Ideal BMI by Age — What Changes and What Doesn't

Searches for an ideal BMI by age usually expect a table with a row for each decade. There isn’t one. In the US ranges, every adult aged 20 and older is read against the same band, 18.5 to 24.9. Age changes the reading in exactly two places — before 20, and, as a caveat, after 65. Calculate your number first:

The one adult table, and who it covers

Age group	How BMI is read
Under 20	Percentile for age and sex, from a growth chart
20 and older	18.5–24.9 healthy · 25–29.9 overweight · 30+ obesity
65 and older	Same cutoffs, with a documented caveat (below)

The NIDDK publishes those adult ranges for “adults ages 20 and older” without any further split by age. So a 27-year-old and a 58-year-old with identical numbers get identical labels, and the common belief that the bar loosens as you age has no basis in the published US cutoffs.

What genuinely changes across those decades is body composition. Lean mass tends to fall and fat tends to rise while the scale stays put, and BMI is blind to that swap by construction. That is an argument for adding waist circumference in midlife, not for adjusting the BMI table.

Under 20: a percentile, not a number

Children and teens are still growing, so a fixed adult cutoff would be meaningless — a healthy BMI for a 7-year-old is not a healthy BMI for a 17-year-old. Their number goes onto a growth chart for age and sex, and what matters is where it falls:

• underweight: below the 5th percentile
• healthy weight: 5th to 85th percentile
• overweight: 85th to 95th percentile
• obesity: at or above the 95th percentile

MedlinePlus is blunt about the crossover: do not use an adult BMI calculator to evaluate a child. The arithmetic is the same, but the interpretation is not.

After 65, a caveat rather than a new table

Here is where the American answer and the intuition part company. MedlinePlus notes that if you are 65 or older, a slightly higher BMI may help protect you from thinning of the bones (osteoporosis). That is a reason to be less alarmed by a reading of 26 at that age — not a published range that replaces 18.5 to 24.9.

We checked whether US federal sources publish a distinct older-adult band, of the kind used in some other countries, and did not find one we could read. So this page reports the caveat as a caveat, and leaves the cutoffs where the NIDDK and NHLBI put them.

Use age to interpret, not to recalculate

The arithmetic never changes: weight divided by height squared. What age changes is which sentence you attach to the answer. Find your group, read the result accordingly, and if you are near a boundary, treat it as a prompt for a conversation. The BMI calculator gives the number in pounds and inches, and the weight-by-height chart shows the band you are aiming at.

This content is informational and does not replace evaluation by a health professional.