Maybe your 2-year-old just outgrew a pair of pants in six weeks. Maybe a grandparent keeps saying the baby "has basketball hands." Either way, you typed "how tall will my child be" and got a stack of clinical calculators and percentile charts.

This guide gives you the same math in plain language, the folk rules families swap, and an honest read on how much to trust each one. Then it turns the question into something better than a number: a family prediction night you get to settle years from now.

The math guess: how a mid-parental height calculator works

Almost every mid parental height calculator online runs the same short formula. You can do it on a napkin.

1. Add both biological parents' heights together, in inches.
2. For a boy, add 5 inches. For a girl, subtract 5 inches.
3. Divide by 2.

The 5 inches stands in for the typical height gap between adult men and women. If you work in centimeters, use 13 instead of 5.

Worked example. Say the parents are 5'5" (65 inches) and 5'11" (71 inches). Together that's 136 inches. For a son: 136 + 5 = 141, divided by 2 = 70.5 inches, or about 5'10½". For a daughter: 136 − 5 = 131, divided by 2 = 65.5 inches, or about 5'5½".

Two quick tips make the math guess better:

• Measure the grown-ups tonight. Heights from memory or a driver's license tend to be generous. Stand barefoot against a wall and use the same flat-book trick you use for the kids.
• Write it as a range. Put a few inches above and below your answer. Doctors who use this formula treat it as the middle of a range, not a single number, and your card should too.

If you don't know both heights, skip this one. Adoption, donor conception, or a parent who isn't in the picture all leave the formula without its inputs, and that's completely fine. The other guesses below still make a great prediction night.

Other ways families guess, from folk rule to pure fun

The formula isn't the only option. Here are the other height predictors for kids that families actually use, roughly in order from "sort of scientific" to "total folklore."

The double-at-two rule

Take your child's height at age 2 and double it. Many versions of the rule use 18 months for girls instead. A boy who's 35 inches at 2 would land around 70 inches (5'10"), and a girl who's 32 inches at 18 months would land around 64 inches (5'4").

Use the number from the checkup if you have it. Getting a wiggly toddler to stand still for a measurement is its own sport.

Follow the percentile line

A growth chart calculator, the kind based on the charts pediatricians use, tells you where your child sits compared with other kids the same age and sex. It's a snapshot of right now, not a forecast. Still, some families trace their child's percentile line out to age 20 and read off the adult height at the end.

This only works, even for fun, if your child has followed a steady line for a while. Babies and toddlers often shift lines before they settle, and puberty can move things again. The CDC growth charts page has printable charts if you want to try it.

Online predictors that use current height

Some online height predictors go a step further. Along with the parents' heights, they ask for your child's current age, height, and weight. One widely used version is called the Khamis-Roche method, made for kids about 4 and older.

These use more information than the napkin formula, but they're still estimates. Plug in a fresh measurement, not one from last year.

The family lookalike guess

Who does your child take after? Pull out old photos of parents, aunts, uncles, and grandparents at the same age and see who matches. If your 3-year-old has Uncle Dan's exact build, Uncle Dan's height becomes a pretty fun guess.

The hands-and-feet guess

Family lore says big hands or big feet mean a tall grown-up, the way a puppy's paws hint at a big dog. There's no reliable track record behind it. Count it strictly for laughs.

Everyone's gut guess

Grandparents, aunts, older siblings, the kid themselves: everyone has an opinion. Gut guesses aren't accurate, and that's the point. They say something about the people guessing, and they make the reveal far more fun than a single number from a formula.

How accurate is any of this?

In short: close enough for a family game, not precise enough to plan around. Height comes from both sides of the family, and it's shaped by nutrition, sleep, overall health, and when puberty starts. Siblings with the same two parents can end up inches apart.

Your pediatrician measures your child at checkups and watches the trend over time, which tells them far more than any prediction. If your child seems to have stopped growing, or has dropped well below the line they used to follow, bring it up at the next visit or call sooner. HealthyChildren.org, from the American Academy of Pediatrics, is a good plain-language place to start with growth questions.

Host a prediction night: the activity card

Here's the fun version of the whole question. Everyone writes down a guess, you add the math guess, and the card gets sealed until your child is grown. It takes about 20 minutes and works at any age, including babies who sleep through it.

What you'll need

• ☐  One copy of the card below per child (an index card works)
• ☐  A pen for every guesser
• ☐  A tape measure and a flat, hardcover book for measuring
• ☐  One envelope per child
• ☐  Tape or a wax seal to close it (sealing is a grown-up job, and hot wax is adults-only)
• ☐  A phone for photos
• ☐  Optional: a roll of painter's tape for marking guesses on the wall

If there's an under-3 in the room, keep pens, caps, tape, and sealing supplies up high between turns.

