Short answer
BMI 15 is considered severe underweight in adults and requires medical assessment. In children, the interpretation depends on age and sex — for a younger child (3–5 years), a raw value of 15 may fall within the normal range, whereas the same figure in a teenager indicates underweight. It is iso-BMI, not the raw value, that is used clinically.
BMI 15 in adults
For adults, fixed BMI thresholds are used regardless of age and sex:
| BMI | Classification |
|---|---|
| Under 16 | Severe underweight |
| 16–17 | Moderate underweight |
| 17–18.5 | Mild underweight |
| 18.5–25 | Normal weight |
| 25–30 | Overweight |
| 30+ | Obesity |
A BMI of 15 falls into the category of severe underweight. This involves an increased risk of nutritional deficiencies, hormonal disturbances (absence of menstruation in women), osteoporosis, a weakened immune system, and cardiac problems. It can be a sign of an eating disorder, malabsorption, or another underlying medical condition — and requires medical assessment.
If you or someone close to you has a BMI of 15, contact your health centre (vårdcentral). Where an eating disorder is suspected, specialised units are available in all healthcare regions.
BMI 15 in children
For children, BMI works differently. The raw value must always be interpreted against age and sex. A BMI of 15 means entirely different things depending on the child's age:
| Age | BMI 15 corresponds approximately to |
|---|---|
| 2–3 years | Normal weight (in the lower range) |
| 5 years | Normal weight |
| 7 years | Normal weight, possibly mild underweight |
| 10 years | Mild underweight |
| 12 years | Moderate underweight |
| 15 years | Severe underweight |
Clinically, iso-BMI is used — an age- and sex-adjusted BMI measure according to Cole 2000. You can calculate the iso-BMI for your child using our iso-BMI calculator.
When should you seek help for a child?
Contact BVC (child health centre), the school health service (elevhälsan), or your health centre if:
- The child's BMI curve is dropping or flattening out.
- The iso-BMI is below the equivalent of 18.5 — this is classified as underweight.
- The child shows reduced or restrictive eating, avoids meals, or has an altered relationship with food or their body.
- The child displays symptoms such as fatigue, feeling cold, hair loss, or absence of menstruation.
Eating disorders in children and adolescents must be taken very seriously. Early treatment offers the best prognosis.
Related BMI values
- BMI 14 — borderline between severe underweight and critically low weight.
- BMI for children — calculator and interpretation guide.
- Iso-BMI for children.
Related questions
- What does a BMI of 14 mean?A BMI of 14 is critically low in adults — requires immediate medical attention.
- How much should an 11-year-old weigh?There is no single correct weight for an 11-year-old — how iso-BMI works at 11 years of age.
- 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.
Sources
- WHO — BMI classification
- Cole TJ et al. — Establishing a standard definition for child overweight and obesity (BMJ 2000)
- 1177 Vårdguiden — BMI
Last updated: June 2026