BMI: what it is, a table, overweight risks by age, and an accurate BMI calculator

In short: BMI (Body Mass Index) is a simple calculation based on weight (kg) and height (m) used as a screening tool for weight status in adults. It’s not a diagnosis, but a quick guide—used this way by the WHO and the CDC, too. The WHO defines overweight starting at BMI 25 and obesity starting at BMI 30; in children and teenagers, BMI is always assessed by age and sex using WHO/CDC percentile charts. (WHO; CDC)

What is BMI (formula, who it applies to)

Formula:
BMI = weight (kg) / [height (m)]²

Mainly applies to: adults aged 20+. For children aged 5–19, BMI-for-age is used (percentiles by age and sex). (WHO; CDC)

Note: BMI doesn’t distinguish muscle from fat, doesn’t account for fat distribution, and its interpretation varies by age, sex, and ethnicity. That’s why it should be complemented with other measures (see below). (CDC)

BMI calculator: accurate calculation and a quick example

1. Measure your height in meters (to 2 decimal places).
2. Weigh yourself in kilograms.
3. Calculate: BMI = kg / m².

Example: 74 kg and 1.78 m → 74 / (1.78 × 1.78) = 23.4 → healthy weight.

Want to check it online? You can find the official calculator here:
BMI Calculator (CDC)

BMI table for adults (according to the CDC)

Category	BMI (kg/m²)
Underweight	< 18.5
Healthy weight	18.5 – 24.9
Overweight	25.0 – 29.9
Obesity Class I	30.0 – 34.9
Obesity Class II	35.0 – 39.9
Obesity Class III	≥ 40.0

Note: In adults, BMI categories do not depend on age—the interpretation is the same for ages 20+. (CDC)

Quick overview of weight ranges by height (approximate)

Ranges in kilograms for healthy weight (BMI 18.5–24.9) and overweight (BMI 25.0–29.9).

Height	Healthy weight	Overweight
160 cm	47.4 – 63.7 kg	64.0 – 76.5 kg
170 cm	53.5 – 72.0 kg	72.2 – 86.4 kg
180 cm	59.9 – 80.7 kg	81.0 – 96.9 kg

The cutoffs are calculated precisely from the BMI formula; small differences are due to rounding.

BMI in children and teenagers (ages 5–19)

In younger age groups, BMI is never interpreted using adult numbers. Instead, percentile-based assessment (BMI-for-age) is used, where for example:

• overweight ≈ > +1 SD (at age 19 this corresponds to BMI ~25),
• obesity ≈ > +2 SD (at age 19 this corresponds to BMI ~30).
  Official charts and tables are available. (WHO)

Overweight risks by age (what to watch besides BMI)

Important: BMI is the same for all adults, but the risk profile changes in the details with age—especially fat distribution and muscle loss.
So along with BMI, always track waist fat (visceral fat) and muscle strength/fitness.

Ages 18–39

• Overweight is often linked to insulin resistance and later to hypertension or dyslipidemia.
• Monitor waist circumference / waist-to-height ratio (WHtR). (NICE)

Ages 40–64

• The cardiometabolic risks of overweight/obesity increase; fat accumulation around the abdomen is particularly important.
• A practical rule: your waist should be < half your height (WHtR < 0.5). (NICE)

Ages 65+

• Muscle loss (sarcopenia) is more common—so BMI may underestimate risk if fat is concentrated around the abdomen.
• That makes it even more important to track your waist and functional measures (strength, walking speed), not just body weight. (CDC; NICE)

Limits of BMI and what to measure alongside it

• It doesn’t distinguish muscle and fat (an athlete can have a “high” BMI but low body fat). (CDC)
• It doesn’t account for fat distribution—fat around the abdomen carries higher risk. (CDC)
• Age/ethnicity/sex can change the BMI ↔ risk relationship. (CDC)
• Additional metrics:
  • Waist circumference (easy to do at home),
  • Waist-to-height ratio (WHtR) – target < 0.5 for adults (a practical rule recommended by NICE),
  • For children, always use BMI-for-age percentiles (WHO/CDC).

How to measure your waist correctly (practical)

• Stand relaxed and place the tape midway between the bottom rib and the top of the hip bone, just above the navel; measure after exhaling.
• Compare your waist to your height: waist < 0.5 × height = a better risk indicator than BMI alone (for adults with BMI < 35). (NICE)

Videos to watch (you can embed them by simply pasting the plain URL)

What is BMI (NHS – short explanation):

How to measure your waist correctly (Diabetes UK – instructions):

Why the waist-to-height ratio matters (explanation and BMI limitations):

Frequently asked questions

Is “just over 25” automatically a problem?
No. BMI is a screening tool. Consider your waist (WHtR), blood pressure, glucose, lipids, and overall fitness.

I’m muscular and my BMI comes out high—what should I do?
Check your body composition (at least waist circumference/WHtR). BMI may overestimate your risk. (CDC)

Children and BMI?
For children and teenagers, use BMI-for-age percentiles only—never adult cutoffs. (WHO/CDC)

Sources

1. WHO – Obesity and overweight (fact sheet) – https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight
2. CDC – Adult BMI Categories & About BMI – https://www.cdc.gov/bmi/adult-calculator/bmi-categories.html and https://www.cdc.gov/bmi/about/index.html
3. NICE – Keep the size of your waist to less than half of your height (recommendation) – https://www.nice.org.uk/news/articles/keep-the-size-of-your-waist-to-less-than-half-of-your-height-nice–recommends
4. WHO – BMI-for-age (5–19 years) reference – https://www.who.int/tools/growth-reference-data-for-5to19-years/indicators/bmi-for-age