What is the difference between overweight and obesity in children?

The definitions

For adults, the cut-offs are straightforward: BMI 25 = overweight, BMI 30 = obesity. For children, the cut-off shifts with age because body composition changes throughout childhood and adolescence. The solution is iso-BMI — the child's BMI is adjusted to what it would correspond to in an adult.

• Iso-BMI 25–30 = overweight
• Iso-BMI 30 or higher = obesity

Want to know where your child sits? The calculator is available at /iso-bmi-barn.

Why the cut-off shifts with age

A 4-year-old child with a BMI of 18 has overweight. A 14-year-old child with the same BMI is within the normal weight range. This is because children naturally have more body fat as toddlers, less during preschool age, and then body fat increases ahead of puberty. The iso-BMI system (Cole 2000) translates the child's BMI into a comparable adult cut-off regardless of age and sex.

What the difference means in practice

Overweight (iso-BMI 25–30)

• Not a disease. Classified as a risk factor.
• Monitored at BVC (child health centre) or by the school nurse with more frequent measurements.
• The family works independently on everyday habits: sleep, meal routines, everyday physical activity, screen time.
• Risk of progressing to obesity if the pattern continues — which is why healthcare services follow up.

Obesity (iso-BMI ≥ 30)

• Classified as a chronic disease (childhood obesity) by the WHO and Socialstyrelsen (National Board of Health and Welfare).
• Must be investigated with blood tests (glucose, lipids, ALT, TSH) and blood pressure — see the BUMM assessment.
• Treated at BUMM (paediatric outpatient clinic) with family-based behavioural treatment for at least two years.
• In severe obesity (iso-BMI ≥ 35), medication (GLP-1 analogues) or, for adolescents, metabolic surgery may be considered.

Why the difference matters

Two examples:

• A child with an iso-BMI of 27 does not require specialist healthcare contact beyond routine check-ups — but the family should begin reviewing everyday habits now, to avoid crossing into obesity.
• A child with an iso-BMI of 31 has a disease that, left untreated, increases the risk of type 2 diabetes, fatty liver, high blood pressure, and cardiovascular disease in adulthood. They should be referred onwards, not left to wait and see.

What if your child is right on the borderline?

Iso-BMI is not absolute — a measurement can fluctuate by a couple of decimals depending on day-to-day variation, fluid balance, and clothing. What matters is the trend across multiple measurements, not a single value. If the child remains at an iso-BMI of 29–31 over time, healthcare services should be involved regardless of which side of the cut-off the most recent measurement falls on.

Related questions

• What is a normal weight for a 10-year-old child?There is no single 'normal' weight figure — here is how iso-BMI works for a 10-year-old.
• When is overweight in children something to take seriously?The patterns that signal overweight needs investigation — and where to seek help.
• What is overweight for a 10-year-old?BMI thresholds for a 10-year-old explained — why iso-BMI is not the same as the adult scale.

See all questions and answers →