Quick Answer: What is BMI? Body mass index is your weight in kilograms divided by your height in meters squared, and it works as a screening measure.
For adults aged 20 and over, CDC lists 18.5 to under 25 as a healthy weight and 30 or above as obesity. BMI is a rough guide to weight loss progress at best. It cannot see muscle or fat distribution, so pair it with waist-to-height ratio.
What Is BMI and How Is It Calculated?
BMI divides your weight in kilograms by your height in meters squared (kg/m²). For example, a person who weighs 70 kg and stands 1.75 m tall has a BMI of about 22.9. The sum is 70 divided by 3.06, which is 1.75 times 1.75.
You can check yours with our free BMI calculator. However, treat the result carefully. CDC describes BMI as a screening measure that should be considered with other factors, not as a diagnosis.
What BMI Categories Mean for Adults
| Category | BMI (kg/m²) |
|---|---|
| Underweight | Less than 18.5 |
| Healthy weight | 18.5 to less than 25 |
| Overweight | 25 to less than 30 |
| Obesity, class 1 | 30 to less than 35 |
| Obesity, class 2 | 35 to less than 40 |
| Obesity, class 3 | 40 or greater |
Note that these categories do not apply to children and teens. If you are asking about a child, speak to a pediatrician or GP instead.
Lower BMI Cut-Offs for Some Family Backgrounds
The CDC table lists a single set of cut-offs. However, UK guidance differs. NICE guideline NG246 applies lower thresholds for people with South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds. NICE explains that cardiometabolic risk occurs at a lower BMI in these groups.
| Group | Overweight | Obesity |
|---|---|---|
| CDC adult categories (all adults) | 25 to 29.9 | 30 or above |
| NICE: the family backgrounds listed above | 23 to 27.4 | 27.5 or above |
So if this applies to you, ask your doctor which threshold they use. Our height and weight chart for women also compares standard and Asian BMI cut-offs.
What BMI Cannot Tell You
BMI uses only height and weight. As a result, it has clear blind spots:
- Fat distribution. NICE says BMI is not a direct measure of central adiposity (fat around the waist).
- Muscle. NICE advises caution when interpreting BMI in adults with high muscle mass.
- Older age. NICE advises caution from age 65, because a slightly higher BMI may be protective when older.
- A healthy reading. A person in the healthy category may still carry central adiposity.
- Health markers. Blood pressure, cholesterol and blood glucose do not enter the formula.
Still, BMI remains a quick starting point. In fact, it is a practical measure that NICE and CDC both use, with the caveats above.
What to Use Alongside BMI
For adults with a BMI below 35, NICE recommends using waist-to-height ratio alongside BMI. Divide your waist measurement by your height, using the same units. NICE advises keeping your waist to less than half your height, which means a ratio under 0.5.
| Waist-to-height ratio | What NICE says |
|---|---|
| Under 0.5 | The target range |
| 0.5 to 0.59 | Increased central adiposity, with increased health risks |
| 0.6 or more | High central adiposity, with further increased health risks |
Besides the ratio, the NHS also gives waist circumference thresholds. Health risk rises above 94 cm (37 in) for men and 80 cm (31.5 in) for women. It is higher still above 102 cm (40 in) for men and 88 cm (34.5 in) for women.
What about body fat scales? In short, NICE advises against using bioimpedance as a substitute for BMI as a measure of general adiposity in adults. Treat these scales and our body fat percentage calculator as rough estimates only.
What the Lancet Commission Changed in 2025
In January 2025, a Commission in The Lancet Diabetes and Endocrinology published diagnostic criteria for clinical obesity. Rubino and colleagues, a group of 56 authors, separated preclinical obesity, a risk factor, from clinical obesity, treated as a standalone illness. News coverage described the report as moving beyond BMI alone. The full paper is linked in the sources below.
Is BMI a Good Indicator of Weight Loss Success?
Not on its own. That is because BMI changes in proportion to weight, so a 5 percent weight loss lowers BMI by 5 percent. CDC notes that losing 5 percent of body weight, which is 10 pounds for a 200-pound person, can improve blood pressure, cholesterol and blood sugar.
| Starting BMI | After 5% weight loss | After 10% weight loss |
|---|---|---|
| 35 | 33.3 (class 1 obesity) | 31.5 (class 1 obesity) |
| 32 | 30.4 (class 1 obesity) | 28.8 (overweight) |
| 28 | 26.6 (overweight) | 25.2 (overweight) |
In other words, your health markers can improve while your BMI category stays the same. Judging progress by category alone can feel discouraging. Instead, add your waist measurement, strength, energy and the checks your care team runs.
