How to use the BMI calculator
Pick pounds and feet or kilograms and centimeters, then enter your height and current weight. Your BMI updates as you type. You’ll see your number, the category it falls in, and the weight range that keeps BMI between 18.5 and 24.9 at your height.
For a better picture, tap Add your waist. Measure around your middle, just above the hip bones, after a normal breath out. The calculator then shows your waist-to-height ratio, which tells you more about belly fat than BMI can.
Nothing you enter is saved or sent anywhere. The math runs in your browser.
How BMI is calculated
BMI compares your weight to your height. It’s the same formula the CDC and World Health Organization use for adults 20 and older.
BMI = weight (kg) ÷ height (m)²Healthy range at your height = 18.5 × height (m)² to 24.9 × height (m)²If you use pounds, the calculator converts for you (1 lb = 0.4536 kg, 1 inch = 2.54 cm). Example: 5 feet 6 inches (1.68 m) and 180 lb (81.6 kg) gives 81.6 ÷ 2.82 = a BMI of about 29.
BMI categories for adults
The calculator uses the standard adult bands. Each band starts at the lower number and runs up to, but not including, the next.
| BMI | Category | What it usually means |
|---|---|---|
| Under 18.5 | Underweight | Worth a check for nutrition, thyroid or other causes of low weight |
| 18.5 to 24.9 | Healthy weight | Lower weight-related risk; still check waist, strength and labs |
| 25 to 29.9 | Overweight | Risk starts to rise, especially with a large waist |
| 30 to 34.9 | Obesity, class 1 | Higher risk of diabetes, high blood pressure and joint pain |
| 35 to 39.9 | Obesity, class 2 | Medical weight loss support usually helps more than effort alone |
| 40 and up | Obesity, class 3 | Highest risk; very treatable with medical care |
These bands are for adults. Children and teens use BMI-for-age percentiles instead, so this calculator is not meant for anyone under 18.
What BMI can’t tell you
BMI is fast, free and useful for whole populations. For one person, it’s a starting point. It only knows two things about you: height and weight. The CDC notes that it doesn’t tell fat, muscle and bone apart, and it doesn’t show where fat sits.
That leads to real misses:
- Muscular people can land in “overweight” with little body fat.
- Older adults can have a “healthy” BMI while losing muscle and gaining fat inside the belly.
- Belly fat carries more risk than fat on the hips and thighs, and BMI can’t see the difference.
- Background matters. People of South Asian, East Asian and some other backgrounds tend to face weight-related risk at lower BMIs.
In 2025, a global Lancet Commission on obesity said BMI alone should not be used to diagnose obesity in a person. It recommends confirming excess body fat with a waist measure or a direct body-fat measurement, and then looking at whether weight is actually affecting how organs work. Above a BMI of 40, extra fat can be assumed.
Waist-to-height ratio: the check BMI needs
UK health guidance from NICE (2022) gives a simple rule: keep your waist to less than half your height. The calculator divides your waist by your height to show this ratio.
| Ratio | Belly fat risk | Example at 5 ft 6 in (66 in) |
|---|---|---|
| Under 0.5 | No increased risk | Waist under 33 in |
| 0.5 to 0.59 | Increased risk | Waist 33 to 39 in |
| 0.6 and up | High risk | Waist 39.6 in or more |
If your BMI says “healthy” but your ratio is 0.5 or higher, that’s worth a conversation with a clinician. If your BMI says “overweight” but your ratio is under 0.5 and you lift weights, your number may be mostly muscle.
What to do with your result
A number only helps if it leads somewhere. Here is a practical order:
- Find how many calories you burn with the TDEE calculator.
- Set a safe daily target and timeline with the calorie deficit calculator.
- Protect muscle while you lose with the protein calculator.
- Tried all that before? Take the why can’t I lose weight quiz to see what may be holding you back.
Losing even 5% of your weight can improve blood pressure, cholesterol and blood sugar, according to the CDC. You don’t need to reach the “healthy” band to get real health benefits.
How GENEVA looks beyond BMI
At GENEVA Primary Care & Med Spa in Alpharetta, our physicians use BMI as one line on the chart, not the verdict. A visit can include a body-composition scan that shows fat and muscle separately, plus lab work for blood sugar, cholesterol, thyroid and hormones.
From there we build a plan that fits your body: nutrition and protein targets, strength and sleep habits, and a look at medicines that may be adding weight. Women can pair it with women’s health care for perimenopause and PCOS. Men can check for low testosterone through our men’s health team.
Prescription weight loss medicine is generally for adults with a BMI of 30 or more, or 27 or more with a weight-related condition such as high blood pressure, type 2 diabetes or sleep apnea. A clinician decides if it’s right for you. Learn more about medical weight loss and GLP-1 medicines at GENEVA. Results vary.
Frequently asked questions
What is a healthy BMI for adults?
Is BMI accurate for muscular people?
Is BMI different for men and women?
Does BMI change with age?
What BMI qualifies for weight loss medicine?
What is a better measure than BMI?
How much weight do I need to lose to change my BMI category?
Sources
- Centers for Disease Control and Prevention. Adult BMI categories.
- Centers for Disease Control and Prevention. (2025). About body mass index (BMI).
- Rubino, F., et al. (2025). Definition and diagnostic criteria of clinical obesity. The Lancet Diabetes & Endocrinology, 13(3), 221–262.
- National Institute for Health and Care Excellence. (2022). Keep the size of your waist to less than half of your height, NICE recommends.
- Centers for Disease Control and Prevention. (2025). Steps for losing weight.
This tool gives general estimates for healthy adults and is not medical advice, a diagnosis or a treatment plan. It is not meant for anyone pregnant, under 18, or managing a medical condition without their clinician. Talk to a GENEVA provider before making big changes to how you eat, drink or exercise.