Standard BMI does not work for children — body proportions change rapidly as they grow. Instead, iso-BMI is used, an age-adjusted measure that converts the child's BMI into the corresponding adult value taking into account age and sex. The calculator below does the calculation for you.
Calculate BMI for your child
What is iso-BMI?
BMI (body mass index) is calculated as weight in kilograms divided by height in metres squared. For adults, fixed thresholds apply: BMI 25 = overweight, BMI 30 = obesity. But for children, body composition changes from year to year, so fixed thresholds do not work.
Iso-BMI is based on the reference system developed by Cole et al. 20001 — an international standard that translates the child's BMI into the corresponding adult value, taking age and sex into account. This makes it possible to use the same cut-off values (25 and 30) regardless of the child's age.
Thresholds for children
- Iso-BMI below 25 — normal weight
- Iso-BMI 25–30 — overweight
- Iso-BMI 30–35 — obesity
- Iso-BMI over 35 — severe obesity
How should the result be interpreted?
- Iso-BMI is a starting point, not a diagnosis.
- Some children have higher muscle mass or a different body composition.
- A single measurement says less than the trend over time.
- Do not weigh the child at home on your own so that the number becomes the focus. If measuring at home is part of the treatment, it is done in consultation with healthcare.
Other measures used clinically
- BMI-SDS (standard deviations) — more common in clinical follow-up, showing how much the child's BMI deviates from the average.
- Growth charts from BVC — shows the weight trend over time.
- Waist circumference — can provide additional information about abdominal obesity.
What to do with the result?
- For overweight: small changes in everyday life, contact BVC or the school health service if you would like support.
- For obesity or severe obesity: contact healthcare for an assessment.
See also our more in-depth guide on BMI for children with interpretation, age-adjusted tables and common misunderstandings. Read more about how to seek care and about what you as a parent can do.
Frequently asked questions
How do you calculate BMI for children?
BMI for children is calculated in the same way as for adults — weight in kilograms divided by height in metres squared (kg/m²). The difference is that the result is then converted into iso-BMI taking into account the child's age and sex, as body composition changes as they grow. The calculator above does both steps for you.
What is a normal BMI for children?
There is no fixed figure — the normal BMI range changes with age. An iso-BMI below 25 is considered normal weight, 25–30 overweight, and 30 or higher obesity. Iso-BMI converts the child's BMI into the corresponding adult value so that the same thresholds can be used regardless of age.
What is iso-BMI?
Iso-BMI is an age-adjusted BMI measure developed by Cole et al. (2000). It converts the child's BMI into the corresponding adult value taking into account age and sex, and is used in Swedish and international healthcare to assess overweight and obesity in children aged 2–18.
From what age can you calculate iso-BMI?
Iso-BMI is used from 2 years of age up to 18 years. For children under 2 years, growth is instead assessed using BVC growth charts.
Sources
- Cole TJ, Lobstein T. Extended international (IOTF) body mass index cut-offs for thinness, overweight and obesity. Pediatric Obesity 2012;7:284–294.
- Cole TJ et al. Establishing a standard definition for child overweight and obesity worldwide: international survey. BMJ 2000;320:1240.
- Socialstyrelsen. National guidelines for obesity care, 2023.
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Last updated: april 2026