A BMI calculator kg with age asks for your age because the meaning of BMI changes for children and teens, not for adults. From 18 onward, one set of bands applies to every age and both sexes. Under 18, the number is ranked on growth charts against children of matching age and sex.
Enter your age, sex, height and weight above. If you are 18 or older, you get the adult bands. If you are younger, the tool sends you to the BMI for kids calculator, which reads the result by age.
Quick numbers
Ages 18 and over
same bands for every ageAges 2 to 17
BMI for age percentilesHealthy adult, India
18.5 to 22.9Over 65
slightly higher BMI often acceptedGender
same adult bands for men and women
Why age is asked in a BMI calculator by age and gender
Growing bodies change height and fat from year to year, so a BMI of 17 can be healthy for a 6 year old and underweight for a 16 year old. Girls and boys also follow different curves through puberty. That is why the age and the sex both matter below 18.
For adults, height has stopped changing and the bands were set from large studies of risk across all adult ages. The calculator still uses age so it can tell which set of rules to apply and to give age notes.
BMI calculator kg with age chart for adults
| Age group | Healthy BMI (India) | Healthy BMI (WHO) | What else to check |
|---|---|---|---|
| 18 to 39 | 18.5 to 22.9 | 18.5 to 24.9 | Waist, activity |
| 40 to 59 | 18.5 to 22.9 | 18.5 to 24.9 | Waist, blood sugar, blood pressure |
| 60 to 74 | 18.5 to 22.9, many doctors accept up to 25 | 18.5 to 24.9 | Muscle strength, unplanned weight loss |
| 75 and above | Judged case by case | Judged case by case | Appetite, falls, frailty |
Notice that the numbers barely move. What changes with age is what else you should look at alongside BMI.
BMI for children and teenagers
For ages 2 to 17, the result is a percentile: the child's rank among 100 peers matched for age and sex. Indian paediatricians rely on the IAP growth charts, which mark the lines that match an adult BMI of 23 and 27. Values below the 5th percentile suggest underweight.
The kids BMI tool explains these charts in full, and ideal weight for kids lists typical weights by age.
BMI after 60
Older adults lose muscle and some height, so BMI can stay the same while body fat rises. Many geriatric doctors prefer that people over 65 do not drift below a BMI of about 22, since some spare weight cushions a hospital stay or a fall. Losing weight without trying is a warning sign at this age.
Strength exercise, enough protein (about 1.0 to 1.2 g per kg a day) and staying active protect muscle. See the protein calculator for a gram target.
Does gender change adult BMI?
No. The adult bands are shared. A woman's fat share runs higher than a man's at matching BMI; that is expected and already allowed for. Read the female BMI page or the male BMI page for notes specific to each.
Body age and metabolic age
Some scales and apps show a body age or metabolic age. This usually compares your resting energy use with the average for people of different ages. It is a marketing figure rather than a medical one, and different devices give very different answers. Your BMI, waist size, blood pressure and blood tests are more useful.
BMI calculator without age
For an adult, age does not change the BMI number at all; it is still kilograms over metres squared. If you leave age out, the result is the same. The main BMI calculator works without it.
For a child under 18 who seems very thin, very heavy, or whose weight has changed quickly, a paediatrician should plot height and weight on a growth chart. Do not put children on a diet based on an adult BMI reading.
Frequently asked questions
Does BMI change with age?
Adult bands hold steady from 18 onward. Younger people are ranked against peers of matching age and sex instead.
What is a normal BMI at 50?
The same as at 25: 18.5 to 22.9 for Indian adults, or 18.5 to 24.9 by WHO bands. At 50, also check your waist and blood sugar.
Why does the BMI calculator ask for age?
To decide whether adult bands or child growth charts apply, and to add notes for older adults.
Is BMI different for older people?
The formula is the same, but many doctors accept a slightly higher BMI after 65 because some extra reserve protects health.
What is BMI for age?
It is the way BMI is read for children and teens, as a percentile on a growth chart for their age and sex.
Can I use the adult BMI calculator for a 15 year old?
No. Teens are still growing, so their number has to be read on a percentile chart; the kids version does that.
Sources
- World Health Organization BMI classification
- Indian Academy of Pediatrics growth charts