home more often than once a week is not helpful. Babies gain in spurts, so daily numbers jump around and can worry you for no reason.

Is a low percentile bad?

No. A percentile is a rank, not a score. Half of all healthy babies are below the 50th percentile.

A baby at the 10th percentile who keeps growing along that line is doing fine. Doctors watch the pattern, not one number.

Why did my baby's percentile change since last time?

Small moves are normal. Common reasons include:

A wet diaper or clothes left on the scale
A feed right before weighing
A different scale or tape measure
A wiggly baby during the length check

A drop across two or more curves is what to ask your doctor about.

What does the z-score in the results mean?

A z-score tells how far your baby is from the average, counted in standard deviations.

z = 0 means right at the 50th percentile
z = +1 is about the 84th percentile
z = -1 is about the 16th percentile

The calculator turns the z-score into the percentile you see.

Can I use this calculator for a child older than 24 months?

No. The WHO standards built into this tool stop at 24 months, so the calculator blocks later dates.

For older kids, use a child growth or child BMI tool instead.

Why do I have to enter both weight and length?

You do not have to. You can fill in just one box. The calculator gives a percentile for whatever you enter and shows a dash for the rest.

Entering all three gives a fuller picture of growth.

Do WHO charts work for formula-fed babies?

Yes. The WHO charts are used for all babies from birth to 24 months in most countries.

They were built from breastfed babies, so formula-fed babies may sit a bit higher after a few months. That is expected and usually not a problem.

Why are boys and girls scored on different charts?

Boys are, on average, a little heavier, longer, and have slightly bigger heads than girls in the first two years.

Using one chart for both would push girls' percentiles down unfairly. Picking the right sex gives a fair comparison.

How many measurements can I add?

Up to 24. Each entry needs its own measurement date.

Every entry appears in the table and as a dot on the growth chart, so you can see the trend over time.

Which entry do the result cards use?

The result cards and the step-by-step math use your most recent measurement date.

All other entries still show up in the table and on the chart.

What do the shaded parts of the percentile bar mean?

The green middle section is the 5th to 95th percentile, where most healthy babies sit.

The orange ends mark below the 5th and above the 95th. Babies there can still be healthy, but it is worth mentioning to your doctor.

Why does the chart show pounds as a decimal like 17.5?

Charts need one number per point, so pounds and ounces are joined into a decimal.

17.5 lb means 17 lb 8 oz. The result cards and table still show the full lb + oz form.

What does 'below the 1st' or 'above the 99th' mean?

It means the measurement is past the ends of the normal chart range.

Very high or very low results can happen from a typing mistake or a wrong unit. Check your numbers first, then talk to your doctor if the value is right.

How do I measure my baby's head correctly?

Use a soft tape, not a stiff ruler. Wrap it around the widest part of the head, just above the eyebrows and ears.

Pull it snug, take the reading, then measure twice more and use the biggest number.

How much weight should a baby gain each month?

Rough guides for full-term babies:

0 to 3 months: about 5 to 7 oz (150 to 200 g) a week
3 to 6 months: about 4 to 5 oz (110 to 140 g) a week
6 to 12 months: about 2 to 3 oz (55 to 85 g) a week

Most babies double birth weight by about 5 months and triple it by 1 year.

Until what age should I use corrected age for a preemie?

Most doctors use corrected age until about 2 years old. Some stop earlier for babies born only a few weeks early.

Leave the premature box ticked while you are still tracking within the first 24 months.

Does the calculator save my baby's information?

No. Everything runs in your browser and nothing is stored or sent anywhere.

If you refresh or close the page, your entries are gone. Write down or screenshot results you want to keep.

Can this calculator tell me if my baby is healthy?

No. It is a tracking tool, not a diagnosis.

Growth is only one part of health. Your doctor also checks feeding, wet diapers, energy, milestones, and family size before deciding anything.

Why is my birth weight percentile different from what the hospital told me?

Hospitals often use birth-size charts based on weeks of pregnancy, not WHO age charts.

This tool uses the WHO day-zero standard, so the number can differ a little. Both are correct for what they measure.

Can I switch units after I type a measurement?

Yes. Change the unit dropdown next to the box and the value is converted for you.

The display dropdowns at the top change only how results and the chart are shown, not what you typed.