What Is the Average Waist Size for a 13-Year-Old?

There is no single universal “average waist size” for a 13-year-old, because the number depends on sex, ethnicity, country, and how rapidly puberty is progressing. The best available U.S. data, drawn from national health surveys, puts the mean waist circumference for adolescents aged 12 to 17 at roughly 80 cm (about 31.5 inches) for boys and 79 cm (about 31 inches) for girls, though those figures bundle several ages together and have been climbing over time. What matters more than chasing a single “normal” number is understanding the wide range that healthy 13-year-olds fall into and knowing when waist size signals a health concern worth discussing with a doctor.

What the National Survey Data Show

The most commonly cited U.S. numbers come from the National Health and Nutrition Examination Survey (NHANES), which physically measures thousands of children and adults every few years. Between the early 1990s and the early 2000s, the average waist circumference for boys aged 12 to 17 rose from about 76.8 cm to 79.8 cm, while for girls in the same age range it went from roughly 75.0 cm to 78.9 cm.¹ Those are means for the entire 12-to-17 bracket, so a typical 13-year-old would sit slightly below those averages, since older teens in the group pull the number upward.

Percentile charts give a better sense of the spread. For context, one study using NHANES data found that the 75th percentile for a 13-year-old girl was around 78.8 cm (roughly 31 inches), meaning three out of four girls that age had a smaller waist.² The 50th percentile, the true midpoint, sits meaningfully lower. No single number printed on a chart can tell you whether your child’s measurement is healthy; it has to be read against sex, age, height, and the growth trajectory over time.

Why Boys and Girls Diverge Around Age 13

Puberty reshuffles where the body stores fat, and 13 is right in the thick of that process. Even before puberty, girls carry slightly less fat around the waist relative to the hips than boys do. Research tracking fat distribution from childhood into adulthood found that prepubertal girls had about 5 percent less waist fat than boys, and by early adulthood the gap had widened to roughly 48 percent less.³ That gap starts becoming more visible during the years around 13.

Girls going through puberty tend to add fat in the hips, thighs, and extremities, while boys deposit more of their fat around the abdomen, particularly deep visceral fat surrounding the organs.⁴ This means that at 13, a boy and a girl with the same total body fat can have noticeably different waist measurements. Boys often look thicker through the middle even when they are not carrying more fat overall. For the same BMI, boys and white children tended to have higher waist circumferences than girls and Black children in one large analysis of youth anthropometric data.⁵

Ethnicity Changes the Numbers

Waist circumference percentiles in U.S. adolescents vary significantly by ethnicity, a fact that complicates any attempt to give one universal “average.” Data from NHANES found that Mexican American boys were roughly twice as likely as non-Hispanic white boys to have a waist measurement above the 90th percentile.⁶ Among girls aged 11 and older, non-Hispanic Black, Mexican American, and non-Hispanic white girls at the 90th percentile all had waist sizes in the range of roughly 88 to 89 cm, which already meets the adult threshold for elevated metabolic risk.

Separate research, controlling for BMI and age, found that white boys consistently had higher waist circumferences than Black boys across most age groups, with no such difference in girls.⁷ These differences are not just statistical curiosities. They reflect genuine variation in how different populations store fat, and they are one reason researchers have called for population-specific reference charts rather than one-size-fits-all cutoffs.

How These Numbers Have Shifted Over Time

If you compare a 13-year-old today to one from the early 1990s, the modern child almost certainly has a larger waist. A study combining multiple waves of NHANES data showed that over roughly 20 years, average waist circumference grew by about 3.7 cm in boys and 6.0 cm in girls.⁸ Among boys, the average went from about 70.6 cm to 74.3 cm, an increase of over 5 percent. Among girls, the jump was almost 9 percent. Most of this increase happened between the late 1980s and the early 2000s, with the rate of change slowing after that.

This trend is not limited to the United States. A study of English adolescents found that waist circumference increased for both boys and girls between 1997 and 2005–2007, even when BMI did not change much.⁹ In other words, children’s midsections were getting bigger faster than their overall weight was increasing, a sign that fat distribution itself was shifting toward the abdomen. That matters because abdominal fat, rather than fat stored elsewhere, is more closely tied to metabolic problems.

International Variation

A 13-year-old in Seoul and a 13-year-old in Sofia will have meaningfully different typical waist sizes even at the same percentile. An international effort pooling measurements from over 113,000 children across eight countries, including China, Iran, Korea, Poland, and Switzerland, developed unified waist circumference percentile cutoffs, but the researchers themselves noted that the variation between populations was substantial enough that country-specific charts remain important for clinical use.¹⁰ German reference curves, for example, fall roughly in the middle of available international data.¹¹ Turkish children’s 50th-percentile waist measurements sit above British and Japanese values but below data from a large U.S. cohort.¹²

The upshot is that looking up “average waist size for a 13-year-old” and landing on a number from a different country or ethnic group can be genuinely misleading. A value at the 50th percentile in one population might land at the 65th or the 40th in another. If you are trying to assess a specific child, the most useful comparison is a chart built from their own population’s data.

