Knowledge
BMI and iso-BMI in children
BMI in children needs to be interpreted differently from BMI in adults. As children's bodies change with age and growth, age- and sex-adjusted limits, often called iso-BMI, are used. Here we explain how BMI and iso-BMI are used in paediatric care and why the development over time is more important than a single value.
How BMI is calculated for children
The BMI formula itself is the same for children and adults:
BMI = weight in kilograms / height in metres²
For example, a child weighing 40 kg and 1.45 metres tall has:
40 / (1.45 × 1.45) = BMI 19.0
But the value 19.0 cannot be interpreted in the same way for a child as for an adult.
Children's normal BMI changes during growth and also differs between boys and girls. Therefore, the value needs to be compared with limits that take into account the child's age and sex.
What is iso-BMI?
Iso-BMI is the Swedish term for age- and sex-adjusted BMI limits based on the International Obesity Task Force, IOTF.
The terms iso-BMI 25, 30, 35 and 40 do not mean that the child's actual BMI is always 25, 30, 35 or 40. Instead, they indicate the BMI curve that, at 18 years of age, corresponds to these adult limits.
In the national care programme, the following are used:
iso-BMI 25
overweight threshold
iso-BMI 30
obesity threshold
iso-BMI 35
severe obesity
iso-BMI 40
very severe obesity
Table: BMI limits for overweight and obesity in children
The table shows the IOTF limits for full years between 2 and 18 years. The limits correspond to iso-BMI 25 for overweight and iso-BMI 30 for obesity.
| Age | Boys: overweight | Boys: obesity | Girls: overweight | Girls: obesity |
|---|---|---|---|---|
| 2 years | 18.36 | 19.99 | 18.09 | 19.81 |
| 3 years | 17.85 | 19.50 | 17.64 | 19.38 |
| 4 years | 17.52 | 19.23 | 17.36 | 19.16 |
| 5 years | 17.39 | 19.27 | 17.23 | 19.20 |
| 6 years | 17.52 | 19.76 | 17.33 | 19.62 |
| 7 years | 17.88 | 20.59 | 17.69 | 20.39 |
| 8 years | 18.41 | 21.56 | 18.28 | 21.44 |
| 9 years | 19.07 | 22.71 | 18.99 | 22.66 |
| 10 years | 19.80 | 23.96 | 19.78 | 23.97 |
| 11 years | 20.51 | 25.07 | 20.66 | 25.25 |
| 12 years | 21.20 | 26.02 | 21.59 | 26.47 |
| 13 years | 21.89 | 26.87 | 22.49 | 27.57 |
| 14 years | 22.60 | 27.64 | 23.27 | 28.42 |
| 15 years | 23.28 | 28.32 | 23.89 | 29.01 |
| 16 years | 23.89 | 28.88 | 24.34 | 29.40 |
| 17 years | 24.46 | 29.43 | 24.70 | 29.70 |
| 18 years | 25.00 | 30.00 | 25.00 | 30.00 |
Important: The table is a classification aid. The assessment of an individual child also needs to take into account the growth curve and how BMI has changed over time.
Why a single BMI value is not enough
In paediatric care, development is often more important than a single measurement.
The national care programme emphasises that the BMI curve should be assessed longitudinally. A rapid increase can be clinically relevant even before a certain threshold value has been passed.
Height and weight also need to be assessed together. A flattened height growth combined with weight gain can, for example, be a reason for further investigation.
It is therefore better to think “How is the child's BMI curve developing?” and “What is the child’s BMI today?”
When does iso-BMI indicate obesity?
Obesity is diagnosed from iso-BMI 30.
The national care programme states that children with an iso-BMI over 30 should be investigated. How the path forward looks – for example via child health services, school health services, primary care or specialist paediatric and adolescent medical care – varies between regions and with the child's age and overall health.
In cases of rapid weight gain, abnormal height growth or suspected other illness, an investigation may be needed sooner.
Read more about how overweight and obesity are detected in children
BMI is a tool – not a description of the child
BMI and iso-BMI are used to identify and monitor health risks at group and individual level. They do not describe the child's value, motivation or lifestyle.
The care programme therefore emphasises the importance of a respectful and non-stigmatising approach and of speaking neutrally about growth and weight.
How BMI development is monitored with Evira
In childhood obesity treatment, healthcare needs to be able to monitor development over a longer period.
With Evira, the child weighs themselves at home on a connected smart scale that does not display weight in kilograms. The measurement is automatically transferred to the family’s app and to the treatment team’s clinic interface.
For children, weight information needs to be related to, among other things, height, age and expected growth. Evira therefore visualises development over time and enables the treatment team to monitor the patient between scheduled clinic visits.
The national care programme describes Evira as a digital treatment support in combination with lifestyle treatment. Read more about the national care programme and KLB
Do you work with childhood obesity?
See how Evira can complement regular treatment with follow-up between visits.