The card (copy this)

PREDICTION NIGHT

Name: ____________ Date: ____________

Age tonight: ______ Height tonight: ______

The math guess: ______ (our range: ______ to ______)

Other rules we tried: ____________

Our guesses (name / guess / why):
1. ____________
2. ____________
3. ____________
4. ____________
5. ____________

The kid's own guess: "When I grow up, I'll be ______ tall, tall enough to reach ______."

Closest guess wins: ____________

Open on: ____________

Signed: ____________

How to run it

1. Pick a night when people are already together. A birthday, a holiday dinner, or your yearly measuring day all work. A Thanksgiving height check at Grandma's house is a natural fit, since the grandparents' guesses are half the fun.
2. Measure and mark first. Shoes off, heels to the wall, flat book on the head, mark, date. For a baby who can't stand yet, use the length from the most recent checkup. If you're new to this, our guide on when to start tracking your baby's height covers how to get a clean mark at every age. Write tonight's height on the card.
3. Collect guesses before anyone does the math. Once people hear the formula, every guess drifts toward it. Have everyone write privately, then read them out loud.
4. Let the kid guess too. A 4-year-old who says "as tall as the ceiling" gets it written down exactly like that.
5. Add the math guess. Run the mid-parental formula and write it with a range a few inches on either side. Add the double-at-two result if your child is the right age.
6. Choose the prize. Decide tonight what the closest guesser wins: picking the celebration dinner, a trophy made from a paper towel roll, or bragging rights in the family group chat.
7. Make the guesses visible. If you have painter's tape, an adult sticks a short strip on the wall at each guess, labeled with initials. Photograph your child standing under the whole ladder of guesses (for a baby, take the photo in a grown-up's arms in front of the tape ladder), then peel the tape off.
8. Photograph the card, then seal it. Write "Open on" and the date or event on the front of the envelope.
9. Store it where you'll find it in 15 years. A memory box, the baby book, or the folder with birth certificates all work. Set a far-off phone reminder and keep the card photo in a shared album as a backup.

A sample filled-in card

Here's how the guesses might look for a 3-year-old boy whose parents are 5'5" and 5'11", the same family from the math example above.

When to open the envelope

Most kids finish growing sometime in their teens, and girls usually finish earlier than boys. Pick an opening moment that fits your family, and write it on the envelope so nobody argues later.

• The day they pass a parent. If you measure every year, this one sneaks up on you in the best way.
• A milestone birthday, like 16 or 18.
• High school graduation, right alongside the cap-and-gown photos.
• When they reach the top of the chart. Once there's no room for next year's mark, it's reveal time.
• When the yearly marks stop climbing. Once a new measurement lands right on top of last year's, it's a good time to check the guesses.

You can also hold a second prediction night around age 10 or 12 and keep both envelopes. Comparing the baby-era guesses with the preteen ones shows how much a family's picture of a kid changes along the way.

Make the growth chart your scoreboard

A growth chart can't predict anything. What it can do is hold every real answer along the way: one dated mark per year, climbing toward the guesses in that envelope. On reveal day, one last measurement settles who guessed closest.

A few habits keep the marks trustworthy over the years:

• Measure on the same day each year. If you already do a New Year's Day height check, that's your date.
• Use the same method every time: shoes off, heels to the wall, flat book, eyes level with the mark.
• Hang the chart securely, away from the crib. Our guide to hanging a canvas growth chart without wrecking your wall covers placement and the baseboard offset in a couple of minutes.

Door-frame marks work until you move, and a sealed envelope may outlast a house or two. Our Canvas Kids Growth Chart ($44) is 100% cotton canvas with a fabric-safe pen included. It starts at the 6-inch mark so it clears the baseboard, comes off the wall when you move, and can be personalized with a name. If you'd rather have color, both designs are in our canvas growth charts collection.

Canvas Kids Growth Chart

$44.00

View the Canvas Kids Growth Chart

On reveal day, measure one last time and write the final height and the winner's name on the card, and beside the top mark if there's room. The chart and the card end up telling the same story from two sides.

Keep it light: a few ground rules

• Guesses, not wishes. Skip lines like "fingers crossed he's tall." Kids hear more than we think, and height isn't something they can work on.
• No ranking siblings. If you make cards for more than one child, keep each envelope separate and give each reveal its own moment.
• There's no bad outcome. A final height below every guess is still a win for whoever guessed lowest. The story matters more than the number.
• Real worries go to the pediatrician. Prediction night is for fun. Questions about whether your child is growing well belong at a checkup.

If you only do one thing, measure the grown-ups, run the formula, and write the answer on a card next to everyone's gut guesses. Seal it, mark the chart, and put a date on the envelope. Years from now, the number will matter a lot less than the handwriting.

Frequently asked questions