Body mass index (BMI) is weight in kilograms divided by the square of height in meters. In US units it is 703 × pounds ÷ inches². An adult who is 5 ft 9 in and weighs 160 lb has a BMI of 23.6, which the CDC classifies as a healthy weight. For that height the healthy range (BMI 18.5 to under 25) is about 125 to 168 lb.
The CDC calls BMI a screening measure, not a diagnosis. It does not distinguish muscle from fat or show where fat is carried, so it should be considered with other factors. A very muscular person can have a high BMI with little body fat. For a closer look at body composition, try the body fat calculator (u.s. navy method).
How to use the bmi calculator
- Choose US or metric units.
- Enter your height (feet and inches, or centimeters) and weight without heavy clothing or shoes.
- Read your BMI, the adult category and the weight range that corresponds to a BMI from 18.5 to under 25 at your height.
- For children and teens (2–19), use a pediatric BMI-for-age percentile tool instead of these adult categories.
Formula
The factor 703 converts pounds and inches to kilograms and meters (0.45359237 kg per lb ÷ 0.0254² m² per in², rounded). Healthy-weight range = 18.5 × height² to 25 × height², in kg with height in meters.
Worked example
Height 5 ft 9 in (69 in), weight 160 lb.
- BMI = 703 × 160 ÷ 69² = 112,480 ÷ 4,761 = 23.6.
- 23.6 is between 18.5 and 25, so the category is healthy weight.
- Range for 69 in: 18.5 × 4,761 ÷ 703 = 125 lb up to just under 25 × 4,761 ÷ 703 = 169 lb, matching the CDC's own 5 ft 9 in example of 125 to 168 lb.
CDC adult BMI categories
| BMI (kg/m²) | Category |
|---|---|
| Less than 18.5 | Underweight |
| 18.5 to less than 25 | Healthy weight |
| 25 to less than 30 | Overweight |
| 30 to less than 35 | Obesity, class 1 |
| 35 to less than 40 | Obesity, class 2 |
| 40 or greater | Obesity, class 3 (severe) |
The CDC applies these categories to adults 20 and older regardless of age, sex or race. If you want to change your weight, estimate your daily energy needs with the tdee calculator and plan a safe rate with the calorie deficit calculator. The ideal weight calculator compares BMI ranges with clinical ideal-body-weight formulas.
Weight range for BMI 18.5–24.9 by height
| Height | Weight range |
|---|---|
| 5 ft 0 in | 95–127 lb |
| 5 ft 4 in | 108–145 lb |
| 5 ft 8 in | 122–164 lb |
| 6 ft 0 in | 136–184 lb |
| 6 ft 4 in | 152–205 lb |
Caveats and assumptions
- BMI is a screening tool; it does not measure body fat directly or diagnose any condition.
- It can overestimate fat in muscular people and underestimate it in older adults who have lost muscle.
- Adult categories do not apply to children, teens or pregnant people.
- Not medical advice; discuss your weight and health with a clinician.
Frequently asked questions
How is BMI calculated?
Weight in kilograms divided by height in meters squared, or 703 × weight in pounds ÷ height in inches squared.
What is a healthy BMI?
For adults the CDC defines healthy weight as a BMI of 18.5 to less than 25.
What is the healthy weight for 5 ft 9 in?
About 125 to 168 lb, which corresponds to a BMI of 18.5 to under 25.
Is BMI accurate?
It is a quick screening measure that works reasonably well for populations, but it does not distinguish fat from muscle. The CDC says it should be considered with other factors.
Does BMI apply to children?
Not with the adult cut-offs. For ages 2–19, BMI is compared with growth charts as a percentile for age and sex.
Sources
- CDC — Adult BMI Categories — BMI formula, categories and the 5 ft 9 in example
- NIST SP 811 Appendix B.9 — Conversion factors — pound and inch to SI
- NIDDK — Body Weight Planner — healthy weight range warnings for goal weights
This calculator gives general estimates for healthy adults and is not medical advice, diagnosis or treatment. Equations are averages and individuals differ. Talk to a qualified health professional before changing your diet, exercise or medication, and during pregnancy. See our disclaimer and how we build and check calculators.