Age selects the reference system
The calculation uses height and weight; choosing a reference is a separate step. WHO describes its general adult BMI references as independent of age and the same for both sexes. This does not make a category a complete assessment of an individual.
bmiatlas applies adult bands from age 18. That is this site's calculator policy. It does not mean every child growth reference ends at that age: the WHO growth reference covers ages 5 to 19.
Children need growth references
A child's BMI needs an age- and sex-specific comparison. WHO provides z-score charts for school-age children and adolescents. A z-score expresses distance from the centre of that reference distribution.
In Germany, AGA recommends the Kromeyer-Hauschild percentile curves. These use a different reference population and different classification boundaries. They are described here as the German system, not as a US standard. AGA percentiles and WHO z-scores must not be combined into a single classification.
Ask your child's doctor which reference they use and how the result relates to previous growth measurements. This site does not calculate child percentiles or z-scores. Entering an age below the site's adult threshold shows the numerical BMI but hides adult bands and the corresponding weight range. A missing or invalid age also leaves the adult interpretation hidden.
Older adults need more context
From age 65, NICE NG246 recommends interpreting BMI cautiously, taking account of other conditions and functional limitations. Winter et al.'s 2014 meta-analysis found an association between lower BMI and higher all-cause mortality in older adults.
An association in observational studies is not a personal weight prescription. It does not show that deliberately gaining weight will extend your life. This page therefore gives no separate BMI target or range for older adults.
The calculator retains its adult display and adds an older-adult note. Discuss the result alongside weight changes, nutrition and your ability to manage everyday activities. If you keep losing weight without trying, contact your doctor.
Make the result useful for a conversation
- Keep the measurement date with height and weight so that changes are traceable.
- For a child, bring earlier growth records and ask which reference chart applies.
- Note questions about changes in appetite, weight or everyday mobility.
- Ask what additional information is needed before drawing a personal conclusion.
The adult BMI chart lists the general bands. The body fat page explains the limits of estimates of body composition. You can also use the main BMI calculator.