Why Waist Size Tells You Something BMI Does Not

Most parents are familiar with BMI percentiles, but waist circumference captures something BMI misses: how much fat sits around the organs in the abdomen. A study in Black and white youth found that waist circumference was a stronger predictor of visceral fat and insulin resistance than BMI percentile. After controlling for waist size, BMI no longer predicted visceral fat or markers of insulin resistance, but the reverse was not true. Waist circumference still mattered even after BMI was accounted for.¹³

Research evaluating where to set alarm bells has generally landed on the 75th or 90th percentile for age and sex as clinically meaningful thresholds. One analysis found that a waist circumference at or above the 75th or 90th percentile was appropriate for identifying adolescents with insulin resistance across racial and ethnic groups.¹⁴ It is worth noting, though, that no single body measurement is a perfect screening tool. A study comparing BMI, waist circumference, and waist-to-height ratio in young people concluded that all three performed similarly when it came to identifying those with clustered metabolic risk, and none had excellent accuracy on its own.¹⁵

A Simpler Rule That Skips the Percentile Charts

If pulling up age-and-sex-specific percentile tables feels overwhelming, there is a shortcut that researchers and some clinicians favor: waist-to-height ratio. The rule is straightforward. Keep your waist circumference below half your height. A 13-year-old who stands 160 cm tall, for instance, should aim for a waist measurement under 80 cm. A review of evidence in children and adolescents found that waist-to-height ratio performs about as well as BMI or waist circumference alone for flagging elevated metabolic risk, with the advantage that it does not require age- or sex-specific lookup tables.¹⁶ For a parent doing a quick check at home, it is the most practical yardstick available.

Measuring Correctly Is Harder Than It Sounds

One underappreciated problem with waist circumference in kids is that there is no universally agreed-upon spot to place the tape measure. Some protocols call for measuring at the belly button. Others use the narrowest point of the torso, the top of the hip bone, or the midpoint between the lowest rib and the hip crest. A review of the literature found that no consensus exists on which site best reflects visceral fat or metabolic risk in children and adolescents.¹⁷ The difference between sites can be a centimeter or more, enough to shift a child from one percentile band to another. If you are tracking a child’s waist size over time, the key is to use the same spot each time so the trend is reliable even if the absolute number would differ under a different protocol.

What Drives Waist Size Besides Genetics

Genes play a real role. A twin study analyzing BMI, body-fat percentage, and waist circumference in children found that a highly heritable common factor explained the overlap between all three measures, though waist circumference also had a meaningful chunk of variance, around 14 to 29 percent, driven by factors specific to it rather than to overall body size.¹⁸ That means part of whether a 13-year-old carries fat in the belly versus the hips is inherited, but environment still matters a great deal.

Among modifiable factors, screen time has gotten the most research attention. A study of adolescents found a dose-response relationship between screen time and several measures of abdominal fat, including waist circumference, independent of how much moderate-to-vigorous exercise the teens were getting.¹⁹ Encouragingly, higher cardiorespiratory fitness appears to blunt this effect. A three-year study of children and adolescents found that the link between screen time and waist circumference was weaker in kids who were more aerobically fit.²⁰ In practical terms, a 13-year-old who games for hours but also runs around a lot is in a different metabolic position than one who is sedentary across the board.

Diet composition has also drawn scrutiny. A systematic review of cohort studies in children and adolescents found that higher intake of ultra-processed foods was linked to increases in waist circumference in some studies, alongside increases in BMI and body fat.²¹ Sleep habits round out the picture: a systematic review found a significant association between late bedtimes and higher waist circumference in children.¹⁸ None of these factors act alone, of course. They interact with each other and with genetics, which is why two 13-year-olds eating the same diet and getting the same amount of sleep can still have meaningfully different waist sizes.

Puberty Timing and Waist Size Feed Back on Each Other

An underappreciated wrinkle at age 13 is that the relationship between body fat and puberty runs in both directions. Carrying more abdominal fat can accelerate puberty, and going through puberty earlier can change how fat gets distributed. Research using genetic risk scores found that higher genetic predisposition to larger BMI and waist circumference was associated with earlier puberty in girls, with weaker effects in boys.²² This makes age 13 a particularly messy time to interpret waist measurements, because two girls at different stages of puberty are not really comparable even if they share the same birthday. A girl who started menstruating at 11 has had two more years of hormonal reshuffling of body fat than a late bloomer who has not yet started. Pediatricians factor pubertal stage into their assessments for exactly this reason.

The Food Environment Around Schools

Where a 13-year-old goes to school can influence their waist measurement in a way that has nothing to do with personal choices. A longitudinal study in the UK tracked whether the density of fast-food outlets along a student’s route to school was associated with body size. It found a small but measurable positive association between the number of fast-food outlets on the commute and waist circumference.²³ The effect size was modest — each additional outlet along the route added a tiny fraction to the waist circumference score — but it is a reminder that the environment children navigate daily shapes their metabolic outcomes in ways that individual willpower cannot fully override.