On This Page
- What are the BMI categories?
- How to calculate BMI
- Healthy weight by height
- What weight do I need for a target BMI?
- The two calculator modes
- When BMI misleads
- Checking the calculator's math
- FAQ
What Are the BMI Categories?
BMI is weight divided by height squared. Adolphe Quetelet devised it in the 1830s to describe populations, not to judge one person's health, and it still works best as a quick first screen. For adults 18 and over, men and women alike, the NIH six-tier scale reads:
| BMI range | Category | Health risk |
|---|---|---|
| Below 18.5 | Underweight | Elevated risk of malnutrition, bone loss, and immune issues |
| 18.5 – 24.9 | Normal weight | Lowest risk in the general population |
| 25.0 – 29.9 | Overweight | Moderately elevated risk |
| 30.0 – 34.9 | Obese, Class I | High risk |
| 35.0 – 39.9 | Obese, Class II | Very high risk |
| 40 and above | Obese, Class III | Severely elevated risk |
How to Calculate BMI
US: BMI = weight (lbs) ÷ height (in)² × 703
Metric: a woman who is 165 cm and 62 kg squares 1.65 to get 2.7225, then 62 ÷ 2.7225 = 22.8, Normal weight.
US units: a man who is 5 ft 11 in (71 inches) and 205 lbs: 71² = 5,041, and 205 ÷ 5,041 × 703 = 28.6, Overweight. Reaching BMI 24.9 at his height means 24.9 × 5,041 ÷ 703 = 178.5 lbs, about 26.5 lbs lower.
The 703 factor lines pounds and inches up with the metric scale; older sources sometimes print 704, and both round to the same answer. To run the formula down a whole class list or patient roster, see how to calculate BMI in Excel, which adds a nested IF that labels each category.
Healthy Weight by Height
The Normal range, BMI 18.5 to 24.9, maps to a different weight band at every height. The bands are the same for men and women:
| Height | Healthy weight (lbs) | Healthy weight (kg) |
|---|---|---|
| 5 ft 0 in (152 cm) | 95 – 127 | 43.0 – 57.8 |
| 5 ft 3 in (160 cm) | 104 – 141 | 47.4 – 63.8 |
| 5 ft 5 in (165 cm) | 111 – 150 | 50.4 – 67.9 |
| 5 ft 7 in (170 cm) | 118 – 159 | 53.6 – 72.1 |
| 5 ft 9 in (175 cm) | 125 – 169 | 56.8 – 76.5 |
| 5 ft 11 in (180 cm) | 133 – 179 | 60.2 – 81.0 |
| 6 ft 1 in (185 cm) | 140 – 189 | 63.6 – 85.6 |
| 6 ft 3 in (191 cm) | 148 – 199 | 67.1 – 90.4 |
The cutoffs do not change by sex, but what they mean does: at the same BMI, women typically carry 8–10% more body fat than men.
What Weight Do I Need for a Target BMI?
Flip the formula: target weight (kg) = target BMI × height (m)². At 180 cm, a target of 22 gives 22 × 3.24 = 71.3 kg (157.1 lbs). Useful targets are 18.5 and 24.9 (the edges of Normal), 22 (its midpoint, a common clinical planning figure) and 23 (where the Asian BMI scale starts Overweight).
The Two Calculator Modes
- Calculate BMI: enter height and weight in metric or US units. You get your BMI, its category, the healthy range for your height, and how far you are from it. The optional age field adds a note for anyone under 18 or over 65.
- Find Target Weight: enter height and a target BMI to get the matching weight in kilograms and pounds.
When BMI Misleads
BMI sees weight and height, nothing else, so it goes wrong in predictable places:
- Muscular people: muscle is dense, so rugby players and strength athletes often sit at BMI 27–30 with little body fat. The Lean Body Mass Calculator separates lean mass from fat.
- Belly fat: fat around the abdomen and organs carries more risk than fat elsewhere, and two people with the same BMI can differ a lot. Waist size tracks this more directly.
- Asian populations: metabolic problems tend to start below BMI 25, and WHO guidance suggests screening from 23. The Asian BMI Calculator uses those lower cutoffs.
- Children: under 18 the same number is read against CDC age-and-sex percentiles (below the 5th is underweight, 85th to 94th overweight, 95th and up obese), so the adult table above does not apply.
- Over 65: muscle declines and fat moves to the middle while BMI barely changes. Some guidelines treat BMI 25–27 as acceptable at this age because being underweight carries more risk.
Checking the Calculator's Math
Switch the unit selector from metric to US and the displayed BMI does not change, because pounds and feet/inches get converted to kilograms and metres before the division happens, not adjusted after the fact with a rounded conversion factor. Reverse mode runs the identical equation the other direction: instead of weight divided by height squared, it multiplies a target BMI by height squared to solve for weight, so entering that resulting weight back into forward mode returns the same BMI you started with. The five classification bands (Underweight under 18.5, Normal 18.5 to 24.9, Overweight 25 to 29.9, and two Obese tiers split at 35 and 40) match the WHO Adult BMI cutoffs without any added buffer or rounding at the edges.
Frequently Asked Questions
What is a normal BMI?
A BMI of 18.5 to 24.9 is classified as Normal weight by the WHO and NIH. Below 18.5 is Underweight; 25–29.9 is Overweight; 30 and above is Obese. These cutoffs apply to adults 18 and over and don't change by sex, though guidelines generally accept mild overweight (up to BMI 27) as reasonable in adults over 65.
Is BMI the same for men and women?
The formula and classification thresholds are identical for both sexes. Men and women at the same BMI typically have different body fat percentages, women carry roughly 8–10% more due to physiological differences, but the classification cutoffs themselves don't adjust by sex.
How do I calculate BMI for kids?
Use the same formula, but compare the result to CDC or WHO age- and sex-specific growth charts rather than the adult cutoffs. The fixed thresholds on this page are not valid for anyone under 18.
How accurate is BMI?
It tracks body size reasonably well at the group level, but has real blind spots for individuals. It can classify muscular athletes as overweight and miss high-risk fat patterning in people with a normal-looking BMI. Treat it as a first screen, not a definitive measure. Waist circumference, body fat percentage, and metabolic blood markers together give a much more complete picture, see waist size and waist-to-height ratio for the number BMI misses. Hydration status also affects body weight and therefore BMI readings, the Water Intake Calculator gives a personalised daily fluid target based on your weight, activity level, and climate, worth adjusting further in hot weather per how much water to drink in hot weather.
References
- CDC Adult BMI Calculator: CDC classification guidelines and BMI interpretation for adults.
- NIH BMI reference: National Heart, Lung, and Blood Institute classification tables.
- WHO, Obesity and overweight: Global BMI data and WHO classification thresholds.
- CDC BMI Percentile Calculator for Children and Teens: Age- and sex-specific percentile charts for ages 2–17.