Meanwhile, CDC also says that people who lose weight at a gradual pace, about 1 to 2 pounds a week, are more likely to keep it off. Our sustainable weight loss tips and maintenance guide show how to build that pace.
What to Do With BMI in a Weight Loss Plan
- Check your BMI once with the BMI calculator to set a baseline.
- Add your waist-to-height ratio, and re-measure every few weeks.
- Track progress with strength, stamina and energy, not only the scale.
- Ask for blood pressure, cholesterol and blood glucose checks, especially if you have a family history. Our guide to diabetes and heart disease explains why they matter.
- Set food habits you can keep. For ideas, read about how to balance protein, carbs and fats or whether you can lose weight without exercise.
Safety: When to Talk to a Doctor
First, see a doctor if your BMI is under 18.5, if your weight changes without explanation, or if your waist measurement is in the higher-risk range. Ask a pediatrician or GP about children and teens. People who are pregnant should follow advice from their midwife or doctor.
Also, frequent weighing and measuring can worsen anxiety for some people. If numbers feel stressful, or you have a history of an eating disorder, talk to a clinician. In the US, the National Alliance for Eating Disorders offers a helpline. In a crisis, call or text 988 in the US, or call Samaritans on 116 123 in the UK.
What the Evidence Says About Common BMI Claims
| Claim | Where it stands | What to know |
|---|---|---|
| A healthy BMI means you are healthy | Overstated | NICE notes that a person in the healthy category may still have central adiposity. |
| BMI is useless | Overstated | CDC and NICE both use BMI as a screening or practical measure, alongside other checks. |
| Everyone uses the same cut-offs | Not supported | NICE applies lower thresholds for several family backgrounds. |
| Body fat scales replace BMI | Not supported | NICE advises against using bioimpedance as a substitute for BMI in adults. |
| Weight loss only counts if your BMI category changes | Not supported | CDC says losing 5% of body weight can improve blood pressure, cholesterol and blood sugar. |
Frequently Asked Questions
BMI Basics and Categories
For adults aged 20 and over, CDC lists 18.5 to less than 25 as healthy weight. NICE uses lower overweight and obesity cut-offs for people with South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds.
Divide weight in kilograms by height in meters squared. A person who weighs 70 kg and is 1.75 m tall has a BMI of about 22.9.
NICE advises caution when interpreting BMI in adults with high muscle mass. Waist-to-height ratio and a clinician assessment give a clearer picture.
NICE advises keeping your waist under half your height, a ratio under 0.5. A ratio of 0.5 to 0.59 means increased central adiposity, and 0.6 or more means high central adiposity.
The NHS gives more than 94 cm (37 in) for men and 80 cm (31.5 in) for women as increased risk. Above 102 cm (40 in) for men and 88 cm (34.5 in) for women, risk is higher.
Weight Loss, Age and Safety Questions
BMI falls in proportion to the weight you lose. A 5 percent loss takes a BMI of 32 down to 30.4. CDC says that 5 percent can still improve blood pressure, cholesterol and blood sugar.
NICE advises against using bioimpedance as a substitute for BMI as a measure of general adiposity in adults. Treat the reading as a rough extra data point.
NICE advises caution from age 65, because a slightly higher BMI may be protective when older. Discuss your results with your doctor.
Speak to a doctor if your BMI is under 18.5, your weight changes without explanation, or your waist measurement is in the higher-risk range. Also seek help if tracking numbers affects your mood or eating.
Related Reading
- Sustainable weight loss tips for lasting results
- Height and weight chart for women: healthy BMI ranges explained
- How to balance protein, carbs and fats for weight loss
- Can you lose weight without exercise?
Sources
- CDC: Adult BMI categories
- NICE NG246: Identifying and assessing overweight, obesity and central adiposity
- NHS: Why is my waist size important?
- CDC: Losing weight
- Rubino et al. (2025), The Lancet Diabetes and Endocrinology: Definition and diagnostic criteria of clinical obesity
- Drug Topics: Lancet Commission redefines clinical obesity, moving beyond BMI
Written and fact-checked by the HealthCoachJP editorial team. No clinician has reviewed this article. See our sourcing policy. Last reviewed and rewritten October 2026.
Disclaimer: This article is general information and not medical advice. BMI is a screening measure and cannot diagnose a condition. Speak to a doctor about your own results.
Sources:
We value truthful content. 3 sources were referenced during research to write this content.
- World Health Organization (WHO). (n.d.). World Health Organization (WHO). Retrieved from https://www.who.int/
- (n.d.). Abdominal Muscles: Anatomy & Function. Retrieved from https://my.clevelandclinic.org/health/body/21755-abdominal-muscles
- (n.d.). Maintain a Healthy Weight. Retrieved from https://www.nhlbi.nih.gov/health/educational/lose_wt/index.htm
